Sync usage with man page.

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This is flex.info, produced by makeinfo version 4.8 from flex.texi.

INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* flex: (flex).      Fast lexical analyzer generator (lex replacement).
END-INFO-DIR-ENTRY

   The flex manual is placed under the same licensing conditions as the
rest of flex:

   Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007 The Flex
Project.

   Copyright (C) 1990, 1997 The Regents of the University of California.
All rights reserved.

   This code is derived from software contributed to Berkeley by Vern
Paxson.

   The United States Government has rights in this work pursuant to
contract no. DE-AC03-76SF00098 between the United States Department of
Energy and the University of California.

   Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

  1.  Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.

  2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the
     distribution.

   Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.

   THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.

\x1f
File: flex.info,  Node: Top,  Next: Copyright,  Prev: (dir),  Up: (dir)

flex
****

This manual describes `flex', a tool for generating programs that
perform pattern-matching on text.  The manual includes both tutorial and
reference sections.

   This edition of `The flex Manual' documents `flex' version 2.5.35.
It was last updated on 10 September 2007.

   This manual was written by Vern Paxson, Will Estes and John Millaway.

* Menu:

* Copyright::
* Reporting Bugs::
* Introduction::
* Simple Examples::
* Format::
* Patterns::
* Matching::
* Actions::
* Generated Scanner::
* Start Conditions::
* Multiple Input Buffers::
* EOF::
* Misc Macros::
* User Values::
* Yacc::
* Scanner Options::
* Performance::
* Cxx::
* Reentrant::
* Lex and Posix::
* Memory Management::
* Serialized Tables::
* Diagnostics::
* Limitations::
* Bibliography::
* FAQ::
* Appendices::
* Indices::

 --- The Detailed Node Listing ---

Format of the Input File

* Definitions Section::
* Rules Section::
* User Code Section::
* Comments in the Input::

Scanner Options

* Options for Specifying Filenames::
* Options Affecting Scanner Behavior::
* Code-Level And API Options::
* Options for Scanner Speed and Size::
* Debugging Options::
* Miscellaneous Options::

Reentrant C Scanners

* Reentrant Uses::
* Reentrant Overview::
* Reentrant Example::
* Reentrant Detail::
* Reentrant Functions::

The Reentrant API in Detail

* Specify Reentrant::
* Extra Reentrant Argument::
* Global Replacement::
* Init and Destroy Functions::
* Accessor Methods::
* Extra Data::
* About yyscan_t::

Memory Management

* The Default Memory Management::
* Overriding The Default Memory Management::
* A Note About yytext And Memory::

Serialized Tables

* Creating Serialized Tables::
* Loading and Unloading Serialized Tables::
* Tables File Format::

FAQ

* When was flex born?::
* How do I expand backslash-escape sequences in C-style quoted strings?::
* Why do flex scanners call fileno if it is not ANSI compatible?::
* Does flex support recursive pattern definitions?::
* How do I skip huge chunks of input (tens of megabytes) while using flex?::
* Flex is not matching my patterns in the same order that I defined them.::
* My actions are executing out of order or sometimes not at all.::
* How can I have multiple input sources feed into the same scanner at the same time?::
* Can I build nested parsers that work with the same input file?::
* How can I match text only at the end of a file?::
* How can I make REJECT cascade across start condition boundaries?::
* Why cant I use fast or full tables with interactive mode?::
* How much faster is -F or -f than -C?::
* If I have a simple grammar cant I just parse it with flex?::
* Why doesn't yyrestart() set the start state back to INITIAL?::
* How can I match C-style comments?::
* The period isn't working the way I expected.::
* Can I get the flex manual in another format?::
* Does there exist a "faster" NDFA->DFA algorithm?::
* How does flex compile the DFA so quickly?::
* How can I use more than 8192 rules?::
* How do I abandon a file in the middle of a scan and switch to a new file?::
* How do I execute code only during initialization (only before the first scan)?::
* How do I execute code at termination?::
* Where else can I find help?::
* Can I include comments in the "rules" section of the file?::
* I get an error about undefined yywrap().::
* How can I change the matching pattern at run time?::
* How can I expand macros in the input?::
* How can I build a two-pass scanner?::
* How do I match any string not matched in the preceding rules?::
* I am trying to port code from AT&T lex that uses yysptr and yysbuf.::
* Is there a way to make flex treat NULL like a regular character?::
* Whenever flex can not match the input it says "flex scanner jammed".::
* Why doesn't flex have non-greedy operators like perl does?::
* Memory leak - 16386 bytes allocated by malloc.::
* How do I track the byte offset for lseek()?::
* How do I use my own I/O classes in a C++ scanner?::
* How do I skip as many chars as possible?::
* deleteme00::
* Are certain equivalent patterns faster than others?::
* Is backing up a big deal?::
* Can I fake multi-byte character support?::
* deleteme01::
* Can you discuss some flex internals?::
* unput() messes up yy_at_bol::
* The | operator is not doing what I want::
* Why can't flex understand this variable trailing context pattern?::
* The ^ operator isn't working::
* Trailing context is getting confused with trailing optional patterns::
* Is flex GNU or not?::
* ERASEME53::
* I need to scan if-then-else blocks and while loops::
* ERASEME55::
* ERASEME56::
* ERASEME57::
* Is there a repository for flex scanners?::
* How can I conditionally compile or preprocess my flex input file?::
* Where can I find grammars for lex and yacc?::
* I get an end-of-buffer message for each character scanned.::
* unnamed-faq-62::
* unnamed-faq-63::
* unnamed-faq-64::
* unnamed-faq-65::
* unnamed-faq-66::
* unnamed-faq-67::
* unnamed-faq-68::
* unnamed-faq-69::
* unnamed-faq-70::
* unnamed-faq-71::
* unnamed-faq-72::
* unnamed-faq-73::
* unnamed-faq-74::
* unnamed-faq-75::
* unnamed-faq-76::
* unnamed-faq-77::
* unnamed-faq-78::
* unnamed-faq-79::
* unnamed-faq-80::
* unnamed-faq-81::
* unnamed-faq-82::
* unnamed-faq-83::
* unnamed-faq-84::
* unnamed-faq-85::
* unnamed-faq-86::
* unnamed-faq-87::
* unnamed-faq-88::
* unnamed-faq-90::
* unnamed-faq-91::
* unnamed-faq-92::
* unnamed-faq-93::
* unnamed-faq-94::
* unnamed-faq-95::
* unnamed-faq-96::
* unnamed-faq-97::
* unnamed-faq-98::
* unnamed-faq-99::
* unnamed-faq-100::
* unnamed-faq-101::
* What is the difference between YYLEX_PARAM and YY_DECL?::
* Why do I get "conflicting types for yylex" error?::
* How do I access the values set in a Flex action from within a Bison action?::

Appendices

* Makefiles and Flex::
* Bison Bridge::
* M4 Dependency::
* Common Patterns::

Indices

* Concept Index::
* Index of Functions and Macros::
* Index of Variables::
* Index of Data Types::
* Index of Hooks::
* Index of Scanner Options::

\x1f
File: flex.info,  Node: Copyright,  Next: Reporting Bugs,  Prev: Top,  Up: Top

1 Copyright
***********

The flex manual is placed under the same licensing conditions as the
rest of flex:

   Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007 The Flex
Project.

   Copyright (C) 1990, 1997 The Regents of the University of California.
All rights reserved.

   This code is derived from software contributed to Berkeley by Vern
Paxson.

   The United States Government has rights in this work pursuant to
contract no. DE-AC03-76SF00098 between the United States Department of
Energy and the University of California.

   Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

  1.  Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.

  2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the
     distribution.

   Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.

   THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.

\x1f
File: flex.info,  Node: Reporting Bugs,  Next: Introduction,  Prev: Copyright,  Up: Top

2 Reporting Bugs
****************

If you find a bug in `flex', please report it using the SourceForge Bug
Tracking facilities which can be found on flex's SourceForge Page
(http://sourceforge.net/projects/flex).

\x1f
File: flex.info,  Node: Introduction,  Next: Simple Examples,  Prev: Reporting Bugs,  Up: Top

3 Introduction
**************

`flex' is a tool for generating "scanners".  A scanner is a program
which recognizes lexical patterns in text.  The `flex' program reads
the given input files, or its standard input if no file names are
given, for a description of a scanner to generate.  The description is
in the form of pairs of regular expressions and C code, called "rules".
`flex' generates as output a C source file, `lex.yy.c' by default,
which defines a routine `yylex()'.  This file can be compiled and
linked with the flex runtime library to produce an executable.  When
the executable is run, it analyzes its input for occurrences of the
regular expressions.  Whenever it finds one, it executes the
corresponding C code.

\x1f
File: flex.info,  Node: Simple Examples,  Next: Format,  Prev: Introduction,  Up: Top

4 Some Simple Examples
**********************

First some simple examples to get the flavor of how one uses `flex'.

   The following `flex' input specifies a scanner which, when it
encounters the string `username' will replace it with the user's login
name:


         %%
         username    printf( "%s", getlogin() );

   By default, any text not matched by a `flex' scanner is copied to
the output, so the net effect of this scanner is to copy its input file
to its output with each occurrence of `username' expanded.  In this
input, there is just one rule.  `username' is the "pattern" and the
`printf' is the "action".  The `%%' symbol marks the beginning of the
rules.

   Here's another simple example:


                 int num_lines = 0, num_chars = 0;

         %%
         \n      ++num_lines; ++num_chars;
         .       ++num_chars;

         %%
         main()
                 {
                 yylex();
                 printf( "# of lines = %d, # of chars = %d\n",
                         num_lines, num_chars );
                 }

   This scanner counts the number of characters and the number of lines
in its input. It produces no output other than the final report on the
character and line counts.  The first line declares two globals,
`num_lines' and `num_chars', which are accessible both inside `yylex()'
and in the `main()' routine declared after the second `%%'.  There are
two rules, one which matches a newline (`\n') and increments both the
line count and the character count, and one which matches any character
other than a newline (indicated by the `.' regular expression).

   A somewhat more complicated example:


         /* scanner for a toy Pascal-like language */

         %{
         /* need this for the call to atof() below */
         #include math.h>
         %}

         DIGIT    [0-9]
         ID       [a-z][a-z0-9]*

         %%

         {DIGIT}+    {
                     printf( "An integer: %s (%d)\n", yytext,
                             atoi( yytext ) );
                     }

         {DIGIT}+"."{DIGIT}*        {
                     printf( "A float: %s (%g)\n", yytext,
                             atof( yytext ) );
                     }

         if|then|begin|end|procedure|function        {
                     printf( "A keyword: %s\n", yytext );
                     }

         {ID}        printf( "An identifier: %s\n", yytext );

         "+"|"-"|"*"|"/"   printf( "An operator: %s\n", yytext );

         "{"[\^{}}\n]*"}"     /* eat up one-line comments */

         [ \t\n]+          /* eat up whitespace */

         .           printf( "Unrecognized character: %s\n", yytext );

         %%

         main( argc, argv )
         int argc;
         char **argv;
             {
             ++argv, --argc;  /* skip over program name */
             if ( argc > 0 )
                     yyin = fopen( argv[0], "r" );
             else
                     yyin = stdin;

             yylex();
             }

   This is the beginnings of a simple scanner for a language like
Pascal.  It identifies different types of "tokens" and reports on what
it has seen.

   The details of this example will be explained in the following
sections.

\x1f
File: flex.info,  Node: Format,  Next: Patterns,  Prev: Simple Examples,  Up: Top

5 Format of the Input File
**************************

The `flex' input file consists of three sections, separated by a line
containing only `%%'.


         definitions
         %%
         rules
         %%
         user code

* Menu:

* Definitions Section::
* Rules Section::
* User Code Section::
* Comments in the Input::

\x1f
File: flex.info,  Node: Definitions Section,  Next: Rules Section,  Prev: Format,  Up: Format

5.1 Format of the Definitions Section
=====================================

The "definitions section" contains declarations of simple "name"
definitions to simplify the scanner specification, and declarations of
"start conditions", which are explained in a later section.

   Name definitions have the form:


         name definition

   The `name' is a word beginning with a letter or an underscore (`_')
followed by zero or more letters, digits, `_', or `-' (dash).  The
definition is taken to begin at the first non-whitespace character
following the name and continuing to the end of the line.  The
definition can subsequently be referred to using `{name}', which will
expand to `(definition)'.  For example,


         DIGIT    [0-9]
         ID       [a-z][a-z0-9]*

   Defines `DIGIT' to be a regular expression which matches a single
digit, and `ID' to be a regular expression which matches a letter
followed by zero-or-more letters-or-digits.  A subsequent reference to


         {DIGIT}+"."{DIGIT}*

   is identical to


         ([0-9])+"."([0-9])*

   and matches one-or-more digits followed by a `.' followed by
zero-or-more digits.

   An unindented comment (i.e., a line beginning with `/*') is copied
verbatim to the output up to the next `*/'.

   Any _indented_ text or text enclosed in `%{' and `%}' is also copied
verbatim to the output (with the %{ and %} symbols removed).  The %{
and %} symbols must appear unindented on lines by themselves.

   A `%top' block is similar to a `%{' ... `%}' block, except that the
code in a `%top' block is relocated to the _top_ of the generated file,
before any flex definitions (1).  The `%top' block is useful when you
want certain preprocessor macros to be defined or certain files to be
included before the generated code.  The single characters, `{'  and
`}' are used to delimit the `%top' block, as show in the example below:


         %top{
             /* This code goes at the "top" of the generated file. */
             #include <stdint.h>
             #include <inttypes.h>
         }

   Multiple `%top' blocks are allowed, and their order is preserved.

   ---------- Footnotes ----------

   (1) Actually, `yyIN_HEADER' is defined before the `%top' block.

\x1f
File: flex.info,  Node: Rules Section,  Next: User Code Section,  Prev: Definitions Section,  Up: Format

5.2 Format of the Rules Section
===============================

The "rules" section of the `flex' input contains a series of rules of
the form:


         pattern   action

   where the pattern must be unindented and the action must begin on
the same line.  *Note Patterns::, for a further description of patterns
and actions.

   In the rules section, any indented or %{ %} enclosed text appearing
before the first rule may be used to declare variables which are local
to the scanning routine and (after the declarations) code which is to be
executed whenever the scanning routine is entered.  Other indented or
%{ %} text in the rule section is still copied to the output, but its
meaning is not well-defined and it may well cause compile-time errors
(this feature is present for POSIX compliance. *Note Lex and Posix::,
for other such features).

   Any _indented_ text or text enclosed in `%{' and `%}' is copied
verbatim to the output (with the %{ and %} symbols removed).  The %{
and %} symbols must appear unindented on lines by themselves.

\x1f
File: flex.info,  Node: User Code Section,  Next: Comments in the Input,  Prev: Rules Section,  Up: Format

5.3 Format of the User Code Section
===================================

The user code section is simply copied to `lex.yy.c' verbatim.  It is
used for companion routines which call or are called by the scanner.
The presence of this section is optional; if it is missing, the second
`%%' in the input file may be skipped, too.

\x1f
File: flex.info,  Node: Comments in the Input,  Prev: User Code Section,  Up: Format

5.4 Comments in the Input
=========================

Flex supports C-style comments, that is, anything between `/*' and `*/'
is considered a comment. Whenever flex encounters a comment, it copies
the entire comment verbatim to the generated source code. Comments may
appear just about anywhere, but with the following exceptions:

   * Comments may not appear in the Rules Section wherever flex is
     expecting a regular expression. This means comments may not appear
     at the beginning of a line, or immediately following a list of
     scanner states.

   * Comments may not appear on an `%option' line in the Definitions
     Section.

   If you want to follow a simple rule, then always begin a comment on a
new line, with one or more whitespace characters before the initial
`/*').  This rule will work anywhere in the input file.

   All the comments in the following example are valid:


     %{
     /* code block */
     %}

     /* Definitions Section */
     %x STATE_X

     %%
         /* Rules Section */
     ruleA   /* after regex */ { /* code block */ } /* after code block */
             /* Rules Section (indented) */
     <STATE_X>{
     ruleC   ECHO;
     ruleD   ECHO;
     %{
     /* code block */
     %}
     }
     %%
     /* User Code Section */

\x1f
File: flex.info,  Node: Patterns,  Next: Matching,  Prev: Format,  Up: Top

6 Patterns
**********

The patterns in the input (see *Note Rules Section::) are written using
an extended set of regular expressions.  These are:

`x'
     match the character 'x'

`.'
     any character (byte) except newline

`[xyz]'
     a "character class"; in this case, the pattern matches either an
     'x', a 'y', or a 'z'

`[abj-oZ]'
     a "character class" with a range in it; matches an 'a', a 'b', any
     letter from 'j' through 'o', or a 'Z'

`[^A-Z]'
     a "negated character class", i.e., any character but those in the
     class.  In this case, any character EXCEPT an uppercase letter.

`[^A-Z\n]'
     any character EXCEPT an uppercase letter or a newline

`[a-z]{-}[aeiou]'
     the lowercase consonants

`r*'
     zero or more r's, where r is any regular expression

`r+'
     one or more r's

`r?'
     zero or one r's (that is, "an optional r")

`r{2,5}'
     anywhere from two to five r's

`r{2,}'
     two or more r's

`r{4}'
     exactly 4 r's

`{name}'
     the expansion of the `name' definition (*note Format::).

`"[xyz]\"foo"'
     the literal string: `[xyz]"foo'

`\X'
     if X is `a', `b', `f', `n', `r', `t', or `v', then the ANSI-C
     interpretation of `\x'.  Otherwise, a literal `X' (used to escape
     operators such as `*')

`\0'
     a NUL character (ASCII code 0)

`\123'
     the character with octal value 123

`\x2a'
     the character with hexadecimal value 2a

`(r)'
     match an `r'; parentheses are used to override precedence (see
     below)

`(?r-s:pattern)'
     apply option `r' and omit option `s' while interpreting pattern.
     Options may be zero or more of the characters `i', `s', or `x'.

     `i' means case-insensitive. `-i' means case-sensitive.

     `s' alters the meaning of the `.' syntax to match any single byte
     whatsoever.  `-s' alters the meaning of `.' to match any byte
     except `\n'.

     `x' ignores comments and whitespace in patterns. Whitespace is
     ignored unless it is backslash-escaped, contained within `""'s, or
     appears inside a character class.

     The following are all valid:


     (?:foo)         same as  (foo)
     (?i:ab7)        same as  ([aA][bB]7)
     (?-i:ab)        same as  (ab)
     (?s:.)          same as  [\x00-\xFF]
     (?-s:.)         same as  [^\n]
     (?ix-s: a . b)  same as  ([Aa][^\n][bB])
     (?x:a  b)       same as  ("ab")
     (?x:a\ b)       same as  ("a b")
     (?x:a" "b)      same as  ("a b")
     (?x:a[ ]b)      same as  ("a b")
     (?x:a
         /* comment */
         b
         c)          same as  (abc)

`(?# comment )'
     omit everything within `()'. The first `)' character encountered
     ends the pattern. It is not possible to for the comment to contain
     a `)' character. The comment may span lines.

`rs'
     the regular expression `r' followed by the regular expression `s';
     called "concatenation"

`r|s'
     either an `r' or an `s'

`r/s'
     an `r' but only if it is followed by an `s'.  The text matched by
     `s' is included when determining whether this rule is the longest
     match, but is then returned to the input before the action is
     executed.  So the action only sees the text matched by `r'.  This
     type of pattern is called "trailing context".  (There are some
     combinations of `r/s' that flex cannot match correctly. *Note
     Limitations::, regarding dangerous trailing context.)

`^r'
     an `r', but only at the beginning of a line (i.e., when just
     starting to scan, or right after a newline has been scanned).

`r$'
     an `r', but only at the end of a line (i.e., just before a
     newline).  Equivalent to `r/\n'.

     Note that `flex''s notion of "newline" is exactly whatever the C
     compiler used to compile `flex' interprets `\n' as; in particular,
     on some DOS systems you must either filter out `\r's in the input
     yourself, or explicitly use `r/\r\n' for `r$'.

`<s>r'
     an `r', but only in start condition `s' (see *Note Start
     Conditions:: for discussion of start conditions).

`<s1,s2,s3>r'
     same, but in any of start conditions `s1', `s2', or `s3'.

`<*>r'
     an `r' in any start condition, even an exclusive one.

`<<EOF>>'
     an end-of-file.

`<s1,s2><<EOF>>'
     an end-of-file when in start condition `s1' or `s2'

   Note that inside of a character class, all regular expression
operators lose their special meaning except escape (`\') and the
character class operators, `-', `]]', and, at the beginning of the
class, `^'.

   The regular expressions listed above are grouped according to
precedence, from highest precedence at the top to lowest at the bottom.
Those grouped together have equal precedence (see special note on the
precedence of the repeat operator, `{}', under the documentation for
the `--posix' POSIX compliance option).  For example,


         foo|bar*

   is the same as


         (foo)|(ba(r*))

   since the `*' operator has higher precedence than concatenation, and
concatenation higher than alternation (`|').  This pattern therefore
matches _either_ the string `foo' _or_ the string `ba' followed by
zero-or-more `r''s.  To match `foo' or zero-or-more repetitions of the
string `bar', use:


         foo|(bar)*

   And to match a sequence of zero or more repetitions of `foo' and
`bar':


         (foo|bar)*

   In addition to characters and ranges of characters, character classes
can also contain "character class expressions".  These are expressions
enclosed inside `[': and `:]' delimiters (which themselves must appear
between the `[' and `]' of the character class. Other elements may
occur inside the character class, too).  The valid expressions are:


         [:alnum:] [:alpha:] [:blank:]
         [:cntrl:] [:digit:] [:graph:]
         [:lower:] [:print:] [:punct:]
         [:space:] [:upper:] [:xdigit:]

   These expressions all designate a set of characters equivalent to the
corresponding standard C `isXXX' function.  For example, `[:alnum:]'
designates those characters for which `isalnum()' returns true - i.e.,
any alphabetic or numeric character.  Some systems don't provide
`isblank()', so flex defines `[:blank:]' as a blank or a tab.

   For example, the following character classes are all equivalent:


         [[:alnum:]]
         [[:alpha:][:digit:]]
         [[:alpha:][0-9]]
         [a-zA-Z0-9]

   A word of caution. Character classes are expanded immediately when
seen in the `flex' input.  This means the character classes are
sensitive to the locale in which `flex' is executed, and the resulting
scanner will not be sensitive to the runtime locale.  This may or may
not be desirable.

   * If your scanner is case-insensitive (the `-i' flag), then
     `[:upper:]' and `[:lower:]' are equivalent to `[:alpha:]'.

   * Character classes with ranges, such as `[a-Z]', should be used with
     caution in a case-insensitive scanner if the range spans upper or
     lowercase characters. Flex does not know if you want to fold all
     upper and lowercase characters together, or if you want the
     literal numeric range specified (with no case folding). When in
     doubt, flex will assume that you meant the literal numeric range,
     and will issue a warning. The exception to this rule is a
     character range such as `[a-z]' or `[S-W]' where it is obvious
     that you want case-folding to occur. Here are some examples with
     the `-i' flag enabled:

     Range        Result      Literal Range        Alternate Range
     `[a-t]'      ok          `[a-tA-T]'           
     `[A-T]'      ok          `[a-tA-T]'           
     `[A-t]'      ambiguous   `[A-Z\[\\\]_`a-t]'   `[a-tA-T]'
     `[_-{]'      ambiguous   `[_`a-z{]'           `[_`a-zA-Z{]'
     `[@-C]'      ambiguous   `[@ABC]'             `[@A-Z\[\\\]_`abc]'

   * A negated character class such as the example `[^A-Z]' above
     _will_ match a newline unless `\n' (or an equivalent escape
     sequence) is one of the characters explicitly present in the
     negated character class (e.g., `[^A-Z\n]').  This is unlike how
     many other regular expression tools treat negated character
     classes, but unfortunately the inconsistency is historically
     entrenched.  Matching newlines means that a pattern like `[^"]*'
     can match the entire input unless there's another quote in the
     input.

     Flex allows negation of character class expressions by prepending
     `^' to the POSIX character class name.


              [:^alnum:] [:^alpha:] [:^blank:]
              [:^cntrl:] [:^digit:] [:^graph:]
              [:^lower:] [:^print:] [:^punct:]
              [:^space:] [:^upper:] [:^xdigit:]

     Flex will issue a warning if the expressions `[:^upper:]' and
     `[:^lower:]' appear in a case-insensitive scanner, since their
     meaning is unclear. The current behavior is to skip them entirely,
     but this may change without notice in future revisions of flex.

   *  The `{-}' operator computes the difference of two character
     classes. For example, `[a-c]{-}[b-z]' represents all the
     characters in the class `[a-c]' that are not in the class `[b-z]'
     (which in this case, is just the single character `a'). The `{-}'
     operator is left associative, so `[abc]{-}[b]{-}[c]' is the same
     as `[a]'. Be careful not to accidentally create an empty set,
     which will never match.

   *  The `{+}' operator computes the union of two character classes.
     For example, `[a-z]{+}[0-9]' is the same as `[a-z0-9]'. This
     operator is useful when preceded by the result of a difference
     operation, as in, `[[:alpha:]]{-}[[:lower:]]{+}[q]', which is
     equivalent to `[A-Zq]' in the "C" locale.

   * A rule can have at most one instance of trailing context (the `/'
     operator or the `$' operator).  The start condition, `^', and
     `<<EOF>>' patterns can only occur at the beginning of a pattern,
     and, as well as with `/' and `$', cannot be grouped inside
     parentheses.  A `^' which does not occur at the beginning of a
     rule or a `$' which does not occur at the end of a rule loses its
     special properties and is treated as a normal character.

   * The following are invalid:


              foo/bar$
              <sc1>foo<sc2>bar

     Note that the first of these can be written `foo/bar\n'.

   * The following will result in `$' or `^' being treated as a normal
     character:


              foo|(bar$)
              foo|^bar

     If the desired meaning is a `foo' or a
     `bar'-followed-by-a-newline, the following could be used (the
     special `|' action is explained below, *note Actions::):


              foo      |
              bar$     /* action goes here */

     A similar trick will work for matching a `foo' or a
     `bar'-at-the-beginning-of-a-line.

\x1f
File: flex.info,  Node: Matching,  Next: Actions,  Prev: Patterns,  Up: Top

7 How the Input Is Matched
**************************

When the generated scanner is run, it analyzes its input looking for
strings which match any of its patterns.  If it finds more than one
match, it takes the one matching the most text (for trailing context
rules, this includes the length of the trailing part, even though it
will then be returned to the input).  If it finds two or more matches of
the same length, the rule listed first in the `flex' input file is
chosen.

   Once the match is determined, the text corresponding to the match
(called the "token") is made available in the global character pointer
`yytext', and its length in the global integer `yyleng'.  The "action"
corresponding to the matched pattern is then executed (*note
Actions::), and then the remaining input is scanned for another match.

   If no match is found, then the "default rule" is executed: the next
character in the input is considered matched and copied to the standard
output.  Thus, the simplest valid `flex' input is:


         %%

   which generates a scanner that simply copies its input (one
character at a time) to its output.

   Note that `yytext' can be defined in two different ways: either as a
character _pointer_ or as a character _array_. You can control which
definition `flex' uses by including one of the special directives
`%pointer' or `%array' in the first (definitions) section of your flex
input.  The default is `%pointer', unless you use the `-l' lex
compatibility option, in which case `yytext' will be an array.  The
advantage of using `%pointer' is substantially faster scanning and no
buffer overflow when matching very large tokens (unless you run out of
dynamic memory).  The disadvantage is that you are restricted in how
your actions can modify `yytext' (*note Actions::), and calls to the
`unput()' function destroys the present contents of `yytext', which can
be a considerable porting headache when moving between different `lex'
versions.

   The advantage of `%array' is that you can then modify `yytext' to
your heart's content, and calls to `unput()' do not destroy `yytext'
(*note Actions::).  Furthermore, existing `lex' programs sometimes
access `yytext' externally using declarations of the form:


         extern char yytext[];

   This definition is erroneous when used with `%pointer', but correct
for `%array'.

   The `%array' declaration defines `yytext' to be an array of `YYLMAX'
characters, which defaults to a fairly large value.  You can change the
size by simply #define'ing `YYLMAX' to a different value in the first
section of your `flex' input.  As mentioned above, with `%pointer'
yytext grows dynamically to accommodate large tokens.  While this means
your `%pointer' scanner can accommodate very large tokens (such as
matching entire blocks of comments), bear in mind that each time the
scanner must resize `yytext' it also must rescan the entire token from
the beginning, so matching such tokens can prove slow.  `yytext'
presently does _not_ dynamically grow if a call to `unput()' results in
too much text being pushed back; instead, a run-time error results.

   Also note that you cannot use `%array' with C++ scanner classes
(*note Cxx::).

\x1f
File: flex.info,  Node: Actions,  Next: Generated Scanner,  Prev: Matching,  Up: Top

8 Actions
*********

Each pattern in a rule has a corresponding "action", which can be any
arbitrary C statement.  The pattern ends at the first non-escaped
whitespace character; the remainder of the line is its action.  If the
action is empty, then when the pattern is matched the input token is
simply discarded.  For example, here is the specification for a program
which deletes all occurrences of `zap me' from its input:


         %%
         "zap me"

   This example will copy all other characters in the input to the
output since they will be matched by the default rule.

   Here is a program which compresses multiple blanks and tabs down to a
single blank, and throws away whitespace found at the end of a line:


         %%
         [ \t]+        putchar( ' ' );
         [ \t]+$       /* ignore this token */

   If the action contains a `{', then the action spans till the
balancing `}' is found, and the action may cross multiple lines.
`flex' knows about C strings and comments and won't be fooled by braces
found within them, but also allows actions to begin with `%{' and will
consider the action to be all the text up to the next `%}' (regardless
of ordinary braces inside the action).

   An action consisting solely of a vertical bar (`|') means "same as
the action for the next rule".  See below for an illustration.

   Actions can include arbitrary C code, including `return' statements
to return a value to whatever routine called `yylex()'.  Each time
`yylex()' is called it continues processing tokens from where it last
left off until it either reaches the end of the file or executes a
return.

   Actions are free to modify `yytext' except for lengthening it
(adding characters to its end-these will overwrite later characters in
the input stream).  This however does not apply when using `%array'
(*note Matching::). In that case, `yytext' may be freely modified in
any way.

   Actions are free to modify `yyleng' except they should not do so if
the action also includes use of `yymore()' (see below).

   There are a number of special directives which can be included
within an action:

`ECHO'
     copies yytext to the scanner's output.

`BEGIN'
     followed by the name of a start condition places the scanner in the
     corresponding start condition (see below).

`REJECT'
     directs the scanner to proceed on to the "second best" rule which
     matched the input (or a prefix of the input).  The rule is chosen
     as described above in *Note Matching::, and `yytext' and `yyleng'
     set up appropriately.  It may either be one which matched as much
     text as the originally chosen rule but came later in the `flex'
     input file, or one which matched less text.  For example, the
     following will both count the words in the input and call the
     routine `special()' whenever `frob' is seen:


                      int word_count = 0;
              %%

              frob        special(); REJECT;
              [^ \t\n]+   ++word_count;

     Without the `REJECT', any occurrences of `frob' in the input would
     not be counted as words, since the scanner normally executes only
     one action per token.  Multiple uses of `REJECT' are allowed, each
     one finding the next best choice to the currently active rule.  For
     example, when the following scanner scans the token `abcd', it will
     write `abcdabcaba' to the output:


              %%
              a        |
              ab       |
              abc      |
              abcd     ECHO; REJECT;
              .|\n     /* eat up any unmatched character */

     The first three rules share the fourth's action since they use the
     special `|' action.

     `REJECT' is a particularly expensive feature in terms of scanner
     performance; if it is used in _any_ of the scanner's actions it
     will slow down _all_ of the scanner's matching.  Furthermore,
     `REJECT' cannot be used with the `-Cf' or `-CF' options (*note
     Scanner Options::).

     Note also that unlike the other special actions, `REJECT' is a
     _branch_.  Code immediately following it in the action will _not_
     be executed.

`yymore()'
     tells the scanner that the next time it matches a rule, the
     corresponding token should be _appended_ onto the current value of
     `yytext' rather than replacing it.  For example, given the input
     `mega-kludge' the following will write `mega-mega-kludge' to the
     output:


              %%
              mega-    ECHO; yymore();
              kludge   ECHO;

     First `mega-' is matched and echoed to the output.  Then `kludge'
     is matched, but the previous `mega-' is still hanging around at the
     beginning of `yytext' so the `ECHO' for the `kludge' rule will
     actually write `mega-kludge'.

   Two notes regarding use of `yymore()'.  First, `yymore()' depends on
the value of `yyleng' correctly reflecting the size of the current
token, so you must not modify `yyleng' if you are using `yymore()'.
Second, the presence of `yymore()' in the scanner's action entails a
minor performance penalty in the scanner's matching speed.

   `yyless(n)' returns all but the first `n' characters of the current
token back to the input stream, where they will be rescanned when the
scanner looks for the next match.  `yytext' and `yyleng' are adjusted
appropriately (e.g., `yyleng' will now be equal to `n').  For example,
on the input `foobar' the following will write out `foobarbar':


         %%
         foobar    ECHO; yyless(3);
         [a-z]+    ECHO;

   An argument of 0 to `yyless()' will cause the entire current input
string to be scanned again.  Unless you've changed how the scanner will
subsequently process its input (using `BEGIN', for example), this will
result in an endless loop.

   Note that `yyless()' is a macro and can only be used in the flex
input file, not from other source files.

   `unput(c)' puts the character `c' back onto the input stream.  It
will be the next character scanned.  The following action will take the
current token and cause it to be rescanned enclosed in parentheses.


         {
         int i;
         /* Copy yytext because unput() trashes yytext */
         char *yycopy = strdup( yytext );
         unput( ')' );
         for ( i = yyleng - 1; i >= 0; --i )
             unput( yycopy[i] );
         unput( '(' );
         free( yycopy );
         }

   Note that since each `unput()' puts the given character back at the
_beginning_ of the input stream, pushing back strings must be done
back-to-front.

   An important potential problem when using `unput()' is that if you
are using `%pointer' (the default), a call to `unput()' _destroys_ the
contents of `yytext', starting with its rightmost character and
devouring one character to the left with each call.  If you need the
value of `yytext' preserved after a call to `unput()' (as in the above
example), you must either first copy it elsewhere, or build your
scanner using `%array' instead (*note Matching::).

   Finally, note that you cannot put back `EOF' to attempt to mark the
input stream with an end-of-file.

   `input()' reads the next character from the input stream.  For
example, the following is one way to eat up C comments:


         %%
         "/*"        {
                     register int c;

                     for ( ; ; )
                         {
                         while ( (c = input()) != '*' &&
                                 c != EOF )
                             ;    /* eat up text of comment */

                         if ( c == '*' )
                             {
                             while ( (c = input()) == '*' )
                                 ;
                             if ( c == '/' )
                                 break;    /* found the end */
                             }

                         if ( c == EOF )
                             {
                             error( "EOF in comment" );
                             break;
                             }
                         }
                     }

   (Note that if the scanner is compiled using `C++', then `input()' is
instead referred to as yyinput(), in order to avoid a name clash with
the `C++' stream by the name of `input'.)

   `YY_FLUSH_BUFFER()' flushes the scanner's internal buffer so that
the next time the scanner attempts to match a token, it will first
refill the buffer using `YY_INPUT()' (*note Generated Scanner::).  This
action is a special case of the more general `yy_flush_buffer()'
function, described below (*note Multiple Input Buffers::)

   `yyterminate()' can be used in lieu of a return statement in an
action.  It terminates the scanner and returns a 0 to the scanner's
caller, indicating "all done".  By default, `yyterminate()' is also
called when an end-of-file is encountered.  It is a macro and may be
redefined.

\x1f
File: flex.info,  Node: Generated Scanner,  Next: Start Conditions,  Prev: Actions,  Up: Top

9 The Generated Scanner
***********************

The output of `flex' is the file `lex.yy.c', which contains the
scanning routine `yylex()', a number of tables used by it for matching
tokens, and a number of auxiliary routines and macros.  By default,
`yylex()' is declared as follows:


         int yylex()
             {
             ... various definitions and the actions in here ...
             }

   (If your environment supports function prototypes, then it will be
`int yylex( void )'.)  This definition may be changed by defining the
`YY_DECL' macro.  For example, you could use:


         #define YY_DECL float lexscan( a, b ) float a, b;

   to give the scanning routine the name `lexscan', returning a float,
and taking two floats as arguments.  Note that if you give arguments to
the scanning routine using a K&R-style/non-prototyped function
declaration, you must terminate the definition with a semi-colon (;).

   `flex' generates `C99' function definitions by default. However flex
does have the ability to generate obsolete, er, `traditional', function
definitions. This is to support bootstrapping gcc on old systems.
Unfortunately, traditional definitions prevent us from using any
standard data types smaller than int (such as short, char, or bool) as
function arguments.  For this reason, future versions of `flex' may
generate standard C99 code only, leaving K&R-style functions to the
historians.  Currently, if you do *not* want `C99' definitions, then
you must use `%option noansi-definitions'.

   Whenever `yylex()' is called, it scans tokens from the global input
file `yyin' (which defaults to stdin).  It continues until it either
reaches an end-of-file (at which point it returns the value 0) or one
of its actions executes a `return' statement.

   If the scanner reaches an end-of-file, subsequent calls are undefined
unless either `yyin' is pointed at a new input file (in which case
scanning continues from that file), or `yyrestart()' is called.
`yyrestart()' takes one argument, a `FILE *' pointer (which can be
NULL, if you've set up `YY_INPUT' to scan from a source other than
`yyin'), and initializes `yyin' for scanning from that file.
Essentially there is no difference between just assigning `yyin' to a
new input file or using `yyrestart()' to do so; the latter is available
for compatibility with previous versions of `flex', and because it can
be used to switch input files in the middle of scanning.  It can also
be used to throw away the current input buffer, by calling it with an
argument of `yyin'; but it would be better to use `YY_FLUSH_BUFFER'
(*note Actions::).  Note that `yyrestart()' does _not_ reset the start
condition to `INITIAL' (*note Start Conditions::).

   If `yylex()' stops scanning due to executing a `return' statement in
one of the actions, the scanner may then be called again and it will
resume scanning where it left off.

   By default (and for purposes of efficiency), the scanner uses
block-reads rather than simple `getc()' calls to read characters from
`yyin'.  The nature of how it gets its input can be controlled by
defining the `YY_INPUT' macro.  The calling sequence for `YY_INPUT()'
is `YY_INPUT(buf,result,max_size)'.  Its action is to place up to
`max_size' characters in the character array `buf' and return in the
integer variable `result' either the number of characters read or the
constant `YY_NULL' (0 on Unix systems) to indicate `EOF'.  The default
`YY_INPUT' reads from the global file-pointer `yyin'.

   Here is a sample definition of `YY_INPUT' (in the definitions
section of the input file):


         %{
         #define YY_INPUT(buf,result,max_size) \
             { \
             int c = getchar(); \
             result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \
             }
         %}

   This definition will change the input processing to occur one
character at a time.

   When the scanner receives an end-of-file indication from YY_INPUT, it
then checks the `yywrap()' function.  If `yywrap()' returns false
(zero), then it is assumed that the function has gone ahead and set up
`yyin' to point to another input file, and scanning continues.  If it
returns true (non-zero), then the scanner terminates, returning 0 to
its caller.  Note that in either case, the start condition remains
unchanged; it does _not_ revert to `INITIAL'.

   If you do not supply your own version of `yywrap()', then you must
either use `%option noyywrap' (in which case the scanner behaves as
though `yywrap()' returned 1), or you must link with `-lfl' to obtain
the default version of the routine, which always returns 1.

   For scanning from in-memory buffers (e.g., scanning strings), see
*Note Scanning Strings::. *Note Multiple Input Buffers::.

   The scanner writes its `ECHO' output to the `yyout' global (default,
`stdout'), which may be redefined by the user simply by assigning it to
some other `FILE' pointer.

\x1f
File: flex.info,  Node: Start Conditions,  Next: Multiple Input Buffers,  Prev: Generated Scanner,  Up: Top

10 Start Conditions
*******************

`flex' provides a mechanism for conditionally activating rules.  Any
rule whose pattern is prefixed with `<sc>' will only be active when the
scanner is in the "start condition" named `sc'.  For example,


         <STRING>[^"]*        { /* eat up the string body ... */
                     ...
                     }

   will be active only when the scanner is in the `STRING' start
condition, and


         <INITIAL,STRING,QUOTE>\.        { /* handle an escape ... */
                     ...
                     }

   will be active only when the current start condition is either
`INITIAL', `STRING', or `QUOTE'.

   Start conditions are declared in the definitions (first) section of
the input using unindented lines beginning with either `%s' or `%x'
followed by a list of names.  The former declares "inclusive" start
conditions, the latter "exclusive" start conditions.  A start condition
is activated using the `BEGIN' action.  Until the next `BEGIN' action
is executed, rules with the given start condition will be active and
rules with other start conditions will be inactive.  If the start
condition is inclusive, then rules with no start conditions at all will
also be active.  If it is exclusive, then _only_ rules qualified with
the start condition will be active.  A set of rules contingent on the
same exclusive start condition describe a scanner which is independent
of any of the other rules in the `flex' input.  Because of this,
exclusive start conditions make it easy to specify "mini-scanners"
which scan portions of the input that are syntactically different from
the rest (e.g., comments).

   If the distinction between inclusive and exclusive start conditions
is still a little vague, here's a simple example illustrating the
connection between the two.  The set of rules:


         %s example
         %%

         <example>foo   do_something();

         bar            something_else();

   is equivalent to


         %x example
         %%

         <example>foo   do_something();

         <INITIAL,example>bar    something_else();

   Without the `<INITIAL,example>' qualifier, the `bar' pattern in the
second example wouldn't be active (i.e., couldn't match) when in start
condition `example'.  If we just used `<example>' to qualify `bar',
though, then it would only be active in `example' and not in `INITIAL',
while in the first example it's active in both, because in the first
example the `example' start condition is an inclusive `(%s)' start
condition.

   Also note that the special start-condition specifier `<*>' matches
every start condition.  Thus, the above example could also have been
written:


         %x example
         %%

         <example>foo   do_something();

         <*>bar    something_else();

   The default rule (to `ECHO' any unmatched character) remains active
in start conditions.  It is equivalent to:


         <*>.|\n     ECHO;

   `BEGIN(0)' returns to the original state where only the rules with
no start conditions are active.  This state can also be referred to as
the start-condition `INITIAL', so `BEGIN(INITIAL)' is equivalent to
`BEGIN(0)'.  (The parentheses around the start condition name are not
required but are considered good style.)

   `BEGIN' actions can also be given as indented code at the beginning
of the rules section.  For example, the following will cause the scanner
to enter the `SPECIAL' start condition whenever `yylex()' is called and
the global variable `enter_special' is true:


                 int enter_special;

         %x SPECIAL
         %%
                 if ( enter_special )
                     BEGIN(SPECIAL);

         <SPECIAL>blahblahblah
         ...more rules follow...

   To illustrate the uses of start conditions, here is a scanner which
provides two different interpretations of a string like `123.456'.  By
default it will treat it as three tokens, the integer `123', a dot
(`.'), and the integer `456'.  But if the string is preceded earlier in
the line by the string `expect-floats' it will treat it as a single
token, the floating-point number `123.456':


         %{
         #include <math.h>
         %}
         %s expect

         %%
         expect-floats        BEGIN(expect);

         <expect>[0-9]+@samp{.}[0-9]+      {
                     printf( "found a float, = %f\n",
                             atof( yytext ) );
                     }
         <expect>\n           {
                     /* that's the end of the line, so
                      * we need another "expect-number"
                      * before we'll recognize any more
                      * numbers
                      */
                     BEGIN(INITIAL);
                     }

         [0-9]+      {
                     printf( "found an integer, = %d\n",
                             atoi( yytext ) );
                     }

         "."         printf( "found a dot\n" );

   Here is a scanner which recognizes (and discards) C comments while
maintaining a count of the current input line.


         %x comment
         %%
                 int line_num = 1;

         "/*"         BEGIN(comment);

         <comment>[^*\n]*        /* eat anything that's not a '*' */
         <comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */
         <comment>\n             ++line_num;
         <comment>"*"+"/"        BEGIN(INITIAL);

   This scanner goes to a bit of trouble to match as much text as
possible with each rule.  In general, when attempting to write a
high-speed scanner try to match as much possible in each rule, as it's
a big win.

   Note that start-conditions names are really integer values and can
be stored as such.  Thus, the above could be extended in the following
fashion:


         %x comment foo
         %%
                 int line_num = 1;
                 int comment_caller;

         "/*"         {
                      comment_caller = INITIAL;
                      BEGIN(comment);
                      }

         ...

         <foo>"/*"    {
                      comment_caller = foo;
                      BEGIN(comment);
                      }

         <comment>[^*\n]*        /* eat anything that's not a '*' */
         <comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */
         <comment>\n             ++line_num;
         <comment>"*"+"/"        BEGIN(comment_caller);

   Furthermore, you can access the current start condition using the
integer-valued `YY_START' macro.  For example, the above assignments to
`comment_caller' could instead be written


         comment_caller = YY_START;

   Flex provides `YYSTATE' as an alias for `YY_START' (since that is
what's used by AT&T `lex').

   For historical reasons, start conditions do not have their own
name-space within the generated scanner. The start condition names are
unmodified in the generated scanner and generated header.  *Note
option-header::. *Note option-prefix::.

   Finally, here's an example of how to match C-style quoted strings
using exclusive start conditions, including expanded escape sequences
(but not including checking for a string that's too long):


         %x str

         %%
                 char string_buf[MAX_STR_CONST];
                 char *string_buf_ptr;


         \"      string_buf_ptr = string_buf; BEGIN(str);

         <str>\"        { /* saw closing quote - all done */
                 BEGIN(INITIAL);
                 *string_buf_ptr = '\0';
                 /* return string constant token type and
                  * value to parser
                  */
                 }

         <str>\n        {
                 /* error - unterminated string constant */
                 /* generate error message */
                 }

         <str>\\[0-7]{1,3} {
                 /* octal escape sequence */
                 int result;

                 (void) sscanf( yytext + 1, "%o", &result );

                 if ( result > 0xff )
                         /* error, constant is out-of-bounds */

                 *string_buf_ptr++ = result;
                 }

         <str>\\[0-9]+ {
                 /* generate error - bad escape sequence; something
                  * like '\48' or '\0777777'
                  */
                 }

         <str>\\n  *string_buf_ptr++ = '\n';
         <str>\\t  *string_buf_ptr++ = '\t';
         <str>\\r  *string_buf_ptr++ = '\r';
         <str>\\b  *string_buf_ptr++ = '\b';
         <str>\\f  *string_buf_ptr++ = '\f';

         <str>\\(.|\n)  *string_buf_ptr++ = yytext[1];

         <str>[^\\\n\"]+        {
                 char *yptr = yytext;

                 while ( *yptr )
                         *string_buf_ptr++ = *yptr++;
                 }

   Often, such as in some of the examples above, you wind up writing a
whole bunch of rules all preceded by the same start condition(s).  Flex
makes this a little easier and cleaner by introducing a notion of start
condition "scope".  A start condition scope is begun with:


         <SCs>{

   where `SCs' is a list of one or more start conditions.  Inside the
start condition scope, every rule automatically has the prefix `SCs>'
applied to it, until a `}' which matches the initial `{'.  So, for
example,


         <ESC>{
             "\\n"   return '\n';
             "\\r"   return '\r';
             "\\f"   return '\f';
             "\\0"   return '\0';
         }

   is equivalent to:


         <ESC>"\\n"  return '\n';
         <ESC>"\\r"  return '\r';
         <ESC>"\\f"  return '\f';
         <ESC>"\\0"  return '\0';

   Start condition scopes may be nested.

   The following routines are available for manipulating stacks of
start conditions:

 -- Function: void yy_push_state ( int `new_state' )
     pushes the current start condition onto the top of the start
     condition stack and switches to `new_state' as though you had used
     `BEGIN new_state' (recall that start condition names are also
     integers).

 -- Function: void yy_pop_state ()
     pops the top of the stack and switches to it via `BEGIN'.

 -- Function: int yy_top_state ()
     returns the top of the stack without altering the stack's contents.

   The start condition stack grows dynamically and so has no built-in
size limitation.  If memory is exhausted, program execution aborts.

   To use start condition stacks, your scanner must include a `%option
stack' directive (*note Scanner Options::).

\x1f
File: flex.info,  Node: Multiple Input Buffers,  Next: EOF,  Prev: Start Conditions,  Up: Top

11 Multiple Input Buffers
*************************

Some scanners (such as those which support "include" files) require
reading from several input streams.  As `flex' scanners do a large
amount of buffering, one cannot control where the next input will be
read from by simply writing a `YY_INPUT()' which is sensitive to the
scanning context.  `YY_INPUT()' is only called when the scanner reaches
the end of its buffer, which may be a long time after scanning a
statement such as an `include' statement which requires switching the
input source.

   To negotiate these sorts of problems, `flex' provides a mechanism
for creating and switching between multiple input buffers.  An input
buffer is created by using:

 -- Function: YY_BUFFER_STATE yy_create_buffer ( FILE *file, int size )

   which takes a `FILE' pointer and a size and creates a buffer
associated with the given file and large enough to hold `size'
characters (when in doubt, use `YY_BUF_SIZE' for the size).  It returns
a `YY_BUFFER_STATE' handle, which may then be passed to other routines
(see below).  The `YY_BUFFER_STATE' type is a pointer to an opaque
`struct yy_buffer_state' structure, so you may safely initialize
`YY_BUFFER_STATE' variables to `((YY_BUFFER_STATE) 0)' if you wish, and
also refer to the opaque structure in order to correctly declare input
buffers in source files other than that of your scanner.  Note that the
`FILE' pointer in the call to `yy_create_buffer' is only used as the
value of `yyin' seen by `YY_INPUT'.  If you redefine `YY_INPUT()' so it
no longer uses `yyin', then you can safely pass a NULL `FILE' pointer to
`yy_create_buffer'.  You select a particular buffer to scan from using:

 -- Function: void yy_switch_to_buffer ( YY_BUFFER_STATE new_buffer )

   The above function switches the scanner's input buffer so subsequent
tokens will come from `new_buffer'.  Note that `yy_switch_to_buffer()'
may be used by `yywrap()' to set things up for continued scanning,
instead of opening a new file and pointing `yyin' at it. If you are
looking for a stack of input buffers, then you want to use
`yypush_buffer_state()' instead of this function. Note also that
switching input sources via either `yy_switch_to_buffer()' or
`yywrap()' does _not_ change the start condition.

 -- Function: void yy_delete_buffer ( YY_BUFFER_STATE buffer )

   is used to reclaim the storage associated with a buffer.  (`buffer'
can be NULL, in which case the routine does nothing.)  You can also
clear the current contents of a buffer using:

 -- Function: void yypush_buffer_state ( YY_BUFFER_STATE buffer )

   This function pushes the new buffer state onto an internal stack.
The pushed state becomes the new current state. The stack is maintained
by flex and will grow as required. This function is intended to be used
instead of `yy_switch_to_buffer', when you want to change states, but
preserve the current state for later use.

 -- Function: void yypop_buffer_state ( )

   This function removes the current state from the top of the stack,
and deletes it by calling `yy_delete_buffer'.  The next state on the
stack, if any, becomes the new current state.

 -- Function: void yy_flush_buffer ( YY_BUFFER_STATE buffer )

   This function discards the buffer's contents, so the next time the
scanner attempts to match a token from the buffer, it will first fill
the buffer anew using `YY_INPUT()'.

 -- Function: YY_BUFFER_STATE yy_new_buffer ( FILE *file, int size )

   is an alias for `yy_create_buffer()', provided for compatibility
with the C++ use of `new' and `delete' for creating and destroying
dynamic objects.

   `YY_CURRENT_BUFFER' macro returns a `YY_BUFFER_STATE' handle to the
current buffer. It should not be used as an lvalue.

   Here are two examples of using these features for writing a scanner
which expands include files (the `<<EOF>>' feature is discussed below).

   This first example uses yypush_buffer_state and yypop_buffer_state.
Flex maintains the stack internally.


         /* the "incl" state is used for picking up the name
          * of an include file
          */
         %x incl
         %%
         include             BEGIN(incl);

         [a-z]+              ECHO;
         [^a-z\n]*\n?        ECHO;

         <incl>[ \t]*      /* eat the whitespace */
         <incl>[^ \t\n]+   { /* got the include file name */
                 yyin = fopen( yytext, "r" );

                 if ( ! yyin )
                     error( ... );

                        yypush_buffer_state(yy_create_buffer( yyin, YY_BUF_SIZE ));

                 BEGIN(INITIAL);
                 }

         <<EOF>> {
                        yypop_buffer_state();

                 if ( !YY_CURRENT_BUFFER )
                     {
                     yyterminate();
                     }
                 }

   The second example, below, does the same thing as the previous
example did, but manages its own input buffer stack manually (instead
of letting flex do it).


         /* the "incl" state is used for picking up the name
          * of an include file
          */
         %x incl

         %{
         #define MAX_INCLUDE_DEPTH 10
         YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
         int include_stack_ptr = 0;
         %}

         %%
         include             BEGIN(incl);

         [a-z]+              ECHO;
         [^a-z\n]*\n?        ECHO;

         <incl>[ \t]*      /* eat the whitespace */
         <incl>[^ \t\n]+   { /* got the include file name */
                 if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
                     {
                     fprintf( stderr, "Includes nested too deeply" );
                     exit( 1 );
                     }

                 include_stack[include_stack_ptr++] =
                     YY_CURRENT_BUFFER;

                 yyin = fopen( yytext, "r" );

                 if ( ! yyin )
                     error( ... );

                 yy_switch_to_buffer(
                     yy_create_buffer( yyin, YY_BUF_SIZE ) );

                 BEGIN(INITIAL);
                 }

         <<EOF>> {
                 if ( --include_stack_ptr  0 )
                     {
                     yyterminate();
                     }

                 else
                     {
                     yy_delete_buffer( YY_CURRENT_BUFFER );
                     yy_switch_to_buffer(
                          include_stack[include_stack_ptr] );
                     }
                 }

   The following routines are available for setting up input buffers for
scanning in-memory strings instead of files.  All of them create a new
input buffer for scanning the string, and return a corresponding
`YY_BUFFER_STATE' handle (which you should delete with
`yy_delete_buffer()' when done with it).  They also switch to the new
buffer using `yy_switch_to_buffer()', so the next call to `yylex()'
will start scanning the string.

 -- Function: YY_BUFFER_STATE yy_scan_string ( const char *str )
     scans a NUL-terminated string.

 -- Function: YY_BUFFER_STATE yy_scan_bytes ( const char *bytes, int
          len )
     scans `len' bytes (including possibly `NUL's) starting at location
     `bytes'.

   Note that both of these functions create and scan a _copy_ of the
string or bytes.  (This may be desirable, since `yylex()' modifies the
contents of the buffer it is scanning.)  You can avoid the copy by
using:

 -- Function: YY_BUFFER_STATE yy_scan_buffer (char *base, yy_size_t
          size)
     which scans in place the buffer starting at `base', consisting of
     `size' bytes, the last two bytes of which _must_ be
     `YY_END_OF_BUFFER_CHAR' (ASCII NUL).  These last two bytes are not
     scanned; thus, scanning consists of `base[0]' through
     `base[size-2]', inclusive.

   If you fail to set up `base' in this manner (i.e., forget the final
two `YY_END_OF_BUFFER_CHAR' bytes), then `yy_scan_buffer()' returns a
NULL pointer instead of creating a new input buffer.

 -- Data type: yy_size_t
     is an integral type to which you can cast an integer expression
     reflecting the size of the buffer.

\x1f
File: flex.info,  Node: EOF,  Next: Misc Macros,  Prev: Multiple Input Buffers,  Up: Top

12 End-of-File Rules
********************

The special rule `<<EOF>>' indicates actions which are to be taken when
an end-of-file is encountered and `yywrap()' returns non-zero (i.e.,
indicates no further files to process).  The action must finish by
doing one of the following things:

   * assigning `yyin' to a new input file (in previous versions of
     `flex', after doing the assignment you had to call the special
     action `YY_NEW_FILE'.  This is no longer necessary.)

   * executing a `return' statement;

   * executing the special `yyterminate()' action.

   * or, switching to a new buffer using `yy_switch_to_buffer()' as
     shown in the example above.

   <<EOF>> rules may not be used with other patterns; they may only be
qualified with a list of start conditions.  If an unqualified <<EOF>>
rule is given, it applies to _all_ start conditions which do not
already have <<EOF>> actions.  To specify an <<EOF>> rule for only the
initial start condition, use:


         <INITIAL><<EOF>>

   These rules are useful for catching things like unclosed comments.
An example:


         %x quote
         %%

         ...other rules for dealing with quotes...

         <quote><<EOF>>   {
                  error( "unterminated quote" );
                  yyterminate();
                  }
        <<EOF>>  {
                  if ( *++filelist )
                      yyin = fopen( *filelist, "r" );
                  else
                     yyterminate();
                  }

\x1f
File: flex.info,  Node: Misc Macros,  Next: User Values,  Prev: EOF,  Up: Top

13 Miscellaneous Macros
***********************

The macro `YY_USER_ACTION' can be defined to provide an action which is
always executed prior to the matched rule's action.  For example, it
could be #define'd to call a routine to convert yytext to lower-case.
When `YY_USER_ACTION' is invoked, the variable `yy_act' gives the
number of the matched rule (rules are numbered starting with 1).
Suppose you want to profile how often each of your rules is matched.
The following would do the trick:


         #define YY_USER_ACTION ++ctr[yy_act]

   where `ctr' is an array to hold the counts for the different rules.
Note that the macro `YY_NUM_RULES' gives the total number of rules
(including the default rule), even if you use `-s)', so a correct
declaration for `ctr' is:


         int ctr[YY_NUM_RULES];

   The macro `YY_USER_INIT' may be defined to provide an action which
is always executed before the first scan (and before the scanner's
internal initializations are done).  For example, it could be used to
call a routine to read in a data table or open a logging file.

   The macro `yy_set_interactive(is_interactive)' can be used to
control whether the current buffer is considered "interactive".  An
interactive buffer is processed more slowly, but must be used when the
scanner's input source is indeed interactive to avoid problems due to
waiting to fill buffers (see the discussion of the `-I' flag in *Note
Scanner Options::).  A non-zero value in the macro invocation marks the
buffer as interactive, a zero value as non-interactive.  Note that use
of this macro overrides `%option always-interactive' or `%option
never-interactive' (*note Scanner Options::).  `yy_set_interactive()'
must be invoked prior to beginning to scan the buffer that is (or is
not) to be considered interactive.

   The macro `yy_set_bol(at_bol)' can be used to control whether the
current buffer's scanning context for the next token match is done as
though at the beginning of a line.  A non-zero macro argument makes
rules anchored with `^' active, while a zero argument makes `^' rules
inactive.

   The macro `YY_AT_BOL()' returns true if the next token scanned from
the current buffer will have `^' rules active, false otherwise.

   In the generated scanner, the actions are all gathered in one large
switch statement and separated using `YY_BREAK', which may be
redefined.  By default, it is simply a `break', to separate each rule's
action from the following rule's.  Redefining `YY_BREAK' allows, for
example, C++ users to #define YY_BREAK to do nothing (while being very
careful that every rule ends with a `break' or a `return'!) to avoid
suffering from unreachable statement warnings where because a rule's
action ends with `return', the `YY_BREAK' is inaccessible.

\x1f
File: flex.info,  Node: User Values,  Next: Yacc,  Prev: Misc Macros,  Up: Top

14 Values Available To the User
*******************************

This chapter summarizes the various values available to the user in the
rule actions.

`char *yytext'
     holds the text of the current token.  It may be modified but not
     lengthened (you cannot append characters to the end).

     If the special directive `%array' appears in the first section of
     the scanner description, then `yytext' is instead declared `char
     yytext[YYLMAX]', where `YYLMAX' is a macro definition that you can
     redefine in the first section if you don't like the default value
     (generally 8KB).  Using `%array' results in somewhat slower
     scanners, but the value of `yytext' becomes immune to calls to
     `unput()', which potentially destroy its value when `yytext' is a
     character pointer.  The opposite of `%array' is `%pointer', which
     is the default.

     You cannot use `%array' when generating C++ scanner classes (the
     `-+' flag).

`int yyleng'
     holds the length of the current token.

`FILE *yyin'
     is the file which by default `flex' reads from.  It may be
     redefined but doing so only makes sense before scanning begins or
     after an EOF has been encountered.  Changing it in the midst of
     scanning will have unexpected results since `flex' buffers its
     input; use `yyrestart()' instead.  Once scanning terminates
     because an end-of-file has been seen, you can assign `yyin' at the
     new input file and then call the scanner again to continue
     scanning.

`void yyrestart( FILE *new_file )'
     may be called to point `yyin' at the new input file.  The
     switch-over to the new file is immediate (any previously
     buffered-up input is lost).  Note that calling `yyrestart()' with
     `yyin' as an argument thus throws away the current input buffer
     and continues scanning the same input file.

`FILE *yyout'
     is the file to which `ECHO' actions are done.  It can be reassigned
     by the user.

`YY_CURRENT_BUFFER'
     returns a `YY_BUFFER_STATE' handle to the current buffer.

`YY_START'
     returns an integer value corresponding to the current start
     condition.  You can subsequently use this value with `BEGIN' to
     return to that start condition.

\x1f
File: flex.info,  Node: Yacc,  Next: Scanner Options,  Prev: User Values,  Up: Top

15 Interfacing with Yacc
************************

One of the main uses of `flex' is as a companion to the `yacc'
parser-generator.  `yacc' parsers expect to call a routine named
`yylex()' to find the next input token.  The routine is supposed to
return the type of the next token as well as putting any associated
value in the global `yylval'.  To use `flex' with `yacc', one specifies
the `-d' option to `yacc' to instruct it to generate the file `y.tab.h'
containing definitions of all the `%tokens' appearing in the `yacc'
input.  This file is then included in the `flex' scanner.  For example,
if one of the tokens is `TOK_NUMBER', part of the scanner might look
like:


         %{
         #include "y.tab.h"
         %}

         %%

         [0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;

\x1f
File: flex.info,  Node: Scanner Options,  Next: Performance,  Prev: Yacc,  Up: Top

16 Scanner Options
******************

The various `flex' options are categorized by function in the following
menu. If you want to lookup a particular option by name, *Note Index of
Scanner Options::.

* Menu:

* Options for Specifying Filenames::
* Options Affecting Scanner Behavior::
* Code-Level And API Options::
* Options for Scanner Speed and Size::
* Debugging Options::
* Miscellaneous Options::

   Even though there are many scanner options, a typical scanner might
only specify the following options:


     %option   8bit reentrant bison-bridge
     %option   warn nodefault
     %option   yylineno
     %option   outfile="scanner.c" header-file="scanner.h"

   The first line specifies the general type of scanner we want. The
second line specifies that we are being careful. The third line asks
flex to track line numbers. The last line tells flex what to name the
files. (The options can be specified in any order. We just divided
them.)

   `flex' also provides a mechanism for controlling options within the
scanner specification itself, rather than from the flex command-line.
This is done by including `%option' directives in the first section of
the scanner specification.  You can specify multiple options with a
single `%option' directive, and multiple directives in the first
section of your flex input file.

   Most options are given simply as names, optionally preceded by the
word `no' (with no intervening whitespace) to negate their meaning.
The names are the same as their long-option equivalents (but without the
leading `--' ).

   `flex' scans your rule actions to determine whether you use the
`REJECT' or `yymore()' features.  The `REJECT' and `yymore' options are
available to override its decision as to whether you use the options,
either by setting them (e.g., `%option reject)' to indicate the feature
is indeed used, or unsetting them to indicate it actually is not used
(e.g., `%option noyymore)'.

   A number of options are available for lint purists who want to
suppress the appearance of unneeded routines in the generated scanner.
Each of the following, if unset (e.g., `%option nounput'), results in
the corresponding routine not appearing in the generated scanner:


         input, unput
         yy_push_state, yy_pop_state, yy_top_state
         yy_scan_buffer, yy_scan_bytes, yy_scan_string

         yyget_extra, yyset_extra, yyget_leng, yyget_text,
         yyget_lineno, yyset_lineno, yyget_in, yyset_in,
         yyget_out, yyset_out, yyget_lval, yyset_lval,
         yyget_lloc, yyset_lloc, yyget_debug, yyset_debug

   (though `yy_push_state()' and friends won't appear anyway unless you
use `%option stack)'.

\x1f
File: flex.info,  Node: Options for Specifying Filenames,  Next: Options Affecting Scanner Behavior,  Prev: Scanner Options,  Up: Scanner Options

16.1 Options for Specifying Filenames
=====================================

`--header-file=FILE, `%option header-file="FILE"''
     instructs flex to write a C header to `FILE'. This file contains
     function prototypes, extern variables, and types used by the
     scanner.  Only the external API is exported by the header file.
     Many macros that are usable from within scanner actions are not
     exported to the header file. This is due to namespace problems and
     the goal of a clean external API.

     While in the header, the macro `yyIN_HEADER' is defined, where `yy'
     is substituted with the appropriate prefix.

     The `--header-file' option is not compatible with the `--c++'
     option, since the C++ scanner provides its own header in
     `yyFlexLexer.h'.

`-oFILE, --outfile=FILE, `%option outfile="FILE"''
     directs flex to write the scanner to the file `FILE' instead of
     `lex.yy.c'.  If you combine `--outfile' with the `--stdout' option,
     then the scanner is written to `stdout' but its `#line' directives
     (see the `-l' option above) refer to the file `FILE'.

`-t, --stdout, `%option stdout''
     instructs `flex' to write the scanner it generates to standard
     output instead of `lex.yy.c'.

`-SFILE, --skel=FILE'
     overrides the default skeleton file from which `flex' constructs
     its scanners.  You'll never need this option unless you are doing
     `flex' maintenance or development.

`--tables-file=FILE'
     Write serialized scanner dfa tables to FILE. The generated scanner
     will not contain the tables, and requires them to be loaded at
     runtime.  *Note serialization::.

`--tables-verify'
     This option is for flex development. We document it here in case
     you stumble upon it by accident or in case you suspect some
     inconsistency in the serialized tables.  Flex will serialize the
     scanner dfa tables but will also generate the in-code tables as it
     normally does. At runtime, the scanner will verify that the
     serialized tables match the in-code tables, instead of loading
     them.


\x1f
File: flex.info,  Node: Options Affecting Scanner Behavior,  Next: Code-Level And API Options,  Prev: Options for Specifying Filenames,  Up: Scanner Options

16.2 Options Affecting Scanner Behavior
=======================================

`-i, --case-insensitive, `%option case-insensitive''
     instructs `flex' to generate a "case-insensitive" scanner.  The
     case of letters given in the `flex' input patterns will be ignored,
     and tokens in the input will be matched regardless of case.  The
     matched text given in `yytext' will have the preserved case (i.e.,
     it will not be folded).  For tricky behavior, see *Note case and
     character ranges::.

`-l, --lex-compat, `%option lex-compat''
     turns on maximum compatibility with the original AT&T `lex'
     implementation.  Note that this does not mean _full_ compatibility.
     Use of this option costs a considerable amount of performance, and
     it cannot be used with the `--c++', `--full', `--fast', `-Cf', or
     `-CF' options.  For details on the compatibilities it provides, see
     *Note Lex and Posix::.  This option also results in the name
     `YY_FLEX_LEX_COMPAT' being `#define''d in the generated scanner.

`-B, --batch, `%option batch''
     instructs `flex' to generate a "batch" scanner, the opposite of
     _interactive_ scanners generated by `--interactive' (see below).
     In general, you use `-B' when you are _certain_ that your scanner
     will never be used interactively, and you want to squeeze a
     _little_ more performance out of it.  If your goal is instead to
     squeeze out a _lot_ more performance, you should be using the
     `-Cf' or `-CF' options, which turn on `--batch' automatically
     anyway.

`-I, --interactive, `%option interactive''
     instructs `flex' to generate an interactive scanner.  An
     interactive scanner is one that only looks ahead to decide what
     token has been matched if it absolutely must.  It turns out that
     always looking one extra character ahead, even if the scanner has
     already seen enough text to disambiguate the current token, is a
     bit faster than only looking ahead when necessary.  But scanners
     that always look ahead give dreadful interactive performance; for
     example, when a user types a newline, it is not recognized as a
     newline token until they enter _another_ token, which often means
     typing in another whole line.

     `flex' scanners default to `interactive' unless you use the `-Cf'
     or `-CF' table-compression options (*note Performance::).  That's
     because if you're looking for high-performance you should be using
     one of these options, so if you didn't, `flex' assumes you'd
     rather trade off a bit of run-time performance for intuitive
     interactive behavior.  Note also that you _cannot_ use
     `--interactive' in conjunction with `-Cf' or `-CF'.  Thus, this
     option is not really needed; it is on by default for all those
     cases in which it is allowed.

     You can force a scanner to _not_ be interactive by using `--batch'

`-7, --7bit, `%option 7bit''
     instructs `flex' to generate a 7-bit scanner, i.e., one which can
     only recognize 7-bit characters in its input.  The advantage of
     using `--7bit' is that the scanner's tables can be up to half the
     size of those generated using the `--8bit'.  The disadvantage is
     that such scanners often hang or crash if their input contains an
     8-bit character.

     Note, however, that unless you generate your scanner using the
     `-Cf' or `-CF' table compression options, use of `--7bit' will
     save only a small amount of table space, and make your scanner
     considerably less portable.  `Flex''s default behavior is to
     generate an 8-bit scanner unless you use the `-Cf' or `-CF', in
     which case `flex' defaults to generating 7-bit scanners unless
     your site was always configured to generate 8-bit scanners (as will
     often be the case with non-USA sites).  You can tell whether flex
     generated a 7-bit or an 8-bit scanner by inspecting the flag
     summary in the `--verbose' output as described above.

     Note that if you use `-Cfe' or `-CFe' `flex' still defaults to
     generating an 8-bit scanner, since usually with these compression
     options full 8-bit tables are not much more expensive than 7-bit
     tables.

`-8, --8bit, `%option 8bit''
     instructs `flex' to generate an 8-bit scanner, i.e., one which can
     recognize 8-bit characters.  This flag is only needed for scanners
     generated using `-Cf' or `-CF', as otherwise flex defaults to
     generating an 8-bit scanner anyway.

     See the discussion of `--7bit' above for `flex''s default behavior
     and the tradeoffs between 7-bit and 8-bit scanners.

`--default, `%option default''
     generate the default rule.

`--always-interactive, `%option always-interactive''
     instructs flex to generate a scanner which always considers its
     input _interactive_.  Normally, on each new input file the scanner
     calls `isatty()' in an attempt to determine whether the scanner's
     input source is interactive and thus should be read a character at
     a time.  When this option is used, however, then no such call is
     made.

`--never-interactive, `--never-interactive''
     instructs flex to generate a scanner which never considers its
     input interactive.  This is the opposite of `always-interactive'.

`-X, --posix, `%option posix''
     turns on maximum compatibility with the POSIX 1003.2-1992
     definition of `lex'.  Since `flex' was originally designed to
     implement the POSIX definition of `lex' this generally involves
     very few changes in behavior.  At the current writing the known
     differences between `flex' and the POSIX standard are:

        * In POSIX and AT&T `lex', the repeat operator, `{}', has lower
          precedence than concatenation (thus `ab{3}' yields `ababab').
          Most POSIX utilities use an Extended Regular Expression (ERE)
          precedence that has the precedence of the repeat operator
          higher than concatenation (which causes `ab{3}' to yield
          `abbb').  By default, `flex' places the precedence of the
          repeat operator higher than concatenation which matches the
          ERE processing of other POSIX utilities.  When either
          `--posix' or `-l' are specified, `flex' will use the
          traditional AT&T and POSIX-compliant precedence for the
          repeat operator where concatenation has higher precedence
          than the repeat operator.

`--stack, `%option stack''
     enables the use of start condition stacks (*note Start
     Conditions::).

`--stdinit, `%option stdinit''
     if set (i.e., %option stdinit) initializes `yyin' and `yyout' to
     `stdin' and `stdout', instead of the default of `NULL'.  Some
     existing `lex' programs depend on this behavior, even though it is
     not compliant with ANSI C, which does not require `stdin' and
     `stdout' to be compile-time constant. In a reentrant scanner,
     however, this is not a problem since initialization is performed
     in `yylex_init' at runtime.

`--yylineno, `%option yylineno''
     directs `flex' to generate a scanner that maintains the number of
     the current line read from its input in the global variable
     `yylineno'.  This option is implied by `%option lex-compat'.  In a
     reentrant C scanner, the macro `yylineno' is accessible regardless
     of the value of `%option yylineno', however, its value is not
     modified by `flex' unless `%option yylineno' is enabled.

`--yywrap, `%option yywrap''
     if unset (i.e., `--noyywrap)', makes the scanner not call
     `yywrap()' upon an end-of-file, but simply assume that there are no
     more files to scan (until the user points `yyin' at a new file and
     calls `yylex()' again).


\x1f
File: flex.info,  Node: Code-Level And API Options,  Next: Options for Scanner Speed and Size,  Prev: Options Affecting Scanner Behavior,  Up: Scanner Options

16.3 Code-Level And API Options
===============================

`--ansi-definitions, `%option ansi-definitions''
     instruct flex to generate ANSI C99 definitions for functions.
     This option is enabled by default.  If `%option
     noansi-definitions' is specified, then the obsolete style is
     generated.

`--ansi-prototypes, `%option ansi-prototypes''
     instructs flex to generate ANSI C99 prototypes for functions.
     This option is enabled by default.  If `noansi-prototypes' is
     specified, then prototypes will have empty parameter lists.

`--bison-bridge, `%option bison-bridge''
     instructs flex to generate a C scanner that is meant to be called
     by a `GNU bison' parser. The scanner has minor API changes for
     `bison' compatibility. In particular, the declaration of `yylex'
     is modified to take an additional parameter, `yylval'.  *Note
     Bison Bridge::.

`--bison-locations, `%option bison-locations''
     instruct flex that `GNU bison' `%locations' are being used.  This
     means `yylex' will be passed an additional parameter, `yylloc'.
     This option implies `%option bison-bridge'.  *Note Bison Bridge::.

`-L, --noline, `%option noline''
     instructs `flex' not to generate `#line' directives.  Without this
     option, `flex' peppers the generated scanner with `#line'
     directives so error messages in the actions will be correctly
     located with respect to either the original `flex' input file (if
     the errors are due to code in the input file), or `lex.yy.c' (if
     the errors are `flex''s fault - you should report these sorts of
     errors to the email address given in *Note Reporting Bugs::).

`-R, --reentrant, `%option reentrant''
     instructs flex to generate a reentrant C scanner.  The generated
     scanner may safely be used in a multi-threaded environment. The
     API for a reentrant scanner is different than for a non-reentrant
     scanner *note Reentrant::).  Because of the API difference between
     reentrant and non-reentrant `flex' scanners, non-reentrant flex
     code must be modified before it is suitable for use with this
     option.  This option is not compatible with the `--c++' option.

     The option `--reentrant' does not affect the performance of the
     scanner.

`-+, --c++, `%option c++''
     specifies that you want flex to generate a C++ scanner class.
     *Note Cxx::, for details.

`--array, `%option array''
     specifies that you want yytext to be an array instead of a char*

`--pointer, `%option pointer''
     specify that  `yytext' should be a `char *', not an array.  This
     default is `char *'.

`-PPREFIX, --prefix=PREFIX, `%option prefix="PREFIX"''
     changes the default `yy' prefix used by `flex' for all
     globally-visible variable and function names to instead be
     `PREFIX'.  For example, `--prefix=foo' changes the name of
     `yytext' to `footext'.  It also changes the name of the default
     output file from `lex.yy.c' to `lex.foo.c'.  Here is a partial
     list of the names affected:


              yy_create_buffer
              yy_delete_buffer
              yy_flex_debug
              yy_init_buffer
              yy_flush_buffer
              yy_load_buffer_state
              yy_switch_to_buffer
              yyin
              yyleng
              yylex
              yylineno
              yyout
              yyrestart
              yytext
              yywrap
              yyalloc
              yyrealloc
              yyfree

     (If you are using a C++ scanner, then only `yywrap' and
     `yyFlexLexer' are affected.)  Within your scanner itself, you can
     still refer to the global variables and functions using either
     version of their name; but externally, they have the modified name.

     This option lets you easily link together multiple `flex' programs
     into the same executable.  Note, though, that using this option
     also renames `yywrap()', so you now _must_ either provide your own
     (appropriately-named) version of the routine for your scanner, or
     use `%option noyywrap', as linking with `-lfl' no longer provides
     one for you by default.

`--main, `%option main''
     directs flex to provide a default `main()' program for the
     scanner, which simply calls `yylex()'.  This option implies
     `noyywrap' (see below).

`--nounistd, `%option nounistd''
     suppresses inclusion of the non-ANSI header file `unistd.h'. This
     option is meant to target environments in which `unistd.h' does
     not exist. Be aware that certain options may cause flex to
     generate code that relies on functions normally found in
     `unistd.h', (e.g. `isatty()', `read()'.)  If you wish to use these
     functions, you will have to inform your compiler where to find
     them.  *Note option-always-interactive::. *Note option-read::.

`--yyclass=NAME, `%option yyclass="NAME"''
     only applies when generating a C++ scanner (the `--c++' option).
     It informs `flex' that you have derived `NAME' as a subclass of
     `yyFlexLexer', so `flex' will place your actions in the member
     function `foo::yylex()' instead of `yyFlexLexer::yylex()'.  It
     also generates a `yyFlexLexer::yylex()' member function that emits
     a run-time error (by invoking `yyFlexLexer::LexerError())' if
     called.  *Note Cxx::.


\x1f
File: flex.info,  Node: Options for Scanner Speed and Size,  Next: Debugging Options,  Prev: Code-Level And API Options,  Up: Scanner Options

16.4 Options for Scanner Speed and Size
=======================================

`-C[aefFmr]'
     controls the degree of table compression and, more generally,
     trade-offs between small scanners and fast scanners.

    `-C'
          A lone `-C' specifies that the scanner tables should be
          compressed but neither equivalence classes nor
          meta-equivalence classes should be used.

    `-Ca, --align, `%option align''
          ("align") instructs flex to trade off larger tables in the
          generated scanner for faster performance because the elements
          of the tables are better aligned for memory access and
          computation.  On some RISC architectures, fetching and
          manipulating longwords is more efficient than with
          smaller-sized units such as shortwords.  This option can
          quadruple the size of the tables used by your scanner.

    `-Ce, --ecs, `%option ecs''
          directs `flex' to construct "equivalence classes", i.e., sets
          of characters which have identical lexical properties (for
          example, if the only appearance of digits in the `flex' input
          is in the character class "[0-9]" then the digits '0', '1',
          ..., '9' will all be put in the same equivalence class).
          Equivalence classes usually give dramatic reductions in the
          final table/object file sizes (typically a factor of 2-5) and
          are pretty cheap performance-wise (one array look-up per
          character scanned).

    `-Cf'
          specifies that the "full" scanner tables should be generated -
          `flex' should not compress the tables by taking advantages of
          similar transition functions for different states.

    `-CF'
          specifies that the alternate fast scanner representation
          (described above under the `--fast' flag) should be used.
          This option cannot be used with `--c++'.

    `-Cm, --meta-ecs, `%option meta-ecs''
          directs `flex' to construct "meta-equivalence classes", which
          are sets of equivalence classes (or characters, if equivalence
          classes are not being used) that are commonly used together.
          Meta-equivalence classes are often a big win when using
          compressed tables, but they have a moderate performance
          impact (one or two `if' tests and one array look-up per
          character scanned).

    `-Cr, --read, `%option read''
          causes the generated scanner to _bypass_ use of the standard
          I/O library (`stdio') for input.  Instead of calling
          `fread()' or `getc()', the scanner will use the `read()'
          system call, resulting in a performance gain which varies
          from system to system, but in general is probably negligible
          unless you are also using `-Cf' or `-CF'.  Using `-Cr' can
          cause strange behavior if, for example, you read from `yyin'
          using `stdio' prior to calling the scanner (because the
          scanner will miss whatever text your previous reads left in
          the `stdio' input buffer).  `-Cr' has no effect if you define
          `YY_INPUT()' (*note Generated Scanner::).

     The options `-Cf' or `-CF' and `-Cm' do not make sense together -
     there is no opportunity for meta-equivalence classes if the table
     is not being compressed.  Otherwise the options may be freely
     mixed, and are cumulative.

     The default setting is `-Cem', which specifies that `flex' should
     generate equivalence classes and meta-equivalence classes.  This
     setting provides the highest degree of table compression.  You can
     trade off faster-executing scanners at the cost of larger tables
     with the following generally being true:


              slowest & smallest
                    -Cem
                    -Cm
                    -Ce
                    -C
                    -C{f,F}e
                    -C{f,F}
                    -C{f,F}a
              fastest & largest

     Note that scanners with the smallest tables are usually generated
     and compiled the quickest, so during development you will usually
     want to use the default, maximal compression.

     `-Cfe' is often a good compromise between speed and size for
     production scanners.

`-f, --full, `%option full''
     specifies "fast scanner".  No table compression is done and
     `stdio' is bypassed.  The result is large but fast.  This option
     is equivalent to `--Cfr'

`-F, --fast, `%option fast''
     specifies that the _fast_ scanner table representation should be
     used (and `stdio' bypassed).  This representation is about as fast
     as the full table representation `--full', and for some sets of
     patterns will be considerably smaller (and for others, larger).  In
     general, if the pattern set contains both _keywords_ and a
     catch-all, _identifier_ rule, such as in the set:


              "case"    return TOK_CASE;
              "switch"  return TOK_SWITCH;
              ...
              "default" return TOK_DEFAULT;
              [a-z]+    return TOK_ID;

     then you're better off using the full table representation.  If
     only the _identifier_ rule is present and you then use a hash
     table or some such to detect the keywords, you're better off using
     `--fast'.

     This option is equivalent to `-CFr'.  It cannot be used with
     `--c++'.


\x1f
File: flex.info,  Node: Debugging Options,  Next: Miscellaneous Options,  Prev: Options for Scanner Speed and Size,  Up: Scanner Options

16.5 Debugging Options
======================

`-b, --backup, `%option backup''
     Generate backing-up information to `lex.backup'.  This is a list of
     scanner states which require backing up and the input characters on
     which they do so.  By adding rules one can remove backing-up
     states.  If _all_ backing-up states are eliminated and `-Cf' or
     `-CF' is used, the generated scanner will run faster (see the
     `--perf-report' flag).  Only users who wish to squeeze every last
     cycle out of their scanners need worry about this option.  (*note
     Performance::).

`-d, --debug, `%option debug''
     makes the generated scanner run in "debug" mode.  Whenever a
     pattern is recognized and the global variable `yy_flex_debug' is
     non-zero (which is the default), the scanner will write to
     `stderr' a line of the form:


              -accepting rule at line 53 ("the matched text")

     The line number refers to the location of the rule in the file
     defining the scanner (i.e., the file that was fed to flex).
     Messages are also generated when the scanner backs up, accepts the
     default rule, reaches the end of its input buffer (or encounters a
     NUL; at this point, the two look the same as far as the scanner's
     concerned), or reaches an end-of-file.

`-p, --perf-report, `%option perf-report''
     generates a performance report to `stderr'.  The report consists of
     comments regarding features of the `flex' input file which will
     cause a serious loss of performance in the resulting scanner.  If
     you give the flag twice, you will also get comments regarding
     features that lead to minor performance losses.

     Note that the use of `REJECT', and variable trailing context
     (*note Limitations::) entails a substantial performance penalty;
     use of `yymore()', the `^' operator, and the `--interactive' flag
     entail minor performance penalties.

`-s, --nodefault, `%option nodefault''
     causes the _default rule_ (that unmatched scanner input is echoed
     to `stdout)' to be suppressed.  If the scanner encounters input
     that does not match any of its rules, it aborts with an error.
     This option is useful for finding holes in a scanner's rule set.

`-T, --trace, `%option trace''
     makes `flex' run in "trace" mode.  It will generate a lot of
     messages to `stderr' concerning the form of the input and the
     resultant non-deterministic and deterministic finite automata.
     This option is mostly for use in maintaining `flex'.

`-w, --nowarn, `%option nowarn''
     suppresses warning messages.

`-v, --verbose, `%option verbose''
     specifies that `flex' should write to `stderr' a summary of
     statistics regarding the scanner it generates.  Most of the
     statistics are meaningless to the casual `flex' user, but the
     first line identifies the version of `flex' (same as reported by
     `--version'), and the next line the flags used when generating the
     scanner, including those that are on by default.

`--warn, `%option warn''
     warn about certain things. In particular, if the default rule can
     be matched but no default rule has been given, the flex will warn
     you.  We recommend using this option always.


\x1f
File: flex.info,  Node: Miscellaneous Options,  Prev: Debugging Options,  Up: Scanner Options

16.6 Miscellaneous Options
==========================

`-c'
     A do-nothing option included for POSIX compliance.

`-h, -?, --help'
     generates a "help" summary of `flex''s options to `stdout' and
     then exits.

`-n'
     Another do-nothing option included for POSIX compliance.

`-V, --version'
     prints the version number to `stdout' and exits.


\x1f
File: flex.info,  Node: Performance,  Next: Cxx,  Prev: Scanner Options,  Up: Top

17 Performance Considerations
*****************************

The main design goal of `flex' is that it generate high-performance
scanners.  It has been optimized for dealing well with large sets of
rules.  Aside from the effects on scanner speed of the table compression
`-C' options outlined above, there are a number of options/actions
which degrade performance.  These are, from most expensive to least:


         REJECT
         arbitrary trailing context

         pattern sets that require backing up
         %option yylineno
         %array

         %option interactive
         %option always-interactive

         @samp{^} beginning-of-line operator
         yymore()

   with the first two all being quite expensive and the last two being
quite cheap.  Note also that `unput()' is implemented as a routine call
that potentially does quite a bit of work, while `yyless()' is a
quite-cheap macro. So if you are just putting back some excess text you
scanned, use `yyless()'.

   `REJECT' should be avoided at all costs when performance is
important.  It is a particularly expensive option.

   There is one case when `%option yylineno' can be expensive. That is
when your patterns match long tokens that could _possibly_ contain a
newline character. There is no performance penalty for rules that can
not possibly match newlines, since flex does not need to check them for
newlines.  In general, you should avoid rules such as `[^f]+', which
match very long tokens, including newlines, and may possibly match your
entire file! A better approach is to separate `[^f]+' into two rules:


     %option yylineno
     %%
         [^f\n]+
         \n+

   The above scanner does not incur a performance penalty.

   Getting rid of backing up is messy and often may be an enormous
amount of work for a complicated scanner.  In principal, one begins by
using the `-b' flag to generate a `lex.backup' file.  For example, on
the input:


         %%
         foo        return TOK_KEYWORD;
         foobar     return TOK_KEYWORD;

   the file looks like:


         State #6 is non-accepting -
          associated rule line numbers:
                2       3
          out-transitions: [ o ]
          jam-transitions: EOF [ \001-n  p-\177 ]

         State #8 is non-accepting -
          associated rule line numbers:
                3
          out-transitions: [ a ]
          jam-transitions: EOF [ \001-`  b-\177 ]

         State #9 is non-accepting -
          associated rule line numbers:
                3
          out-transitions: [ r ]
          jam-transitions: EOF [ \001-q  s-\177 ]

         Compressed tables always back up.

   The first few lines tell us that there's a scanner state in which it
can make a transition on an 'o' but not on any other character, and
that in that state the currently scanned text does not match any rule.
The state occurs when trying to match the rules found at lines 2 and 3
in the input file.  If the scanner is in that state and then reads
something other than an 'o', it will have to back up to find a rule
which is matched.  With a bit of headscratching one can see that this
must be the state it's in when it has seen `fo'.  When this has
happened, if anything other than another `o' is seen, the scanner will
have to back up to simply match the `f' (by the default rule).

   The comment regarding State #8 indicates there's a problem when
`foob' has been scanned.  Indeed, on any character other than an `a',
the scanner will have to back up to accept "foo".  Similarly, the
comment for State #9 concerns when `fooba' has been scanned and an `r'
does not follow.

   The final comment reminds us that there's no point going to all the
trouble of removing backing up from the rules unless we're using `-Cf'
or `-CF', since there's no performance gain doing so with compressed
scanners.

   The way to remove the backing up is to add "error" rules:


         %%
         foo         return TOK_KEYWORD;
         foobar      return TOK_KEYWORD;

         fooba       |
         foob        |
         fo          {
                     /* false alarm, not really a keyword */
                     return TOK_ID;
                     }

   Eliminating backing up among a list of keywords can also be done
using a "catch-all" rule:


         %%
         foo         return TOK_KEYWORD;
         foobar      return TOK_KEYWORD;

         [a-z]+      return TOK_ID;

   This is usually the best solution when appropriate.

   Backing up messages tend to cascade.  With a complicated set of rules
it's not uncommon to get hundreds of messages.  If one can decipher
them, though, it often only takes a dozen or so rules to eliminate the
backing up (though it's easy to make a mistake and have an error rule
accidentally match a valid token.  A possible future `flex' feature
will be to automatically add rules to eliminate backing up).

   It's important to keep in mind that you gain the benefits of
eliminating backing up only if you eliminate _every_ instance of
backing up.  Leaving just one means you gain nothing.

   _Variable_ trailing context (where both the leading and trailing
parts do not have a fixed length) entails almost the same performance
loss as `REJECT' (i.e., substantial).  So when possible a rule like:


         %%
         mouse|rat/(cat|dog)   run();

   is better written:


         %%
         mouse/cat|dog         run();
         rat/cat|dog           run();

   or as


         %%
         mouse|rat/cat         run();
         mouse|rat/dog         run();

   Note that here the special '|' action does _not_ provide any
savings, and can even make things worse (*note Limitations::).

   Another area where the user can increase a scanner's performance (and
one that's easier to implement) arises from the fact that the longer the
tokens matched, the faster the scanner will run.  This is because with
long tokens the processing of most input characters takes place in the
(short) inner scanning loop, and does not often have to go through the
additional work of setting up the scanning environment (e.g., `yytext')
for the action.  Recall the scanner for C comments:


         %x comment
         %%
                 int line_num = 1;

         "/*"         BEGIN(comment);

         <comment>[^*\n]*
         <comment>"*"+[^*/\n]*
         <comment>\n             ++line_num;
         <comment>"*"+"/"        BEGIN(INITIAL);

   This could be sped up by writing it as:


         %x comment
         %%
                 int line_num = 1;

         "/*"         BEGIN(comment);

         <comment>[^*\n]*
         <comment>[^*\n]*\n      ++line_num;
         <comment>"*"+[^*/\n]*
         <comment>"*"+[^*/\n]*\n ++line_num;
         <comment>"*"+"/"        BEGIN(INITIAL);

   Now instead of each newline requiring the processing of another
action, recognizing the newlines is distributed over the other rules to
keep the matched text as long as possible.  Note that _adding_ rules
does _not_ slow down the scanner!  The speed of the scanner is
independent of the number of rules or (modulo the considerations given
at the beginning of this section) how complicated the rules are with
regard to operators such as `*' and `|'.

   A final example in speeding up a scanner: suppose you want to scan
through a file containing identifiers and keywords, one per line and
with no other extraneous characters, and recognize all the keywords.  A
natural first approach is:


         %%
         asm      |
         auto     |
         break    |
         ... etc ...
         volatile |
         while    /* it's a keyword */

         .|\n     /* it's not a keyword */

   To eliminate the back-tracking, introduce a catch-all rule:


         %%
         asm      |
         auto     |
         break    |
         ... etc ...
         volatile |
         while    /* it's a keyword */

         [a-z]+   |
         .|\n     /* it's not a keyword */

   Now, if it's guaranteed that there's exactly one word per line, then
we can reduce the total number of matches by a half by merging in the
recognition of newlines with that of the other tokens:


         %%
         asm\n    |
         auto\n   |
         break\n  |
         ... etc ...
         volatile\n |
         while\n  /* it's a keyword */

         [a-z]+\n |
         .|\n     /* it's not a keyword */

   One has to be careful here, as we have now reintroduced backing up
into the scanner.  In particular, while _we_ know that there will never
be any characters in the input stream other than letters or newlines,
`flex' can't figure this out, and it will plan for possibly needing to
back up when it has scanned a token like `auto' and then the next
character is something other than a newline or a letter.  Previously it
would then just match the `auto' rule and be done, but now it has no
`auto' rule, only a `auto\n' rule.  To eliminate the possibility of
backing up, we could either duplicate all rules but without final
newlines, or, since we never expect to encounter such an input and
therefore don't how it's classified, we can introduce one more
catch-all rule, this one which doesn't include a newline:


         %%
         asm\n    |
         auto\n   |
         break\n  |
         ... etc ...
         volatile\n |
         while\n  /* it's a keyword */

         [a-z]+\n |
         [a-z]+   |
         .|\n     /* it's not a keyword */

   Compiled with `-Cf', this is about as fast as one can get a `flex'
scanner to go for this particular problem.

   A final note: `flex' is slow when matching `NUL's, particularly when
a token contains multiple `NUL's.  It's best to write rules which match
_short_ amounts of text if it's anticipated that the text will often
include `NUL's.

   Another final note regarding performance: as mentioned in *Note
Matching::, dynamically resizing `yytext' to accommodate huge tokens is
a slow process because it presently requires that the (huge) token be
rescanned from the beginning.  Thus if performance is vital, you should
attempt to match "large" quantities of text but not "huge" quantities,
where the cutoff between the two is at about 8K characters per token.

\x1f
File: flex.info,  Node: Cxx,  Next: Reentrant,  Prev: Performance,  Up: Top

18 Generating C++ Scanners
**************************

*IMPORTANT*: the present form of the scanning class is _experimental_
and may change considerably between major releases.

   `flex' provides two different ways to generate scanners for use with
C++.  The first way is to simply compile a scanner generated by `flex'
using a C++ compiler instead of a C compiler.  You should not encounter
any compilation errors (*note Reporting Bugs::).  You can then use C++
code in your rule actions instead of C code.  Note that the default
input source for your scanner remains `yyin', and default echoing is
still done to `yyout'.  Both of these remain `FILE *' variables and not
C++ _streams_.

   You can also use `flex' to generate a C++ scanner class, using the
`-+' option (or, equivalently, `%option c++)', which is automatically
specified if the name of the `flex' executable ends in a '+', such as
`flex++'.  When using this option, `flex' defaults to generating the
scanner to the file `lex.yy.cc' instead of `lex.yy.c'.  The generated
scanner includes the header file `FlexLexer.h', which defines the
interface to two C++ classes.

   The first class, `FlexLexer', provides an abstract base class
defining the general scanner class interface.  It provides the
following member functions:

`const char* YYText()'
     returns the text of the most recently matched token, the
     equivalent of `yytext'.

`int YYLeng()'
     returns the length of the most recently matched token, the
     equivalent of `yyleng'.

`int lineno() const'
     returns the current input line number (see `%option yylineno)', or
     `1' if `%option yylineno' was not used.

`void set_debug( int flag )'
     sets the debugging flag for the scanner, equivalent to assigning to
     `yy_flex_debug' (*note Scanner Options::).  Note that you must
     build the scanner using `%option debug' to include debugging
     information in it.

`int debug() const'
     returns the current setting of the debugging flag.

   Also provided are member functions equivalent to
`yy_switch_to_buffer()', `yy_create_buffer()' (though the first
argument is an `istream*' object pointer and not a `FILE*)',
`yy_flush_buffer()', `yy_delete_buffer()', and `yyrestart()' (again,
the first argument is a `istream*' object pointer).

   The second class defined in `FlexLexer.h' is `yyFlexLexer', which is
derived from `FlexLexer'.  It defines the following additional member
functions:

`yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )'
     constructs a `yyFlexLexer' object using the given streams for input
     and output.  If not specified, the streams default to `cin' and
     `cout', respectively.

`virtual int yylex()'
     performs the same role is `yylex()' does for ordinary `flex'
     scanners: it scans the input stream, consuming tokens, until a
     rule's action returns a value.  If you derive a subclass `S' from
     `yyFlexLexer' and want to access the member functions and variables
     of `S' inside `yylex()', then you need to use `%option
     yyclass="S"' to inform `flex' that you will be using that subclass
     instead of `yyFlexLexer'.  In this case, rather than generating
     `yyFlexLexer::yylex()', `flex' generates `S::yylex()' (and also
     generates a dummy `yyFlexLexer::yylex()' that calls
     `yyFlexLexer::LexerError()' if called).

`virtual void switch_streams(istream* new_in = 0, ostream* new_out = 0)'
     reassigns `yyin' to `new_in' (if non-null) and `yyout' to
     `new_out' (if non-null), deleting the previous input buffer if
     `yyin' is reassigned.

`int yylex( istream* new_in, ostream* new_out = 0 )'
     first switches the input streams via `switch_streams( new_in,
     new_out )' and then returns the value of `yylex()'.

   In addition, `yyFlexLexer' defines the following protected virtual
functions which you can redefine in derived classes to tailor the
scanner:

`virtual int LexerInput( char* buf, int max_size )'
     reads up to `max_size' characters into `buf' and returns the
     number of characters read.  To indicate end-of-input, return 0
     characters.  Note that `interactive' scanners (see the `-B' and
     `-I' flags in *Note Scanner Options::) define the macro
     `YY_INTERACTIVE'.  If you redefine `LexerInput()' and need to take
     different actions depending on whether or not the scanner might be
     scanning an interactive input source, you can test for the
     presence of this name via `#ifdef' statements.

`virtual void LexerOutput( const char* buf, int size )'
     writes out `size' characters from the buffer `buf', which, while
     `NUL'-terminated, may also contain internal `NUL's if the
     scanner's rules can match text with `NUL's in them.

`virtual void LexerError( const char* msg )'
     reports a fatal error message.  The default version of this
     function writes the message to the stream `cerr' and exits.

   Note that a `yyFlexLexer' object contains its _entire_ scanning
state.  Thus you can use such objects to create reentrant scanners, but
see also *Note Reentrant::.  You can instantiate multiple instances of
the same `yyFlexLexer' class, and you can also combine multiple C++
scanner classes together in the same program using the `-P' option
discussed above.

   Finally, note that the `%array' feature is not available to C++
scanner classes; you must use `%pointer' (the default).

   Here is an example of a simple C++ scanner:


             // An example of using the flex C++ scanner class.

         %{
         int mylineno = 0;
         %}

         string  \"[^\n"]+\"

         ws      [ \t]+

         alpha   [A-Za-z]
         dig     [0-9]
         name    ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
         num1    [-+]?{dig}+\.?([eE][-+]?{dig}+)?
         num2    [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
         number  {num1}|{num2}

         %%

         {ws}    /* skip blanks and tabs */

         "/*"    {
                 int c;

                 while((c = yyinput()) != 0)
                     {
                     if(c == '\n')
                         ++mylineno;

                     else if(c == @samp{*})
                         {
                         if((c = yyinput()) == '/')
                             break;
                         else
                             unput(c);
                         }
                     }
                 }

         {number}  cout  "number "  YYText()  '\n';

         \n        mylineno++;

         {name}    cout  "name "  YYText()  '\n';

         {string}  cout  "string "  YYText()  '\n';

         %%

         int main( int /* argc */, char** /* argv */ )
             {
             @code{flex}Lexer* lexer = new yyFlexLexer;
             while(lexer->yylex() != 0)
                 ;
             return 0;
             }

   If you want to create multiple (different) lexer classes, you use the
`-P' flag (or the `prefix=' option) to rename each `yyFlexLexer' to
some other `xxFlexLexer'.  You then can include `<FlexLexer.h>' in your
other sources once per lexer class, first renaming `yyFlexLexer' as
follows:


         #undef yyFlexLexer
         #define yyFlexLexer xxFlexLexer
         #include <FlexLexer.h>

         #undef yyFlexLexer
         #define yyFlexLexer zzFlexLexer
         #include <FlexLexer.h>

   if, for example, you used `%option prefix="xx"' for one of your
scanners and `%option prefix="zz"' for the other.

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File: flex.info,  Node: Reentrant,  Next: Lex and Posix,  Prev: Cxx,  Up: Top

19 Reentrant C Scanners
***********************

`flex' has the ability to generate a reentrant C scanner. This is
accomplished by specifying `%option reentrant' (`-R') The generated
scanner is both portable, and safe to use in one or more separate
threads of control.  The most common use for reentrant scanners is from
within multi-threaded applications.  Any thread may create and execute
a reentrant `flex' scanner without the need for synchronization with
other threads.

* Menu:

* Reentrant Uses::
* Reentrant Overview::
* Reentrant Example::
* Reentrant Detail::
* Reentrant Functions::

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File: flex.info,  Node: Reentrant Uses,  Next: Reentrant Overview,  Prev: Reentrant,  Up: Reentrant

19.1 Uses for Reentrant Scanners
================================

However, there are other uses for a reentrant scanner.  For example, you
could scan two or more files simultaneously to implement a `diff' at
the token level (i.e., instead of at the character level):


         /* Example of maintaining more than one active scanner. */

         do {
             int tok1, tok2;

             tok1 = yylex( scanner_1 );
             tok2 = yylex( scanner_2 );

             if( tok1 != tok2 )
                 printf("Files are different.");

        } while ( tok1 && tok2 );

   Another use for a reentrant scanner is recursion.  (Note that a
recursive scanner can also be created using a non-reentrant scanner and
buffer states. *Note Multiple Input Buffers::.)

   The following crude scanner supports the `eval' command by invoking
another instance of itself.


         /* Example of recursive invocation. */

         %option reentrant

         %%
         "eval(".+")"  {
                           yyscan_t scanner;
                           YY_BUFFER_STATE buf;

                           yylex_init( &scanner );
                           yytext[yyleng-1] = ' ';

                           buf = yy_scan_string( yytext + 5, scanner );
                           yylex( scanner );

                           yy_delete_buffer(buf,scanner);
                           yylex_destroy( scanner );
                      }
         ...
         %%

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File: flex.info,  Node: Reentrant Overview,  Next: Reentrant Example,  Prev: Reentrant Uses,  Up: Reentrant

19.2 An Overview of the Reentrant API
=====================================

The API for reentrant scanners is different than for non-reentrant
scanners. Here is a quick overview of the API:

     `%option reentrant' must be specified.

   * All functions take one additional argument: `yyscanner'

   * All global variables are replaced by their macro equivalents.  (We
     tell you this because it may be important to you during debugging.)

   * `yylex_init' and `yylex_destroy' must be called before and after
     `yylex', respectively.

   * Accessor methods (get/set functions) provide access to common
     `flex' variables.

   * User-specific data can be stored in `yyextra'.

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File: flex.info,  Node: Reentrant Example,  Next: Reentrant Detail,  Prev: Reentrant Overview,  Up: Reentrant

19.3 Reentrant Example
======================

First, an example of a reentrant scanner: 

         /* This scanner prints "//" comments. */

         %option reentrant stack noyywrap
         %x COMMENT

         %%

         "//"                 yy_push_state( COMMENT, yyscanner);
         .|\n

         <COMMENT>\n          yy_pop_state( yyscanner );
         <COMMENT>[^\n]+      fprintf( yyout, "%s\n", yytext);

         %%

         int main ( int argc, char * argv[] )
         {
             yyscan_t scanner;

             yylex_init ( &scanner );
             yylex ( scanner );
             yylex_destroy ( scanner );
         return 0;
        }

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File: flex.info,  Node: Reentrant Detail,  Next: Reentrant Functions,  Prev: Reentrant Example,  Up: Reentrant

19.4 The Reentrant API in Detail
================================

Here are the things you need to do or know to use the reentrant C API of
`flex'.

* Menu:

* Specify Reentrant::
* Extra Reentrant Argument::
* Global Replacement::
* Init and Destroy Functions::
* Accessor Methods::
* Extra Data::
* About yyscan_t::

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File: flex.info,  Node: Specify Reentrant,  Next: Extra Reentrant Argument,  Prev: Reentrant Detail,  Up: Reentrant Detail

19.4.1 Declaring a Scanner As Reentrant
---------------------------------------

%option reentrant (-reentrant) must be specified.

   Notice that `%option reentrant' is specified in the above example
(*note Reentrant Example::. Had this option not been specified, `flex'
would have happily generated a non-reentrant scanner without
complaining. You may explicitly specify `%option noreentrant', if you
do _not_ want a reentrant scanner, although it is not necessary. The
default is to generate a non-reentrant scanner.

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File: flex.info,  Node: Extra Reentrant Argument,  Next: Global Replacement,  Prev: Specify Reentrant,  Up: Reentrant Detail

19.4.2 The Extra Argument
-------------------------

All functions take one additional argument: `yyscanner'.

   Notice that the calls to `yy_push_state' and `yy_pop_state' both
have an argument, `yyscanner' , that is not present in a non-reentrant
scanner.  Here are the declarations of `yy_push_state' and
`yy_pop_state' in the reentrant scanner:


         static void yy_push_state  ( int new_state , yyscan_t yyscanner ) ;
         static void yy_pop_state  ( yyscan_t yyscanner  ) ;

   Notice that the argument `yyscanner' appears in the declaration of
both functions.  In fact, all `flex' functions in a reentrant scanner
have this additional argument.  It is always the last argument in the
argument list, it is always of type `yyscan_t' (which is typedef'd to
`void *') and it is always named `yyscanner'.  As you may have guessed,
`yyscanner' is a pointer to an opaque data structure encapsulating the
current state of the scanner.  For a list of function declarations, see
*Note Reentrant Functions::. Note that preprocessor macros, such as
`BEGIN', `ECHO', and `REJECT', do not take this additional argument.

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File: flex.info,  Node: Global Replacement,  Next: Init and Destroy Functions,  Prev: Extra Reentrant Argument,  Up: Reentrant Detail

19.4.3 Global Variables Replaced By Macros
------------------------------------------

All global variables in traditional flex have been replaced by macro
equivalents.

   Note that in the above example, `yyout' and `yytext' are not plain
variables. These are macros that will expand to their equivalent lvalue.
All of the familiar `flex' globals have been replaced by their macro
equivalents. In particular, `yytext', `yyleng', `yylineno', `yyin',
`yyout', `yyextra', `yylval', and `yylloc' are macros. You may safely
use these macros in actions as if they were plain variables. We only
tell you this so you don't expect to link to these variables
externally. Currently, each macro expands to a member of an internal
struct, e.g.,


     #define yytext (((struct yyguts_t*)yyscanner)->yytext_r)

   One important thing to remember about `yytext' and friends is that
`yytext' is not a global variable in a reentrant scanner, you can not
access it directly from outside an action or from other functions. You
must use an accessor method, e.g., `yyget_text', to accomplish this.
(See below).

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File: flex.info,  Node: Init and Destroy Functions,  Next: Accessor Methods,  Prev: Global Replacement,  Up: Reentrant Detail

19.4.4 Init and Destroy Functions
---------------------------------

`yylex_init' and `yylex_destroy' must be called before and after
`yylex', respectively.


         int yylex_init ( yyscan_t * ptr_yy_globals ) ;
         int yylex_init_extra ( YY_EXTRA_TYPE user_defined, yyscan_t * ptr_yy_globals ) ;
         int yylex ( yyscan_t yyscanner ) ;
         int yylex_destroy ( yyscan_t yyscanner ) ;

   The function `yylex_init' must be called before calling any other
function. The argument to `yylex_init' is the address of an
uninitialized pointer to be filled in by `yylex_init', overwriting any
previous contents. The function `yylex_init_extra' may be used instead,
taking as its first argument a variable of type `YY_EXTRA_TYPE'.  See
the section on yyextra, below, for more details.

   The value stored in `ptr_yy_globals' should thereafter be passed to
`yylex' and `yylex_destroy'.  Flex does not save the argument passed to
`yylex_init', so it is safe to pass the address of a local pointer to
`yylex_init' so long as it remains in scope for the duration of all
calls to the scanner, up to and including the call to `yylex_destroy'.

   The function `yylex' should be familiar to you by now. The reentrant
version takes one argument, which is the value returned (via an
argument) by `yylex_init'.  Otherwise, it behaves the same as the
non-reentrant version of `yylex'.

   Both `yylex_init' and `yylex_init_extra' returns 0 (zero) on success,
or non-zero on failure, in which case errno is set to one of the
following values:

   * ENOMEM Memory allocation error. *Note memory-management::.

   * EINVAL Invalid argument.

   The function `yylex_destroy' should be called to free resources used
by the scanner. After `yylex_destroy' is called, the contents of
`yyscanner' should not be used.  Of course, there is no need to destroy
a scanner if you plan to reuse it.  A `flex' scanner (both reentrant
and non-reentrant) may be restarted by calling `yyrestart'.

   Below is an example of a program that creates a scanner, uses it,
then destroys it when done:


         int main ()
         {
             yyscan_t scanner;
             int tok;

             yylex_init(&scanner);

             while ((tok=yylex()) > 0)
                 printf("tok=%d  yytext=%s\n", tok, yyget_text(scanner));

             yylex_destroy(scanner);
             return 0;
         }

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File: flex.info,  Node: Accessor Methods,  Next: Extra Data,  Prev: Init and Destroy Functions,  Up: Reentrant Detail

19.4.5 Accessing Variables with Reentrant Scanners
--------------------------------------------------

Accessor methods (get/set functions) provide access to common `flex'
variables.

   Many scanners that you build will be part of a larger project.
Portions of your project will need access to `flex' values, such as
`yytext'.  In a non-reentrant scanner, these values are global, so
there is no problem accessing them. However, in a reentrant scanner,
there are no global `flex' values. You can not access them directly.
Instead, you must access `flex' values using accessor methods (get/set
functions). Each accessor method is named `yyget_NAME' or `yyset_NAME',
where `NAME' is the name of the `flex' variable you want. For example:


         /* Set the last character of yytext to NULL. */
         void chop ( yyscan_t scanner )
         {
             int len = yyget_leng( scanner );
             yyget_text( scanner )[len - 1] = '\0';
         }

   The above code may be called from within an action like this:


         %%
         .+\n    { chop( yyscanner );}

   You may find that `%option header-file' is particularly useful for
generating prototypes of all the accessor functions. *Note
option-header::.

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File: flex.info,  Node: Extra Data,  Next: About yyscan_t,  Prev: Accessor Methods,  Up: Reentrant Detail

19.4.6 Extra Data
-----------------

User-specific data can be stored in `yyextra'.

   In a reentrant scanner, it is unwise to use global variables to
communicate with or maintain state between different pieces of your
program.  However, you may need access to external data or invoke
external functions from within the scanner actions.  Likewise, you may
need to pass information to your scanner (e.g., open file descriptors,
or database connections).  In a non-reentrant scanner, the only way to
do this would be through the use of global variables.  `Flex' allows
you to store arbitrary, "extra" data in a scanner.  This data is
accessible through the accessor methods `yyget_extra' and `yyset_extra'
from outside the scanner, and through the shortcut macro `yyextra' from
within the scanner itself. They are defined as follows:


         #define YY_EXTRA_TYPE  void*
         YY_EXTRA_TYPE  yyget_extra ( yyscan_t scanner );
         void           yyset_extra ( YY_EXTRA_TYPE arbitrary_data , yyscan_t scanner);

   In addition, an extra form of `yylex_init' is provided,
`yylex_init_extra'. This function is provided so that the yyextra value
can be accessed from within the very first yyalloc, used to allocate
the scanner itself.

   By default, `YY_EXTRA_TYPE' is defined as type `void *'.  You may
redefine this type using `%option extra-type="your_type"' in the
scanner:


         /* An example of overriding YY_EXTRA_TYPE. */
         %{
         #include <sys/stat.h>
         #include <unistd.h>
         %}
         %option reentrant
         %option extra-type="struct stat *"
         %%

         __filesize__     printf( "%ld", yyextra->st_size  );
         __lastmod__      printf( "%ld", yyextra->st_mtime );
         %%
         void scan_file( char* filename )
         {
             yyscan_t scanner;
             struct stat buf;
             FILE *in;

             in = fopen( filename, "r" );
             stat( filename, &buf );

             yylex_init_extra( buf, &scanner );
             yyset_in( in, scanner );
             yylex( scanner );
             yylex_destroy( scanner );

             fclose( in );
        }

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File: flex.info,  Node: About yyscan_t,  Prev: Extra Data,  Up: Reentrant Detail

19.4.7 About yyscan_t
---------------------

`yyscan_t' is defined as:


          typedef void* yyscan_t;

   It is initialized by `yylex_init()' to point to an internal
structure. You should never access this value directly. In particular,
you should never attempt to free it (use `yylex_destroy()' instead.)

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File: flex.info,  Node: Reentrant Functions,  Prev: Reentrant Detail,  Up: Reentrant

19.5 Functions and Macros Available in Reentrant C Scanners
===========================================================

The following Functions are available in a reentrant scanner:


         char *yyget_text ( yyscan_t scanner );
         int yyget_leng ( yyscan_t scanner );
         FILE *yyget_in ( yyscan_t scanner );
         FILE *yyget_out ( yyscan_t scanner );
         int yyget_lineno ( yyscan_t scanner );
         YY_EXTRA_TYPE yyget_extra ( yyscan_t scanner );
         int  yyget_debug ( yyscan_t scanner );

         void yyset_debug ( int flag, yyscan_t scanner );
         void yyset_in  ( FILE * in_str , yyscan_t scanner );
         void yyset_out  ( FILE * out_str , yyscan_t scanner );
         void yyset_lineno ( int line_number , yyscan_t scanner );
         void yyset_extra ( YY_EXTRA_TYPE user_defined , yyscan_t scanner );

   There are no "set" functions for yytext and yyleng. This is
intentional.

   The following Macro shortcuts are available in actions in a reentrant
scanner:


         yytext
         yyleng
         yyin
         yyout
         yylineno
         yyextra
         yy_flex_debug

   In a reentrant C scanner, support for yylineno is always present
(i.e., you may access yylineno), but the value is never modified by
`flex' unless `%option yylineno' is enabled. This is to allow the user
to maintain the line count independently of `flex'.

   The following functions and macros are made available when `%option
bison-bridge' (`--bison-bridge') is specified:


         YYSTYPE * yyget_lval ( yyscan_t scanner );
         void yyset_lval ( YYSTYPE * yylvalp , yyscan_t scanner );
         yylval

   The following functions and macros are made available when `%option
bison-locations' (`--bison-locations') is specified:


         YYLTYPE *yyget_lloc ( yyscan_t scanner );
         void yyset_lloc ( YYLTYPE * yyllocp , yyscan_t scanner );
         yylloc

   Support for yylval assumes that `YYSTYPE' is a valid type.  Support
for yylloc assumes that `YYSLYPE' is a valid type.  Typically, these
types are generated by `bison', and are included in section 1 of the
`flex' input.

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File: flex.info,  Node: Lex and Posix,  Next: Memory Management,  Prev: Reentrant,  Up: Top

20 Incompatibilities with Lex and Posix
***************************************

`flex' is a rewrite of the AT&T Unix _lex_ tool (the two
implementations do not share any code, though), with some extensions and
incompatibilities, both of which are of concern to those who wish to
write scanners acceptable to both implementations.  `flex' is fully
compliant with the POSIX `lex' specification, except that when using
`%pointer' (the default), a call to `unput()' destroys the contents of
`yytext', which is counter to the POSIX specification.  In this section
we discuss all of the known areas of incompatibility between `flex',
AT&T `lex', and the POSIX specification.  `flex''s `-l' option turns on
maximum compatibility with the original AT&T `lex' implementation, at
the cost of a major loss in the generated scanner's performance.  We
note below which incompatibilities can be overcome using the `-l'
option.  `flex' is fully compatible with `lex' with the following
exceptions:

   * The undocumented `lex' scanner internal variable `yylineno' is not
     supported unless `-l' or `%option yylineno' is used.

   * `yylineno' should be maintained on a per-buffer basis, rather than
     a per-scanner (single global variable) basis.

   * `yylineno' is not part of the POSIX specification.

   * The `input()' routine is not redefinable, though it may be called
     to read characters following whatever has been matched by a rule.
     If `input()' encounters an end-of-file the normal `yywrap()'
     processing is done.  A "real" end-of-file is returned by `input()'
     as `EOF'.

   * Input is instead controlled by defining the `YY_INPUT()' macro.

   * The `flex' restriction that `input()' cannot be redefined is in
     accordance with the POSIX specification, which simply does not
     specify any way of controlling the scanner's input other than by
     making an initial assignment to `yyin'.

   * The `unput()' routine is not redefinable.  This restriction is in
     accordance with POSIX.

   * `flex' scanners are not as reentrant as `lex' scanners.  In
     particular, if you have an interactive scanner and an interrupt
     handler which long-jumps out of the scanner, and the scanner is
     subsequently called again, you may get the following message:


              fatal @code{flex} scanner internal error--end of buffer missed

     To reenter the scanner, first use:


              yyrestart( yyin );

     Note that this call will throw away any buffered input; usually
     this isn't a problem with an interactive scanner. *Note
     Reentrant::, for `flex''s reentrant API.

   * Also note that `flex' C++ scanner classes _are_ reentrant, so if
     using C++ is an option for you, you should use them instead.
     *Note Cxx::, and *Note Reentrant::  for details.

   * `output()' is not supported.  Output from the ECHO macro is done
     to the file-pointer `yyout' (default `stdout)'.

   * `output()' is not part of the POSIX specification.

   * `lex' does not support exclusive start conditions (%x), though they
     are in the POSIX specification.

   * When definitions are expanded, `flex' encloses them in parentheses.
     With `lex', the following:


              NAME    [A-Z][A-Z0-9]*
              %%
              foo{NAME}?      printf( "Found it\n" );
              %%

     will not match the string `foo' because when the macro is expanded
     the rule is equivalent to `foo[A-Z][A-Z0-9]*?'  and the precedence
     is such that the `?' is associated with `[A-Z0-9]*'.  With `flex',
     the rule will be expanded to `foo([A-Z][A-Z0-9]*)?' and so the
     string `foo' will match.

   * Note that if the definition begins with `^' or ends with `$' then
     it is _not_ expanded with parentheses, to allow these operators to
     appear in definitions without losing their special meanings.  But
     the `<s>', `/', and `<<EOF>>' operators cannot be used in a `flex'
     definition.

   * Using `-l' results in the `lex' behavior of no parentheses around
     the definition.

   * The POSIX specification is that the definition be enclosed in
     parentheses.

   * Some implementations of `lex' allow a rule's action to begin on a
     separate line, if the rule's pattern has trailing whitespace:


              %%
              foo|bar<space here>
                { foobar_action();}

     `flex' does not support this feature.

   * The `lex' `%r' (generate a Ratfor scanner) option is not
     supported.  It is not part of the POSIX specification.

   * After a call to `unput()', _yytext_ is undefined until the next
     token is matched, unless the scanner was built using `%array'.
     This is not the case with `lex' or the POSIX specification.  The
     `-l' option does away with this incompatibility.

   * The precedence of the `{,}' (numeric range) operator is different.
     The AT&T and POSIX specifications of `lex' interpret `abc{1,3}'
     as match one, two, or three occurrences of `abc'", whereas `flex'
     interprets it as "match `ab' followed by one, two, or three
     occurrences of `c'".  The `-l' and `--posix' options do away with
     this incompatibility.

   * The precedence of the `^' operator is different.  `lex' interprets
     `^foo|bar' as "match either 'foo' at the beginning of a line, or
     'bar' anywhere", whereas `flex' interprets it as "match either
     `foo' or `bar' if they come at the beginning of a line".  The
     latter is in agreement with the POSIX specification.

   * The special table-size declarations such as `%a' supported by
     `lex' are not required by `flex' scanners..  `flex' ignores them.

   * The name `FLEX_SCANNER' is `#define''d so scanners may be written
     for use with either `flex' or `lex'.  Scanners also include
     `YY_FLEX_MAJOR_VERSION',  `YY_FLEX_MINOR_VERSION' and
     `YY_FLEX_SUBMINOR_VERSION' indicating which version of `flex'
     generated the scanner. For example, for the 2.5.22 release, these
     defines would be 2,  5 and 22 respectively. If the version of
     `flex' being used is a beta version, then the symbol `FLEX_BETA'
     is defined.

   * The symbols `[[' and `]]' in the code sections of the input may
     conflict with the m4 delimiters. *Note M4 Dependency::.


   The following `flex' features are not included in `lex' or the POSIX
specification:

   * C++ scanners

   * %option

   * start condition scopes

   * start condition stacks

   * interactive/non-interactive scanners

   * yy_scan_string() and friends

   * yyterminate()

   * yy_set_interactive()

   * yy_set_bol()

   * YY_AT_BOL()    <<EOF>>

   * <*>

   * YY_DECL

   * YY_START

   * YY_USER_ACTION

   * YY_USER_INIT

   * #line directives

   * %{}'s around actions

   * reentrant C API

   * multiple actions on a line

   * almost all of the `flex' command-line options

   The feature "multiple actions on a line" refers to the fact that
with `flex' you can put multiple actions on the same line, separated
with semi-colons, while with `lex', the following:


         foo    handle_foo(); ++num_foos_seen;

   is (rather surprisingly) truncated to


         foo    handle_foo();

   `flex' does not truncate the action.  Actions that are not enclosed
in braces are simply terminated at the end of the line.

\x1f
File: flex.info,  Node: Memory Management,  Next: Serialized Tables,  Prev: Lex and Posix,  Up: Top

21 Memory Management
********************

This chapter describes how flex handles dynamic memory, and how you can
override the default behavior.

* Menu:

* The Default Memory Management::
* Overriding The Default Memory Management::
* A Note About yytext And Memory::

\x1f
File: flex.info,  Node: The Default Memory Management,  Next: Overriding The Default Memory Management,  Prev: Memory Management,  Up: Memory Management

21.1 The Default Memory Management
==================================

Flex allocates dynamic memory during initialization, and once in a
while from within a call to yylex(). Initialization takes place during
the first call to yylex(). Thereafter, flex may reallocate more memory
if it needs to enlarge a buffer. As of version 2.5.9 Flex will clean up
all memory when you call `yylex_destroy' *Note faq-memory-leak::.

   Flex allocates dynamic memory for four purposes, listed below (1)

16kB for the input buffer.
     Flex allocates memory for the character buffer used to perform
     pattern matching.  Flex must read ahead from the input stream and
     store it in a large character buffer.  This buffer is typically
     the largest chunk of dynamic memory flex consumes. This buffer
     will grow if necessary, doubling the size each time.  Flex frees
     this memory when you call yylex_destroy().  The default size of
     this buffer (16384 bytes) is almost always too large.  The ideal
     size for this buffer is the length of the longest token expected,
     in bytes, plus a little more.  Flex will allocate a few extra
     bytes for housekeeping. Currently, to override the size of the
     input buffer you must `#define YY_BUF_SIZE' to whatever number of
     bytes you want. We don't plan to change this in the near future,
     but we reserve the right to do so if we ever add a more robust
     memory management API.

64kb for the REJECT state. This will only be allocated if you use REJECT.
     The size is the large enough to hold the same number of states as
     characters in the input buffer. If you override the size of the
     input buffer (via `YY_BUF_SIZE'), then you automatically override
     the size of this buffer as well.

100 bytes for the start condition stack.
     Flex allocates memory for the start condition stack. This is the
     stack used for pushing start states, i.e., with yy_push_state().
     It will grow if necessary.  Since the states are simply integers,
     this stack doesn't consume much memory.  This stack is not present
     if `%option stack' is not specified.  You will rarely need to tune
     this buffer. The ideal size for this stack is the maximum depth
     expected.  The memory for this stack is automatically destroyed
     when you call yylex_destroy(). *Note option-stack::.

40 bytes for each YY_BUFFER_STATE.
     Flex allocates memory for each YY_BUFFER_STATE. The buffer state
     itself is about 40 bytes, plus an additional large character
     buffer (described above.)  The initial buffer state is created
     during initialization, and with each call to yy_create_buffer().
     You can't tune the size of this, but you can tune the character
     buffer as described above. Any buffer state that you explicitly
     create by calling yy_create_buffer() is _NOT_ destroyed
     automatically. You must call yy_delete_buffer() to free the
     memory. The exception to this rule is that flex will delete the
     current buffer automatically when you call yylex_destroy(). If you
     delete the current buffer, be sure to set it to NULL.  That way,
     flex will not try to delete the buffer a second time (possibly
     crashing your program!) At the time of this writing, flex does not
     provide a growable stack for the buffer states.  You have to
     manage that yourself.  *Note Multiple Input Buffers::.

84 bytes for the reentrant scanner guts
     Flex allocates about 84 bytes for the reentrant scanner structure
     when you call yylex_init(). It is destroyed when the user calls
     yylex_destroy().


   ---------- Footnotes ----------

   (1) The quantities given here are approximate, and may vary due to
host architecture, compiler configuration, or due to future
enhancements to flex.

\x1f
File: flex.info,  Node: Overriding The Default Memory Management,  Next: A Note About yytext And Memory,  Prev: The Default Memory Management,  Up: Memory Management

21.2 Overriding The Default Memory Management
=============================================

Flex calls the functions `yyalloc', `yyrealloc', and `yyfree' when it
needs to allocate or free memory. By default, these functions are
wrappers around the standard C functions, `malloc', `realloc', and
`free', respectively. You can override the default implementations by
telling flex that you will provide your own implementations.

   To override the default implementations, you must do two things:

  1. Suppress the default implementations by specifying one or more of
     the following options:

        * `%option noyyalloc'

        * `%option noyyrealloc'

        * `%option noyyfree'.

  2. Provide your own implementation of the following functions: (1)


          // For a non-reentrant scanner
          void * yyalloc (size_t bytes);
          void * yyrealloc (void * ptr, size_t bytes);
          void   yyfree (void * ptr);

          // For a reentrant scanner
          void * yyalloc (size_t bytes, void * yyscanner);
          void * yyrealloc (void * ptr, size_t bytes, void * yyscanner);
          void   yyfree (void * ptr, void * yyscanner);


   In the following example, we will override all three memory
routines. We assume that there is a custom allocator with garbage
collection. In order to make this example interesting, we will use a
reentrant scanner, passing a pointer to the custom allocator through
`yyextra'.


     %{
     #include "some_allocator.h"
     %}

     /* Suppress the default implementations. */
     %option noyyalloc noyyrealloc noyyfree
     %option reentrant

     /* Initialize the allocator. */
     #define YY_EXTRA_TYPE  struct allocator*
     #define YY_USER_INIT  yyextra = allocator_create();

     %%
     .|\n   ;
     %%

     /* Provide our own implementations. */
     void * yyalloc (size_t bytes, void* yyscanner) {
         return allocator_alloc (yyextra, bytes);
     }

     void * yyrealloc (void * ptr, size_t bytes, void* yyscanner) {
         return allocator_realloc (yyextra, bytes);
     }

     void yyfree (void * ptr, void * yyscanner) {
         /* Do nothing -- we leave it to the garbage collector. */
     }

   ---------- Footnotes ----------

   (1) It is not necessary to override all (or any) of the memory
management routines.  You may, for example, override `yyrealloc', but
not `yyfree' or `yyalloc'.

\x1f
File: flex.info,  Node: A Note About yytext And Memory,  Prev: Overriding The Default Memory Management,  Up: Memory Management

21.3 A Note About yytext And Memory
===================================

When flex finds a match, `yytext' points to the first character of the
match in the input buffer. The string itself is part of the input
buffer, and is _NOT_ allocated separately. The value of yytext will be
overwritten the next time yylex() is called. In short, the value of
yytext is only valid from within the matched rule's action.

   Often, you want the value of yytext to persist for later processing,
i.e., by a parser with non-zero lookahead. In order to preserve yytext,
you will have to copy it with strdup() or a similar function. But this
introduces some headache because your parser is now responsible for
freeing the copy of yytext. If you use a yacc or bison parser,
(commonly used with flex), you will discover that the error recovery
mechanisms can cause memory to be leaked.

   To prevent memory leaks from strdup'd yytext, you will have to track
the memory somehow. Our experience has shown that a garbage collection
mechanism or a pooled memory mechanism will save you a lot of grief
when writing parsers.

\x1f
File: flex.info,  Node: Serialized Tables,  Next: Diagnostics,  Prev: Memory Management,  Up: Top

22 Serialized Tables
********************

A `flex' scanner has the ability to save the DFA tables to a file, and
load them at runtime when needed.  The motivation for this feature is
to reduce the runtime memory footprint.  Traditionally, these tables
have been compiled into the scanner as C arrays, and are sometimes
quite large.  Since the tables are compiled into the scanner, the
memory used by the tables can never be freed.  This is a waste of
memory, especially if an application uses several scanners, but none of
them at the same time.

   The serialization feature allows the tables to be loaded at runtime,
before scanning begins. The tables may be discarded when scanning is
finished.

* Menu:

* Creating Serialized Tables::
* Loading and Unloading Serialized Tables::
* Tables File Format::

\x1f
File: flex.info,  Node: Creating Serialized Tables,  Next: Loading and Unloading Serialized Tables,  Prev: Serialized Tables,  Up: Serialized Tables

22.1 Creating Serialized Tables
===============================

You may create a scanner with serialized tables by specifying:


         %option tables-file=FILE
     or
         --tables-file=FILE

   These options instruct flex to save the DFA tables to the file FILE.
The tables will _not_ be embedded in the generated scanner. The scanner
will not function on its own. The scanner will be dependent upon the
serialized tables. You must load the tables from this file at runtime
before you can scan anything.

   If you do not specify a filename to `--tables-file', the tables will
be saved to `lex.yy.tables', where `yy' is the appropriate prefix.

   If your project uses several different scanners, you can concatenate
the serialized tables into one file, and flex will find the correct set
of tables, using the scanner prefix as part of the lookup key. An
example follows:


     $ flex --tables-file --prefix=cpp cpp.l
     $ flex --tables-file --prefix=c   c.l
     $ cat lex.cpp.tables lex.c.tables  >  all.tables

   The above example created two scanners, `cpp', and `c'. Since we did
not specify a filename, the tables were serialized to `lex.c.tables' and
`lex.cpp.tables', respectively. Then, we concatenated the two files
together into `all.tables', which we will distribute with our project.
At runtime, we will open the file and tell flex to load the tables from
it.  Flex will find the correct tables automatically. (See next
section).

\x1f
File: flex.info,  Node: Loading and Unloading Serialized Tables,  Next: Tables File Format,  Prev: Creating Serialized Tables,  Up: Serialized Tables

22.2 Loading and Unloading Serialized Tables
============================================

If you've built your scanner with `%option tables-file', then you must
load the scanner tables at runtime. This can be accomplished with the
following function:

 -- Function: int yytables_fload (FILE* FP [, yyscan_t SCANNER])
     Locates scanner tables in the stream pointed to by FP and loads
     them.  Memory for the tables is allocated via `yyalloc'.  You must
     call this function before the first call to `yylex'. The argument
     SCANNER only appears in the reentrant scanner.  This function
     returns `0' (zero) on success, or non-zero on error.

   The loaded tables are *not* automatically destroyed (unloaded) when
you call `yylex_destroy'. The reason is that you may create several
scanners of the same type (in a reentrant scanner), each of which needs
access to these tables.  To avoid a nasty memory leak, you must call
the following function:

 -- Function: int yytables_destroy ([yyscan_t SCANNER])
     Unloads the scanner tables. The tables must be loaded again before
     you can scan any more data.  The argument SCANNER only appears in
     the reentrant scanner.  This function returns `0' (zero) on
     success, or non-zero on error.

   *The functions `yytables_fload' and `yytables_destroy' are not
thread-safe.* You must ensure that these functions are called exactly
once (for each scanner type) in a threaded program, before any thread
calls `yylex'.  After the tables are loaded, they are never written to,
and no thread protection is required thereafter - until you destroy
them.

\x1f
File: flex.info,  Node: Tables File Format,  Prev: Loading and Unloading Serialized Tables,  Up: Serialized Tables

22.3 Tables File Format
=======================

This section defines the file format of serialized `flex' tables.

   The tables format allows for one or more sets of tables to be
specified, where each set corresponds to a given scanner. Scanners are
indexed by name, as described below. The file format is as follows:


                      TABLE SET 1
                     +-------------------------------+
             Header  | uint32          th_magic;     |
                     | uint32          th_hsize;     |
                     | uint32          th_ssize;     |
                     | uint16          th_flags;     |
                     | char            th_version[]; |
                     | char            th_name[];    |
                     | uint8           th_pad64[];   |
                     +-------------------------------+
             Table 1 | uint16          td_id;        |
                     | uint16          td_flags;     |
                     | uint32          td_lolen;     |
                     | uint32          td_hilen;     |
                     | void            td_data[];    |
                     | uint8           td_pad64[];   |
                     +-------------------------------+
             Table 2 |                               |
                .    .                               .
                .    .                               .
                .    .                               .
                .    .                               .
             Table n |                               |
                     +-------------------------------+
                      TABLE SET 2
                           .
                           .
                           .
                      TABLE SET N

   The above diagram shows that a complete set of tables consists of a
header followed by multiple individual tables. Furthermore, multiple
complete sets may be present in the same file, each set with its own
header and tables. The sets are contiguous in the file. The only way to
know if another set follows is to check the next four bytes for the
magic number (or check for EOF). The header and tables sections are
padded to 64-bit boundaries. Below we describe each field in detail.
This format does not specify how the scanner will expand the given
data, i.e., data may be serialized as int8, but expanded to an int32
array at runtime. This is to reduce the size of the serialized data
where possible.  Remember, _all integer values are in network byte
order_.

Fields of a table header:

`th_magic'
     Magic number, always 0xF13C57B1.

`th_hsize'
     Size of this entire header, in bytes, including all fields plus
     any padding.

`th_ssize'
     Size of this entire set, in bytes, including the header, all
     tables, plus any padding.

`th_flags'
     Bit flags for this table set. Currently unused.

`th_version[]'
     Flex version in NULL-terminated string format. e.g., `2.5.13a'.
     This is the version of flex that was used to create the serialized
     tables.

`th_name[]'
     Contains the name of this table set. The default is `yytables',
     and is prefixed accordingly, e.g., `footables'. Must be
     NULL-terminated.

`th_pad64[]'
     Zero or more NULL bytes, padding the entire header to the next
     64-bit boundary as calculated from the beginning of the header.

Fields of a table:

`td_id'
     Specifies the table identifier. Possible values are:
    `YYTD_ID_ACCEPT (0x01)'
          `yy_accept'

    `YYTD_ID_BASE   (0x02)'
          `yy_base'

    `YYTD_ID_CHK    (0x03)'
          `yy_chk'

    `YYTD_ID_DEF    (0x04)'
          `yy_def'

    `YYTD_ID_EC     (0x05)'
          `yy_ec '

    `YYTD_ID_META   (0x06)'
          `yy_meta'

    `YYTD_ID_NUL_TRANS (0x07)'
          `yy_NUL_trans'

    `YYTD_ID_NXT (0x08)'
          `yy_nxt'. This array may be two dimensional. See the
          `td_hilen' field below.

    `YYTD_ID_RULE_CAN_MATCH_EOL (0x09)'
          `yy_rule_can_match_eol'

    `YYTD_ID_START_STATE_LIST (0x0A)'
          `yy_start_state_list'. This array is handled specially
          because it is an array of pointers to structs. See the
          `td_flags' field below.

    `YYTD_ID_TRANSITION (0x0B)'
          `yy_transition'. This array is handled specially because it
          is an array of structs. See the `td_lolen' field below.

    `YYTD_ID_ACCLIST (0x0C)'
          `yy_acclist'

`td_flags'
     Bit flags describing how to interpret the data in `td_data'.  The
     data arrays are one-dimensional by default, but may be two
     dimensional as specified in the `td_hilen' field.

    `YYTD_DATA8 (0x01)'
          The data is serialized as an array of type int8.

    `YYTD_DATA16 (0x02)'
          The data is serialized as an array of type int16.

    `YYTD_DATA32 (0x04)'
          The data is serialized as an array of type int32.

    `YYTD_PTRANS (0x08)'
          The data is a list of indexes of entries in the expanded
          `yy_transition' array.  Each index should be expanded to a
          pointer to the corresponding entry in the `yy_transition'
          array. We count on the fact that the `yy_transition' array
          has already been seen.

    `YYTD_STRUCT (0x10)'
          The data is a list of yy_trans_info structs, each of which
          consists of two integers. There is no padding between struct
          elements or between structs.  The type of each member is
          determined by the `YYTD_DATA*' bits.

`td_lolen'
     Specifies the number of elements in the lowest dimension array. If
     this is a one-dimensional array, then it is simply the number of
     elements in this array.  The element size is determined by the
     `td_flags' field.

`td_hilen'
     If `td_hilen' is non-zero, then the data is a two-dimensional
     array.  Otherwise, the data is a one-dimensional array. `td_hilen'
     contains the number of elements in the higher dimensional array,
     and `td_lolen' contains the number of elements in the lowest
     dimension.

     Conceptually, `td_data' is either `sometype td_data[td_lolen]', or
     `sometype td_data[td_hilen][td_lolen]', where `sometype' is
     specified by the `td_flags' field.  It is possible for both
     `td_lolen' and `td_hilen' to be zero, in which case `td_data' is a
     zero length array, and no data is loaded, i.e., this table is
     simply skipped. Flex does not currently generate tables of zero
     length.

`td_data[]'
     The table data. This array may be a one- or two-dimensional array,
     of type `int8', `int16', `int32', `struct yy_trans_info', or
     `struct yy_trans_info*',  depending upon the values in the
     `td_flags', `td_lolen', and `td_hilen' fields.

`td_pad64[]'
     Zero or more NULL bytes, padding the entire table to the next
     64-bit boundary as calculated from the beginning of this table.

\x1f
File: flex.info,  Node: Diagnostics,  Next: Limitations,  Prev: Serialized Tables,  Up: Top

23 Diagnostics
**************

The following is a list of `flex' diagnostic messages:

   * `warning, rule cannot be matched' indicates that the given rule
     cannot be matched because it follows other rules that will always
     match the same text as it.  For example, in the following `foo'
     cannot be matched because it comes after an identifier "catch-all"
     rule:


              [a-z]+    got_identifier();
              foo       got_foo();

     Using `REJECT' in a scanner suppresses this warning.

   * `warning, -s option given but default rule can be matched' means
     that it is possible (perhaps only in a particular start condition)
     that the default rule (match any single character) is the only one
     that will match a particular input.  Since `-s' was given,
     presumably this is not intended.

   * `reject_used_but_not_detected undefined' or
     `yymore_used_but_not_detected undefined'. These errors can occur
     at compile time.  They indicate that the scanner uses `REJECT' or
     `yymore()' but that `flex' failed to notice the fact, meaning that
     `flex' scanned the first two sections looking for occurrences of
     these actions and failed to find any, but somehow you snuck some in
     (via a #include file, for example).  Use `%option reject' or
     `%option yymore' to indicate to `flex' that you really do use
     these features.

   * `flex scanner jammed'. a scanner compiled with `-s' has
     encountered an input string which wasn't matched by any of its
     rules.  This error can also occur due to internal problems.

   * `token too large, exceeds YYLMAX'. your scanner uses `%array' and
     one of its rules matched a string longer than the `YYLMAX'
     constant (8K bytes by default).  You can increase the value by
     #define'ing `YYLMAX' in the definitions section of your `flex'
     input.

   * `scanner requires -8 flag to use the character 'x''. Your scanner
     specification includes recognizing the 8-bit character `'x'' and
     you did not specify the -8 flag, and your scanner defaulted to
     7-bit because you used the `-Cf' or `-CF' table compression
     options.  See the discussion of the `-7' flag, *Note Scanner
     Options::, for details.

   * `flex scanner push-back overflow'. you used `unput()' to push back
     so much text that the scanner's buffer could not hold both the
     pushed-back text and the current token in `yytext'.  Ideally the
     scanner should dynamically resize the buffer in this case, but at
     present it does not.

   * `input buffer overflow, can't enlarge buffer because scanner uses
     REJECT'.  the scanner was working on matching an extremely large
     token and needed to expand the input buffer.  This doesn't work
     with scanners that use `REJECT'.

   * `fatal flex scanner internal error--end of buffer missed'. This can
     occur in a scanner which is reentered after a long-jump has jumped
     out (or over) the scanner's activation frame.  Before reentering
     the scanner, use:

              yyrestart( yyin );
     or, as noted above, switch to using the C++ scanner class.

   * `too many start conditions in <> construct!'  you listed more start
     conditions in a <> construct than exist (so you must have listed at
     least one of them twice).

\x1f
File: flex.info,  Node: Limitations,  Next: Bibliography,  Prev: Diagnostics,  Up: Top

24 Limitations
**************

Some trailing context patterns cannot be properly matched and generate
warning messages (`dangerous trailing context').  These are patterns
where the ending of the first part of the rule matches the beginning of
the second part, such as `zx*/xy*', where the 'x*' matches the 'x' at
the beginning of the trailing context.  (Note that the POSIX draft
states that the text matched by such patterns is undefined.)  For some
trailing context rules, parts which are actually fixed-length are not
recognized as such, leading to the abovementioned performance loss.  In
particular, parts using `|' or `{n}' (such as `foo{3}') are always
considered variable-length.  Combining trailing context with the
special `|' action can result in _fixed_ trailing context being turned
into the more expensive _variable_ trailing context.  For example, in
the following:


         %%
         abc      |
         xyz/def

   Use of `unput()' invalidates yytext and yyleng, unless the `%array'
directive or the `-l' option has been used.  Pattern-matching of `NUL's
is substantially slower than matching other characters.  Dynamic
resizing of the input buffer is slow, as it entails rescanning all the
text matched so far by the current (generally huge) token.  Due to both
buffering of input and read-ahead, you cannot intermix calls to
`<stdio.h>' routines, such as, getchar(), with `flex' rules and expect
it to work.  Call `input()' instead.  The total table entries listed by
the `-v' flag excludes the number of table entries needed to determine
what rule has been matched.  The number of entries is equal to the
number of DFA states if the scanner does not use `REJECT', and somewhat
greater than the number of states if it does.  `REJECT' cannot be used
with the `-f' or `-F' options.

   The `flex' internal algorithms need documentation.

\x1f
File: flex.info,  Node: Bibliography,  Next: FAQ,  Prev: Limitations,  Up: Top

25 Additional Reading
*********************

You may wish to read more about the following programs:
   * lex

   * yacc

   * sed

   * awk

   The following books may contain material of interest:

   John Levine, Tony Mason, and Doug Brown, _Lex & Yacc_, O'Reilly and
Associates.  Be sure to get the 2nd edition.

   M. E. Lesk and E. Schmidt, _LEX - Lexical Analyzer Generator_

   Alfred Aho, Ravi Sethi and Jeffrey Ullman, _Compilers: Principles,
Techniques and Tools_, Addison-Wesley (1986).  Describes the
pattern-matching techniques used by `flex' (deterministic finite
automata).

\x1f
File: flex.info,  Node: FAQ,  Next: Appendices,  Prev: Bibliography,  Up: Top

FAQ
***

From time to time, the `flex' maintainer receives certain questions.
Rather than repeat answers to well-understood problems, we publish them
here.

* Menu:

* When was flex born?::
* How do I expand backslash-escape sequences in C-style quoted strings?::
* Why do flex scanners call fileno if it is not ANSI compatible?::
* Does flex support recursive pattern definitions?::
* How do I skip huge chunks of input (tens of megabytes) while using flex?::
* Flex is not matching my patterns in the same order that I defined them.::
* My actions are executing out of order or sometimes not at all.::
* How can I have multiple input sources feed into the same scanner at the same time?::
* Can I build nested parsers that work with the same input file?::
* How can I match text only at the end of a file?::
* How can I make REJECT cascade across start condition boundaries?::
* Why cant I use fast or full tables with interactive mode?::
* How much faster is -F or -f than -C?::
* If I have a simple grammar cant I just parse it with flex?::
* Why doesn't yyrestart() set the start state back to INITIAL?::
* How can I match C-style comments?::
* The period isn't working the way I expected.::
* Can I get the flex manual in another format?::
* Does there exist a "faster" NDFA->DFA algorithm?::
* How does flex compile the DFA so quickly?::
* How can I use more than 8192 rules?::
* How do I abandon a file in the middle of a scan and switch to a new file?::
* How do I execute code only during initialization (only before the first scan)?::
* How do I execute code at termination?::
* Where else can I find help?::
* Can I include comments in the "rules" section of the file?::
* I get an error about undefined yywrap().::
* How can I change the matching pattern at run time?::
* How can I expand macros in the input?::
* How can I build a two-pass scanner?::
* How do I match any string not matched in the preceding rules?::
* I am trying to port code from AT&T lex that uses yysptr and yysbuf.::
* Is there a way to make flex treat NULL like a regular character?::
* Whenever flex can not match the input it says "flex scanner jammed".::
* Why doesn't flex have non-greedy operators like perl does?::
* Memory leak - 16386 bytes allocated by malloc.::
* How do I track the byte offset for lseek()?::
* How do I use my own I/O classes in a C++ scanner?::
* How do I skip as many chars as possible?::
* deleteme00::
* Are certain equivalent patterns faster than others?::
* Is backing up a big deal?::
* Can I fake multi-byte character support?::
* deleteme01::
* Can you discuss some flex internals?::
* unput() messes up yy_at_bol::
* The | operator is not doing what I want::
* Why can't flex understand this variable trailing context pattern?::
* The ^ operator isn't working::
* Trailing context is getting confused with trailing optional patterns::
* Is flex GNU or not?::
* ERASEME53::
* I need to scan if-then-else blocks and while loops::
* ERASEME55::
* ERASEME56::
* ERASEME57::
* Is there a repository for flex scanners?::
* How can I conditionally compile or preprocess my flex input file?::
* Where can I find grammars for lex and yacc?::
* I get an end-of-buffer message for each character scanned.::
* unnamed-faq-62::
* unnamed-faq-63::
* unnamed-faq-64::
* unnamed-faq-65::
* unnamed-faq-66::
* unnamed-faq-67::
* unnamed-faq-68::
* unnamed-faq-69::
* unnamed-faq-70::
* unnamed-faq-71::
* unnamed-faq-72::
* unnamed-faq-73::
* unnamed-faq-74::
* unnamed-faq-75::
* unnamed-faq-76::
* unnamed-faq-77::
* unnamed-faq-78::
* unnamed-faq-79::
* unnamed-faq-80::
* unnamed-faq-81::
* unnamed-faq-82::
* unnamed-faq-83::
* unnamed-faq-84::
* unnamed-faq-85::
* unnamed-faq-86::
* unnamed-faq-87::
* unnamed-faq-88::
* unnamed-faq-90::
* unnamed-faq-91::
* unnamed-faq-92::
* unnamed-faq-93::
* unnamed-faq-94::
* unnamed-faq-95::
* unnamed-faq-96::
* unnamed-faq-97::
* unnamed-faq-98::
* unnamed-faq-99::
* unnamed-faq-100::
* unnamed-faq-101::
* What is the difference between YYLEX_PARAM and YY_DECL?::
* Why do I get "conflicting types for yylex" error?::
* How do I access the values set in a Flex action from within a Bison action?::

\x1f
File: flex.info,  Node: When was flex born?,  Next: How do I expand backslash-escape sequences in C-style quoted strings?,  Up: FAQ

When was flex born?
===================

Vern Paxson took over the `Software Tools' lex project from Jef
Poskanzer in 1982.  At that point it was written in Ratfor.  Around
1987 or so, Paxson translated it into C, and a legend was born :-).

\x1f
File: flex.info,  Node: How do I expand backslash-escape sequences in C-style quoted strings?,  Next: Why do flex scanners call fileno if it is not ANSI compatible?,  Prev: When was flex born?,  Up: FAQ

How do I expand backslash-escape sequences in C-style quoted strings?
=====================================================================

A key point when scanning quoted strings is that you cannot (easily)
write a single rule that will precisely match the string if you allow
things like embedded escape sequences and newlines.  If you try to
match strings with a single rule then you'll wind up having to rescan
the string anyway to find any escape sequences.

   Instead you can use exclusive start conditions and a set of rules,
one for matching non-escaped text, one for matching a single escape,
one for matching an embedded newline, and one for recognizing the end
of the string.  Each of these rules is then faced with the question of
where to put its intermediary results.  The best solution is for the
rules to append their local value of `yytext' to the end of a "string
literal" buffer.  A rule like the escape-matcher will append to the
buffer the meaning of the escape sequence rather than the literal text
in `yytext'.  In this way, `yytext' does not need to be modified at all.

\x1f
File: flex.info,  Node: Why do flex scanners call fileno if it is not ANSI compatible?,  Next: Does flex support recursive pattern definitions?,  Prev: How do I expand backslash-escape sequences in C-style quoted strings?,  Up: FAQ

Why do flex scanners call fileno if it is not ANSI compatible?
==============================================================

Flex scanners call `fileno()' in order to get the file descriptor
corresponding to `yyin'. The file descriptor may be passed to
`isatty()' or `read()', depending upon which `%options' you specified.
If your system does not have `fileno()' support, to get rid of the
`read()' call, do not specify `%option read'. To get rid of the
`isatty()' call, you must specify one of `%option always-interactive' or
`%option never-interactive'.

\x1f
File: flex.info,  Node: Does flex support recursive pattern definitions?,  Next: How do I skip huge chunks of input (tens of megabytes) while using flex?,  Prev: Why do flex scanners call fileno if it is not ANSI compatible?,  Up: FAQ

Does flex support recursive pattern definitions?
================================================

e.g.,


     %%
     block   "{"({block}|{statement})*"}"

   No. You cannot have recursive definitions.  The pattern-matching
power of regular expressions in general (and therefore flex scanners,
too) is limited.  In particular, regular expressions cannot "balance"
parentheses to an arbitrary degree.  For example, it's impossible to
write a regular expression that matches all strings containing the same
number of '{'s as '}'s.  For more powerful pattern matching, you need a
parser, such as `GNU bison'.

\x1f
File: flex.info,  Node: How do I skip huge chunks of input (tens of megabytes) while using flex?,  Next: Flex is not matching my patterns in the same order that I defined them.,  Prev: Does flex support recursive pattern definitions?,  Up: FAQ

How do I skip huge chunks of input (tens of megabytes) while using flex?
========================================================================

Use `fseek()' (or `lseek()') to position yyin, then call `yyrestart()'.

\x1f
File: flex.info,  Node: Flex is not matching my patterns in the same order that I defined them.,  Next: My actions are executing out of order or sometimes not at all.,  Prev: How do I skip huge chunks of input (tens of megabytes) while using flex?,  Up: FAQ

Flex is not matching my patterns in the same order that I defined them.
=======================================================================

`flex' picks the rule that matches the most text (i.e., the longest
possible input string).  This is because `flex' uses an entirely
different matching technique ("deterministic finite automata") that
actually does all of the matching simultaneously, in parallel.  (Seems
impossible, but it's actually a fairly simple technique once you
understand the principles.)

   A side-effect of this parallel matching is that when the input
matches more than one rule, `flex' scanners pick the rule that matched
the _most_ text. This is explained further in the manual, in the
section *Note Matching::.

   If you want `flex' to choose a shorter match, then you can work
around this behavior by expanding your short rule to match more text,
then put back the extra:


     data_.*        yyless( 5 ); BEGIN BLOCKIDSTATE;

   Another fix would be to make the second rule active only during the
`<BLOCKIDSTATE>' start condition, and make that start condition
exclusive by declaring it with `%x' instead of `%s'.

   A final fix is to change the input language so that the ambiguity for
`data_' is removed, by adding characters to it that don't match the
identifier rule, or by removing characters (such as `_') from the
identifier rule so it no longer matches `data_'.  (Of course, you might
also not have the option of changing the input language.)

\x1f
File: flex.info,  Node: My actions are executing out of order or sometimes not at all.,  Next: How can I have multiple input sources feed into the same scanner at the same time?,  Prev: Flex is not matching my patterns in the same order that I defined them.,  Up: FAQ

My actions are executing out of order or sometimes not at all.
==============================================================

Most likely, you have (in error) placed the opening `{' of the action
block on a different line than the rule, e.g.,


     ^(foo|bar)
     {  <<<--- WRONG!

     }

   `flex' requires that the opening `{' of an action associated with a
rule begin on the same line as does the rule.  You need instead to
write your rules as follows:


     ^(foo|bar)   {  // CORRECT!

     }

\x1f
File: flex.info,  Node: How can I have multiple input sources feed into the same scanner at the same time?,  Next: Can I build nested parsers that work with the same input file?,  Prev: My actions are executing out of order or sometimes not at all.,  Up: FAQ

How can I have multiple input sources feed into the same scanner at the same time?
==================================================================================

If ...
   * your scanner is free of backtracking (verified using `flex''s `-b'
     flag),

   * AND you run your scanner interactively (`-I' option; default
     unless using special table compression options),

   * AND you feed it one character at a time by redefining `YY_INPUT'
     to do so,

   then every time it matches a token, it will have exhausted its input
buffer (because the scanner is free of backtracking).  This means you
can safely use `select()' at the point and only call `yylex()' for
another token if `select()' indicates there's data available.

   That is, move the `select()' out from the input function to a point
where it determines whether `yylex()' gets called for the next token.

   With this approach, you will still have problems if your input can
arrive piecemeal; `select()' could inform you that the beginning of a
token is available, you call `yylex()' to get it, but it winds up
blocking waiting for the later characters in the token.

   Here's another way:  Move your input multiplexing inside of
`YY_INPUT'.  That is, whenever `YY_INPUT' is called, it `select()''s to
see where input is available.  If input is available for the scanner,
it reads and returns the next byte.  If input is available from another
source, it calls whatever function is responsible for reading from that
source.  (If no input is available, it blocks until some input is
available.)  I've used this technique in an interpreter I wrote that
both reads keyboard input using a `flex' scanner and IPC traffic from
sockets, and it works fine.

\x1f
File: flex.info,  Node: Can I build nested parsers that work with the same input file?,  Next: How can I match text only at the end of a file?,  Prev: How can I have multiple input sources feed into the same scanner at the same time?,  Up: FAQ

Can I build nested parsers that work with the same input file?
==============================================================

This is not going to work without some additional effort.  The reason is
that `flex' block-buffers the input it reads from `yyin'.  This means
that the "outermost" `yylex()', when called, will automatically slurp
up the first 8K of input available on yyin, and subsequent calls to
other `yylex()''s won't see that input.  You might be tempted to work
around this problem by redefining `YY_INPUT' to only return a small
amount of text, but it turns out that that approach is quite difficult.
Instead, the best solution is to combine all of your scanners into one
large scanner, using a different exclusive start condition for each.

\x1f
File: flex.info,  Node: How can I match text only at the end of a file?,  Next: How can I make REJECT cascade across start condition boundaries?,  Prev: Can I build nested parsers that work with the same input file?,  Up: FAQ

How can I match text only at the end of a file?
===============================================

There is no way to write a rule which is "match this text, but only if
it comes at the end of the file".  You can fake it, though, if you
happen to have a character lying around that you don't allow in your
input.  Then you redefine `YY_INPUT' to call your own routine which, if
it sees an `EOF', returns the magic character first (and remembers to
return a real `EOF' next time it's called).  Then you could write:


     <COMMENT>(.|\n)*{EOF_CHAR}    /* saw comment at EOF */

\x1f
File: flex.info,  Node: How can I make REJECT cascade across start condition boundaries?,  Next: Why cant I use fast or full tables with interactive mode?,  Prev: How can I match text only at the end of a file?,  Up: FAQ

How can I make REJECT cascade across start condition boundaries?
================================================================

You can do this as follows.  Suppose you have a start condition `A', and
after exhausting all of the possible matches in `<A>', you want to try
matches in `<INITIAL>'.  Then you could use the following:


     %x A
     %%
     <A>rule_that_is_long    ...; REJECT;
     <A>rule                 ...; REJECT; /* shorter rule */
     <A>etc.
     ...
     <A>.|\n  {
     /* Shortest and last rule in <A>, so
     * cascaded REJECTs will eventually
     * wind up matching this rule.  We want
     * to now switch to the initial state
     * and try matching from there instead.
     */
     yyless(0);    /* put back matched text */
     BEGIN(INITIAL);
     }

\x1f
File: flex.info,  Node: Why cant I use fast or full tables with interactive mode?,  Next: How much faster is -F or -f than -C?,  Prev: How can I make REJECT cascade across start condition boundaries?,  Up: FAQ

Why can't I use fast or full tables with interactive mode?
==========================================================

One of the assumptions flex makes is that interactive applications are
inherently slow (they're waiting on a human after all).  It has to do
with how the scanner detects that it must be finished scanning a token.
For interactive scanners, after scanning each character the current
state is looked up in a table (essentially) to see whether there's a
chance of another input character possibly extending the length of the
match.  If not, the scanner halts.  For non-interactive scanners, the
end-of-token test is much simpler, basically a compare with 0, so no
memory bus cycles.  Since the test occurs in the innermost scanning
loop, one would like to make it go as fast as possible.

   Still, it seems reasonable to allow the user to choose to trade off
a bit of performance in this area to gain the corresponding
flexibility.  There might be another reason, though, why fast scanners
don't support the interactive option.

\x1f
File: flex.info,  Node: How much faster is -F or -f than -C?,  Next: If I have a simple grammar cant I just parse it with flex?,  Prev: Why cant I use fast or full tables with interactive mode?,  Up: FAQ

How much faster is -F or -f than -C?
====================================

Much faster (factor of 2-3).

\x1f
File: flex.info,  Node: If I have a simple grammar cant I just parse it with flex?,  Next: Why doesn't yyrestart() set the start state back to INITIAL?,  Prev: How much faster is -F or -f than -C?,  Up: FAQ

If I have a simple grammar can't I just parse it with flex?
===========================================================

Is your grammar recursive? That's almost always a sign that you're
better off using a parser/scanner rather than just trying to use a
scanner alone.

\x1f
File: flex.info,  Node: Why doesn't yyrestart() set the start state back to INITIAL?,  Next: How can I match C-style comments?,  Prev: If I have a simple grammar cant I just parse it with flex?,  Up: FAQ

Why doesn't yyrestart() set the start state back to INITIAL?
============================================================

There are two reasons.  The first is that there might be programs that
rely on the start state not changing across file changes.  The second
is that beginning with `flex' version 2.4, use of `yyrestart()' is no
longer required, so fixing the problem there doesn't solve the more
general problem.

\x1f
File: flex.info,  Node: How can I match C-style comments?,  Next: The period isn't working the way I expected.,  Prev: Why doesn't yyrestart() set the start state back to INITIAL?,  Up: FAQ

How can I match C-style comments?
=================================

You might be tempted to try something like this:


     "/*".*"*/"       // WRONG!

   or, worse, this:


     "/*"(.|\n)"*/"   // WRONG!

   The above rules will eat too much input, and blow up on things like:


     /* a comment */ do_my_thing( "oops */" );

   Here is one way which allows you to track line information:


     <INITIAL>{
     "/*"              BEGIN(IN_COMMENT);
     }
     <IN_COMMENT>{
     "*/"      BEGIN(INITIAL);
     [^*\n]+   // eat comment in chunks
     "*"       // eat the lone star
     \n        yylineno++;
     }

\x1f
File: flex.info,  Node: The period isn't working the way I expected.,  Next: Can I get the flex manual in another format?,  Prev: How can I match C-style comments?,  Up: FAQ

The '.' isn't working the way I expected.
=========================================

Here are some tips for using `.':

   * A common mistake is to place the grouping parenthesis AFTER an
     operator, when you really meant to place the parenthesis BEFORE
     the operator, e.g., you probably want this `(foo|bar)+' and NOT
     this `(foo|bar+)'.

     The first pattern matches the words `foo' or `bar' any number of
     times, e.g., it matches the text `barfoofoobarfoo'. The second
     pattern matches a single instance of `foo' or a single instance of
     `bar' followed by one or more `r's, e.g., it matches the text
     `barrrr' .

   * A `.' inside `[]''s just means a literal`.' (period), and NOT "any
     character except newline".

   * Remember that `.' matches any character EXCEPT `\n' (and `EOF').
     If you really want to match ANY character, including newlines,
     then use `(.|\n)' Beware that the regex `(.|\n)+' will match your
     entire input!

   * Finally, if you want to match a literal `.' (a period), then use
     `[.]' or `"."'

\x1f
File: flex.info,  Node: Can I get the flex manual in another format?,  Next: Does there exist a "faster" NDFA->DFA algorithm?,  Prev: The period isn't working the way I expected.,  Up: FAQ

Can I get the flex manual in another format?
============================================

The `flex' source distribution  includes a texinfo manual. You are free
to convert that texinfo into whatever format you desire. The `texinfo'
package includes tools for conversion to a number of formats.

\x1f
File: flex.info,  Node: Does there exist a "faster" NDFA->DFA algorithm?,  Next: How does flex compile the DFA so quickly?,  Prev: Can I get the flex manual in another format?,  Up: FAQ

Does there exist a "faster" NDFA->DFA algorithm?
================================================

There's no way around the potential exponential running time - it can
take you exponential time just to enumerate all of the DFA states.  In
practice, though, the running time is closer to linear, or sometimes
quadratic.

\x1f
File: flex.info,  Node: How does flex compile the DFA so quickly?,  Next: How can I use more than 8192 rules?,  Prev: Does there exist a "faster" NDFA->DFA algorithm?,  Up: FAQ

How does flex compile the DFA so quickly?
=========================================

There are two big speed wins that `flex' uses:

  1. It analyzes the input rules to construct equivalence classes for
     those characters that always make the same transitions.  It then
     rewrites the NFA using equivalence classes for transitions instead
     of characters.  This cuts down the NFA->DFA computation time
     dramatically, to the point where, for uncompressed DFA tables, the
     DFA generation is often I/O bound in writing out the tables.

  2. It maintains hash values for previously computed DFA states, so
     testing whether a newly constructed DFA state is equivalent to a
     previously constructed state can be done very quickly, by first
     comparing hash values.

\x1f
File: flex.info,  Node: How can I use more than 8192 rules?,  Next: How do I abandon a file in the middle of a scan and switch to a new file?,  Prev: How does flex compile the DFA so quickly?,  Up: FAQ

How can I use more than 8192 rules?
===================================

`Flex' is compiled with an upper limit of 8192 rules per scanner.  If
you need more than 8192 rules in your scanner, you'll have to recompile
`flex' with the following changes in `flexdef.h':


     <    #define YY_TRAILING_MASK 0x2000
     <    #define YY_TRAILING_HEAD_MASK 0x4000
     --
     >    #define YY_TRAILING_MASK 0x20000000
     >    #define YY_TRAILING_HEAD_MASK 0x40000000

   This should work okay as long as your C compiler uses 32 bit
integers.  But you might want to think about whether using such a huge
number of rules is the best way to solve your problem.

   The following may also be relevant:

   With luck, you should be able to increase the definitions in
flexdef.h for:


     #define JAMSTATE -32766 /* marks a reference to the state that always jams */
     #define MAXIMUM_MNS 31999
     #define BAD_SUBSCRIPT -32767

   recompile everything, and it'll all work.  Flex only has these
16-bit-like values built into it because a long time ago it was
developed on a machine with 16-bit ints.  I've given this advice to
others in the past but haven't heard back from them whether it worked
okay or not...

\x1f
File: flex.info,  Node: How do I abandon a file in the middle of a scan and switch to a new file?,  Next: How do I execute code only during initialization (only before the first scan)?,  Prev: How can I use more than 8192 rules?,  Up: FAQ

How do I abandon a file in the middle of a scan and switch to a new file?
=========================================================================

Just call `yyrestart(newfile)'. Be sure to reset the start state if you
want a "fresh start, since `yyrestart' does NOT reset the start state
back to `INITIAL'.

\x1f
File: flex.info,  Node: How do I execute code only during initialization (only before the first scan)?,  Next: How do I execute code at termination?,  Prev: How do I abandon a file in the middle of a scan and switch to a new file?,  Up: FAQ

How do I execute code only during initialization (only before the first scan)?
==============================================================================

You can specify an initial action by defining the macro `YY_USER_INIT'
(though note that `yyout' may not be available at the time this macro
is executed).  Or you can add to the beginning of your rules section:


     %%
         /* Must be indented! */
         static int did_init = 0;

         if ( ! did_init ){
     do_my_init();
             did_init = 1;
         }

\x1f
File: flex.info,  Node: How do I execute code at termination?,  Next: Where else can I find help?,  Prev: How do I execute code only during initialization (only before the first scan)?,  Up: FAQ

How do I execute code at termination?
=====================================

You can specify an action for the `<<EOF>>' rule.

\x1f
File: flex.info,  Node: Where else can I find help?,  Next: Can I include comments in the "rules" section of the file?,  Prev: How do I execute code at termination?,  Up: FAQ

Where else can I find help?
===========================

You can find the flex homepage on the web at
`http://flex.sourceforge.net/'. See that page for details about flex
mailing lists as well.

\x1f
File: flex.info,  Node: Can I include comments in the "rules" section of the file?,  Next: I get an error about undefined yywrap().,  Prev: Where else can I find help?,  Up: FAQ

Can I include comments in the "rules" section of the file?
==========================================================

Yes, just about anywhere you want to. See the manual for the specific
syntax.

\x1f
File: flex.info,  Node: I get an error about undefined yywrap().,  Next: How can I change the matching pattern at run time?,  Prev: Can I include comments in the "rules" section of the file?,  Up: FAQ

I get an error about undefined yywrap().
========================================

You must supply a `yywrap()' function of your own, or link to `libfl.a'
(which provides one), or use


     %option noyywrap

   in your source to say you don't want a `yywrap()' function.

\x1f
File: flex.info,  Node: How can I change the matching pattern at run time?,  Next: How can I expand macros in the input?,  Prev: I get an error about undefined yywrap().,  Up: FAQ

How can I change the matching pattern at run time?
==================================================

You can't, it's compiled into a static table when flex builds the
scanner.

\x1f
File: flex.info,  Node: How can I expand macros in the input?,  Next: How can I build a two-pass scanner?,  Prev: How can I change the matching pattern at run time?,  Up: FAQ

How can I expand macros in the input?
=====================================

The best way to approach this problem is at a higher level, e.g., in
the parser.

   However, you can do this using multiple input buffers.


     %%
     macro/[a-z]+       {
     /* Saw the macro "macro" followed by extra stuff. */
     main_buffer = YY_CURRENT_BUFFER;
     expansion_buffer = yy_scan_string(expand(yytext));
     yy_switch_to_buffer(expansion_buffer);
     }

     <<EOF>>    {
     if ( expansion_buffer )
     {
     // We were doing an expansion, return to where
     // we were.
     yy_switch_to_buffer(main_buffer);
     yy_delete_buffer(expansion_buffer);
     expansion_buffer = 0;
     }
     else
     yyterminate();
     }

   You probably will want a stack of expansion buffers to allow nested
macros.  From the above though hopefully the idea is clear.

\x1f
File: flex.info,  Node: How can I build a two-pass scanner?,  Next: How do I match any string not matched in the preceding rules?,  Prev: How can I expand macros in the input?,  Up: FAQ

How can I build a two-pass scanner?
===================================

One way to do it is to filter the first pass to a temporary file, then
process the temporary file on the second pass. You will probably see a
performance hit, due to all the disk I/O.

   When you need to look ahead far forward like this, it almost always
means that the right solution is to build a parse tree of the entire
input, then walk it after the parse in order to generate the output.
In a sense, this is a two-pass approach, once through the text and once
through the parse tree, but the performance hit for the latter is
usually an order of magnitude smaller, since everything is already
classified, in binary format, and residing in memory.

\x1f
File: flex.info,  Node: How do I match any string not matched in the preceding rules?,  Next: I am trying to port code from AT&T lex that uses yysptr and yysbuf.,  Prev: How can I build a two-pass scanner?,  Up: FAQ

How do I match any string not matched in the preceding rules?
=============================================================

One way to assign precedence, is to place the more specific rules
first. If two rules would match the same input (same sequence of
characters) then the first rule listed in the `flex' input wins, e.g.,


     %%
     foo[a-zA-Z_]+    return FOO_ID;
     bar[a-zA-Z_]+    return BAR_ID;
     [a-zA-Z_]+       return GENERIC_ID;

   Note that the rule `[a-zA-Z_]+' must come *after* the others.  It
will match the same amount of text as the more specific rules, and in
that case the `flex' scanner will pick the first rule listed in your
scanner as the one to match.

\x1f
File: flex.info,  Node: I am trying to port code from AT&T lex that uses yysptr and yysbuf.,  Next: Is there a way to make flex treat NULL like a regular character?,  Prev: How do I match any string not matched in the preceding rules?,  Up: FAQ

I am trying to port code from AT&T lex that uses yysptr and yysbuf.
===================================================================

Those are internal variables pointing into the AT&T scanner's input
buffer.  I imagine they're being manipulated in user versions of the
`input()' and `unput()' functions.  If so, what you need to do is
analyze those functions to figure out what they're doing, and then
replace `input()' with an appropriate definition of `YY_INPUT'.  You
shouldn't need to (and must not) replace `flex''s `unput()' function.

\x1f
File: flex.info,  Node: Is there a way to make flex treat NULL like a regular character?,  Next: Whenever flex can not match the input it says "flex scanner jammed".,  Prev: I am trying to port code from AT&T lex that uses yysptr and yysbuf.,  Up: FAQ

Is there a way to make flex treat NULL like a regular character?
================================================================

Yes, `\0' and `\x00' should both do the trick.  Perhaps you have an
ancient version of `flex'.  The latest release is version 2.5.35.

\x1f
File: flex.info,  Node: Whenever flex can not match the input it says "flex scanner jammed".,  Next: Why doesn't flex have non-greedy operators like perl does?,  Prev: Is there a way to make flex treat NULL like a regular character?,  Up: FAQ

Whenever flex can not match the input it says "flex scanner jammed".
====================================================================

You need to add a rule that matches the otherwise-unmatched text, e.g.,


     %option yylineno
     %%
     [[a bunch of rules here]]

     .  printf("bad input character '%s' at line %d\n", yytext, yylineno);

   See `%option default' for more information.

\x1f
File: flex.info,  Node: Why doesn't flex have non-greedy operators like perl does?,  Next: Memory leak - 16386 bytes allocated by malloc.,  Prev: Whenever flex can not match the input it says "flex scanner jammed".,  Up: FAQ

Why doesn't flex have non-greedy operators like perl does?
==========================================================

A DFA can do a non-greedy match by stopping the first time it enters an
accepting state, instead of consuming input until it determines that no
further matching is possible (a "jam" state).  This is actually easier
to implement than longest leftmost match (which flex does).

   But it's also much less useful than longest leftmost match.  In
general, when you find yourself wishing for non-greedy matching, that's
usually a sign that you're trying to make the scanner do some parsing.
That's generally the wrong approach, since it lacks the power to do a
decent job.  Better is to either introduce a separate parser, or to
split the scanner into multiple scanners using (exclusive) start
conditions.

   You might have a separate start state once you've seen the `BEGIN'.
In that state, you might then have a regex that will match `END' (to
kick you out of the state), and perhaps `(.|\n)' to get a single
character within the chunk ...

   This approach also has much better error-reporting properties.

\x1f
File: flex.info,  Node: Memory leak - 16386 bytes allocated by malloc.,  Next: How do I track the byte offset for lseek()?,  Prev: Why doesn't flex have non-greedy operators like perl does?,  Up: FAQ

Memory leak - 16386 bytes allocated by malloc.
==============================================

UPDATED 2002-07-10: As of `flex' version 2.5.9, this leak means that
you did not call `yylex_destroy()'. If you are using an earlier version
of `flex', then read on.

   The leak is about 16426 bytes.  That is, (8192 * 2 + 2) for the
read-buffer, and about 40 for `struct yy_buffer_state' (depending upon
alignment). The leak is in the non-reentrant C scanner only (NOT in the
reentrant scanner, NOT in the C++ scanner). Since `flex' doesn't know
when you are done, the buffer is never freed.

   However, the leak won't multiply since the buffer is reused no
matter how many times you call `yylex()'.

   If you want to reclaim the memory when you are completely done
scanning, then you might try this:


     /* For non-reentrant C scanner only. */
     yy_delete_buffer(YY_CURRENT_BUFFER);
     yy_init = 1;

   Note: `yy_init' is an "internal variable", and hasn't been tested in
this situation. It is possible that some other globals may need
resetting as well.

\x1f
File: flex.info,  Node: How do I track the byte offset for lseek()?,  Next: How do I use my own I/O classes in a C++ scanner?,  Prev: Memory leak - 16386 bytes allocated by malloc.,  Up: FAQ

How do I track the byte offset for lseek()?
===========================================


     >   We thought that it would be possible to have this number through the
     >   evaluation of the following expression:
     >
     >   seek_position = (no_buffers)*YY_READ_BUF_SIZE + yy_c_buf_p - YY_CURRENT_BUFFER->yy_ch_buf

   While this is the right idea, it has two problems.  The first is that
it's possible that `flex' will request less than `YY_READ_BUF_SIZE'
during an invocation of `YY_INPUT' (or that your input source will
return less even though `YY_READ_BUF_SIZE' bytes were requested).  The
second problem is that when refilling its internal buffer, `flex' keeps
some characters from the previous buffer (because usually it's in the
middle of a match, and needs those characters to construct `yytext' for
the match once it's done).  Because of this, `yy_c_buf_p -
YY_CURRENT_BUFFER->yy_ch_buf' won't be exactly the number of characters
already read from the current buffer.

   An alternative solution is to count the number of characters you've
matched since starting to scan.  This can be done by using
`YY_USER_ACTION'.  For example,


     #define YY_USER_ACTION num_chars += yyleng;

   (You need to be careful to update your bookkeeping if you use
`yymore('), `yyless()', `unput()', or `input()'.)

\x1f
File: flex.info,  Node: How do I use my own I/O classes in a C++ scanner?,  Next: How do I skip as many chars as possible?,  Prev: How do I track the byte offset for lseek()?,  Up: FAQ

How do I use my own I/O classes in a C++ scanner?
=================================================

When the flex C++ scanning class rewrite finally happens, then this
sort of thing should become much easier.

   You can do this by passing the various functions (such as
`LexerInput()' and `LexerOutput()') NULL `iostream*''s, and then
dealing with your own I/O classes surreptitiously (i.e., stashing them
in special member variables).  This works because the only assumption
about the lexer regarding what's done with the iostream's is that
they're ultimately passed to `LexerInput()' and `LexerOutput', which
then do whatever is necessary with them.

\x1f
File: flex.info,  Node: How do I skip as many chars as possible?,  Next: deleteme00,  Prev: How do I use my own I/O classes in a C++ scanner?,  Up: FAQ

How do I skip as many chars as possible?
========================================

How do I skip as many chars as possible - without interfering with the
other patterns?

   In the example below, we want to skip over characters until we see
the phrase "endskip". The following will _NOT_ work correctly (do you
see why not?)


     /* INCORRECT SCANNER */
     %x SKIP
     %%
     <INITIAL>startskip   BEGIN(SKIP);
     ...
     <SKIP>"endskip"       BEGIN(INITIAL);
     <SKIP>.*             ;

   The problem is that the pattern .* will eat up the word "endskip."
The simplest (but slow) fix is:


     <SKIP>"endskip"      BEGIN(INITIAL);
     <SKIP>.              ;

   The fix involves making the second rule match more, without making
it match "endskip" plus something else.  So for example:


     <SKIP>"endskip"     BEGIN(INITIAL);
     <SKIP>[^e]+         ;
     <SKIP>.                    ;/* so you eat up e's, too */

\x1f
File: flex.info,  Node: deleteme00,  Next: Are certain equivalent patterns faster than others?,  Prev: How do I skip as many chars as possible?,  Up: FAQ

deleteme00
==========


     QUESTION:
     When was flex born?

     Vern Paxson took over
     the Software Tools lex project from Jef Poskanzer in 1982.  At that point it
     was written in Ratfor.  Around 1987 or so, Paxson translated it into C, and
     a legend was born :-).

\x1f
File: flex.info,  Node: Are certain equivalent patterns faster than others?,  Next: Is backing up a big deal?,  Prev: deleteme00,  Up: FAQ

Are certain equivalent patterns faster than others?
===================================================


     To: Adoram Rogel <adoram@orna.hybridge.com>
     Subject: Re: Flex 2.5.2 performance questions
     In-reply-to: Your message of Wed, 18 Sep 96 11:12:17 EDT.
     Date: Wed, 18 Sep 96 10:51:02 PDT
     From: Vern Paxson <vern>

     [Note, the most recent flex release is 2.5.4, which you can get from
     ftp.ee.lbl.gov.  It has bug fixes over 2.5.2 and 2.5.3.]

     > 1. Using the pattern
     >    ([Ff](oot)?)?[Nn](ote)?(\.)?
     >    instead of
     >    (((F|f)oot(N|n)ote)|((N|n)ote)|((N|n)\.)|((F|f)(N|n)(\.)))
     >    (in a very complicated flex program) caused the program to slow from
     >    300K+/min to 100K/min (no other changes were done).

     These two are not equivalent.  For example, the first can match "footnote."
     but the second can only match "footnote".  This is almost certainly the
     cause in the discrepancy - the slower scanner run is matching more tokens,
     and/or having to do more backing up.

     > 2. Which of these two are better: [Ff]oot or (F|f)oot ?

     From a performance point of view, they're equivalent (modulo presumably
     minor effects such as memory cache hit rates; and the presence of trailing
     context, see below).  From a space point of view, the first is slightly
     preferable.

     > 3. I have a pattern that look like this:
     >    pats {p1}|{p2}|{p3}|...|{p50}     (50 patterns ORd)
     >
     >    running yet another complicated program that includes the following rule:
     >    <snext>{and}/{no4}{bb}{pats}
     >
     >    gets me to "too complicated - over 32,000 states"...

     I can't tell from this example whether the trailing context is variable-length
     or fixed-length (it could be the latter if {and} is fixed-length).  If it's
     variable length, which flex -p will tell you, then this reflects a basic
     performance problem, and if you can eliminate it by restructuring your
     scanner, you will see significant improvement.

     >    so I divided {pats} to {pats1}, {pats2},..., {pats5} each consists of about
     >    10 patterns and changed the rule to be 5 rules.
     >    This did compile, but what is the rule of thumb here ?

     The rule is to avoid trailing context other than fixed-length, in which for
     a/b, either the 'a' pattern or the 'b' pattern have a fixed length.  Use
     of the '|' operator automatically makes the pattern variable length, so in
     this case '[Ff]oot' is preferred to '(F|f)oot'.

     > 4. I changed a rule that looked like this:
     >    <snext8>{and}{bb}/{ROMAN}[^A-Za-z] { BEGIN...
     >
     >    to the next 2 rules:
     >    <snext8>{and}{bb}/{ROMAN}[A-Za-z] { ECHO;}
     >    <snext8>{and}{bb}/{ROMAN}         { BEGIN...
     >
     >    Again, I understand the using [^...] will cause a great performance loss

     Actually, it doesn't cause any sort of performance loss.  It's a surprising
     fact about regular expressions that they always match in linear time
     regardless of how complex they are.

     >    but are there any specific rules about it ?

     See the "Performance Considerations" section of the man page, and also
     the example in MISC/fastwc/.

                Vern

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File: flex.info,  Node: Is backing up a big deal?,  Next: Can I fake multi-byte character support?,  Prev: Are certain equivalent patterns faster than others?,  Up: FAQ

Is backing up a big deal?
=========================


     To: Adoram Rogel <adoram@hybridge.com>
     Subject: Re: Flex 2.5.2 performance questions
     In-reply-to: Your message of Thu, 19 Sep 96 10:16:04 EDT.
     Date: Thu, 19 Sep 96 09:58:00 PDT
     From: Vern Paxson <vern>

     > a lot about the backing up problem.
     > I believe that there lies my biggest problem, and I'll try to improve
     > it.

     Since you have variable trailing context, this is a bigger performance
     problem.  Fixing it is usually easier than fixing backing up, which in a
     complicated scanner (yours seems to fit the bill) can be extremely
     difficult to do correctly.

     You also don't mention what flags you are using for your scanner.
     -f makes a large speed difference, and -Cfe buys you nearly as much
     speed but the resulting scanner is considerably smaller.

     > I have an | operator in {and} and in {pats} so both of them are variable
     > length.

     -p should have reported this.

     > Is changing one of them to fixed-length is enough ?

     Yes.

     > Is it possible to change the 32,000 states limit ?

     Yes.  I've appended instructions on how.  Before you make this change,
     though, you should think about whether there are ways to fundamentally
     simplify your scanner - those are certainly preferable!

                Vern

     To increase the 32K limit (on a machine with 32 bit integers), you increase
     the magnitude of the following in flexdef.h:

     #define JAMSTATE -32766 /* marks a reference to the state that always jams */
     #define MAXIMUM_MNS 31999
     #define BAD_SUBSCRIPT -32767
     #define MAX_SHORT 32700

     Adding a 0 or two after each should do the trick.

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File: flex.info,  Node: Can I fake multi-byte character support?,  Next: deleteme01,  Prev: Is backing up a big deal?,  Up: FAQ

Can I fake multi-byte character support?
========================================


     To: Heeman_Lee@hp.com
     Subject: Re: flex - multi-byte support?
     In-reply-to: Your message of Thu, 03 Oct 1996 17:24:04 PDT.
     Date: Fri, 04 Oct 1996 11:42:18 PDT
     From: Vern Paxson <vern>

     >      I assume as long as my *.l file defines the
     >      range of expected character code values (in octal format), flex will
     >      scan the file and read multi-byte characters correctly. But I have no
     >      confidence in this assumption.

     Your lack of confidence is justified - this won't work.

     Flex has in it a widespread assumption that the input is processed
     one byte at a time.  Fixing this is on the to-do list, but is involved,
     so it won't happen any time soon.  In the interim, the best I can suggest
     (unless you want to try fixing it yourself) is to write your rules in
     terms of pairs of bytes, using definitions in the first section:

        X       \xfe\xc2
        ...
        %%
        foo{X}bar       found_foo_fe_c2_bar();

     etc.  Definitely a pain - sorry about that.

     By the way, the email address you used for me is ancient, indicating you
     have a very old version of flex.  You can get the most recent, 2.5.4, from
     ftp.ee.lbl.gov.

                Vern

\x1f
File: flex.info,  Node: deleteme01,  Next: Can you discuss some flex internals?,  Prev: Can I fake multi-byte character support?,  Up: FAQ

deleteme01
==========


     To: moleary@primus.com
     Subject: Re: Flex / Unicode compatibility question
     In-reply-to: Your message of Tue, 22 Oct 1996 10:15:42 PDT.
     Date: Tue, 22 Oct 1996 11:06:13 PDT
     From: Vern Paxson <vern>

     Unfortunately flex at the moment has a widespread assumption within it
     that characters are processed 8 bits at a time.  I don't see any easy
     fix for this (other than writing your rules in terms of double characters -
     a pain).  I also don't know of a wider lex, though you might try surfing
     the Plan 9 stuff because I know it's a Unicode system, and also the PCCT
     toolkit (try searching say Alta Vista for "Purdue Compiler Construction
     Toolkit").

     Fixing flex to handle wider characters is on the long-term to-do list.
     But since flex is a strictly spare-time project these days, this probably
     won't happen for quite a while, unless someone else does it first.

                Vern

\x1f
File: flex.info,  Node: Can you discuss some flex internals?,  Next: unput() messes up yy_at_bol,  Prev: deleteme01,  Up: FAQ

Can you discuss some flex internals?
====================================


     To: Johan Linde <jl@theophys.kth.se>
     Subject: Re: translation of flex
     In-reply-to: Your message of Sun, 10 Nov 1996 09:16:36 PST.
     Date: Mon, 11 Nov 1996 10:33:50 PST
     From: Vern Paxson <vern>

     > I'm working for the Swedish team translating GNU program, and I'm currently
     > working with flex. I have a few questions about some of the messages which
     > I hope you can answer.

     All of the things you're wondering about, by the way, concerning flex
     internals - probably the only person who understands what they mean in
     English is me!  So I wouldn't worry too much about getting them right.
     That said ...

     > #: main.c:545
     > msgid "  %d protos created\n"
     >
     > Does proto mean prototype?

     Yes - prototypes of state compression tables.

     > #: main.c:539
     > msgid "  %d/%d (peak %d) template nxt-chk entries created\n"
     >
     > Here I'm mainly puzzled by 'nxt-chk'. I guess it means 'next-check'. (?)
     > However, 'template next-check entries' doesn't make much sense to me. To be
     > able to find a good translation I need to know a little bit more about it.

     There is a scheme in the Aho/Sethi/Ullman compiler book for compressing
     scanner tables.  It involves creating two pairs of tables.  The first has
     "base" and "default" entries, the second has "next" and "check" entries.
     The "base" entry is indexed by the current state and yields an index into
     the next/check table.  The "default" entry gives what to do if the state
     transition isn't found in next/check.  The "next" entry gives the next
     state to enter, but only if the "check" entry verifies that this entry is
     correct for the current state.  Flex creates templates of series of
     next/check entries and then encodes differences from these templates as a
     way to compress the tables.

     > #: main.c:533
     > msgid "  %d/%d base-def entries created\n"
     >
     > The same problem here for 'base-def'.

     See above.

                Vern

\x1f
File: flex.info,  Node: unput() messes up yy_at_bol,  Next: The | operator is not doing what I want,  Prev: Can you discuss some flex internals?,  Up: FAQ

unput() messes up yy_at_bol
===========================


     To: Xinying Li <xli@npac.syr.edu>
     Subject: Re: FLEX ?
     In-reply-to: Your message of Wed, 13 Nov 1996 17:28:38 PST.
     Date: Wed, 13 Nov 1996 19:51:54 PST
     From: Vern Paxson <vern>

     > "unput()" them to input flow, question occurs. If I do this after I scan
     > a carriage, the variable "YY_CURRENT_BUFFER->yy_at_bol" is changed. That
     > means the carriage flag has gone.

     You can control this by calling yy_set_bol().  It's described in the manual.

     >      And if in pre-reading it goes to the end of file, is anything done
     > to control the end of curren buffer and end of file?

     No, there's no way to put back an end-of-file.

     >      By the way I am using flex 2.5.2 and using the "-l".

     The latest release is 2.5.4, by the way.  It fixes some bugs in 2.5.2 and
     2.5.3.  You can get it from ftp.ee.lbl.gov.

                Vern

\x1f
File: flex.info,  Node: The | operator is not doing what I want,  Next: Why can't flex understand this variable trailing context pattern?,  Prev: unput() messes up yy_at_bol,  Up: FAQ

The | operator is not doing what I want
=======================================


     To: Alain.ISSARD@st.com
     Subject: Re: Start condition with FLEX
     In-reply-to: Your message of Mon, 18 Nov 1996 09:45:02 PST.
     Date: Mon, 18 Nov 1996 10:41:34 PST
     From: Vern Paxson <vern>

     > I am not able to use the start condition scope and to use the | (OR) with
     > rules having start conditions.

     The problem is that if you use '|' as a regular expression operator, for
     example "a|b" meaning "match either 'a' or 'b'", then it must *not* have
     any blanks around it.  If you instead want the special '|' *action* (which
     from your scanner appears to be the case), which is a way of giving two
     different rules the same action:

        foo     |
        bar     matched_foo_or_bar();

     then '|' *must* be separated from the first rule by whitespace and *must*
     be followed by a new line.  You *cannot* write it as:

        foo | bar       matched_foo_or_bar();

     even though you might think you could because yacc supports this syntax.
     The reason for this unfortunately incompatibility is historical, but it's
     unlikely to be changed.

     Your problems with start condition scope are simply due to syntax errors
     from your use of '|' later confusing flex.

     Let me know if you still have problems.

                Vern

\x1f
File: flex.info,  Node: Why can't flex understand this variable trailing context pattern?,  Next: The ^ operator isn't working,  Prev: The | operator is not doing what I want,  Up: FAQ

Why can't flex understand this variable trailing context pattern?
=================================================================


     To: Gregory Margo <gmargo@newton.vip.best.com>
     Subject: Re: flex-2.5.3 bug report
     In-reply-to: Your message of Sat, 23 Nov 1996 16:50:09 PST.
     Date: Sat, 23 Nov 1996 17:07:32 PST
     From: Vern Paxson <vern>

     > Enclosed is a lex file that "real" lex will process, but I cannot get
     > flex to process it.  Could you try it and maybe point me in the right direction?

     Your problem is that some of the definitions in the scanner use the '/'
     trailing context operator, and have it enclosed in ()'s.  Flex does not
     allow this operator to be enclosed in ()'s because doing so allows undefined
     regular expressions such as "(a/b)+".  So the solution is to remove the
     parentheses.  Note that you must also be building the scanner with the -l
     option for AT&T lex compatibility.  Without this option, flex automatically
     encloses the definitions in parentheses.

                Vern

\x1f
File: flex.info,  Node: The ^ operator isn't working,  Next: Trailing context is getting confused with trailing optional patterns,  Prev: Why can't flex understand this variable trailing context pattern?,  Up: FAQ

The ^ operator isn't working
============================


     To: Thomas Hadig <hadig@toots.physik.rwth-aachen.de>
     Subject: Re: Flex Bug ?
     In-reply-to: Your message of Tue, 26 Nov 1996 14:35:01 PST.
     Date: Tue, 26 Nov 1996 11:15:05 PST
     From: Vern Paxson <vern>

     > In my lexer code, i have the line :
     > ^\*.*          { }
     >
     > Thus all lines starting with an astrix (*) are comment lines.
     > This does not work !

     I can't get this problem to reproduce - it works fine for me.  Note
     though that if what you have is slightly different:

        COMMENT ^\*.*
        %%
        {COMMENT}       { }

     then it won't work, because flex pushes back macro definitions enclosed
     in ()'s, so the rule becomes

        (^\*.*)         { }

     and now that the '^' operator is not at the immediate beginning of the
     line, it's interpreted as just a regular character.  You can avoid this
     behavior by using the "-l" lex-compatibility flag, or "%option lex-compat".

                Vern

\x1f
File: flex.info,  Node: Trailing context is getting confused with trailing optional patterns,  Next: Is flex GNU or not?,  Prev: The ^ operator isn't working,  Up: FAQ

Trailing context is getting confused with trailing optional patterns
====================================================================


     To: Adoram Rogel <adoram@hybridge.com>
     Subject: Re: Flex 2.5.4 BOF ???
     In-reply-to: Your message of Tue, 26 Nov 1996 16:10:41 PST.
     Date: Wed, 27 Nov 1996 10:56:25 PST
     From: Vern Paxson <vern>

     >     Organization(s)?/[a-z]
     >
     > This matched "Organizations" (looking in debug mode, the trailing s
     > was matched with trailing context instead of the optional (s) in the
     > end of the word.

     That should only happen with lex.  Flex can properly match this pattern.
     (That might be what you're saying, I'm just not sure.)

     > Is there a way to avoid this dangerous trailing context problem ?

     Unfortunately, there's no easy way.  On the other hand, I don't see why
     it should be a problem.  Lex's matching is clearly wrong, and I'd hope
     that usually the intent remains the same as expressed with the pattern,
     so flex's matching will be correct.

                Vern

\x1f
File: flex.info,  Node: Is flex GNU or not?,  Next: ERASEME53,  Prev: Trailing context is getting confused with trailing optional patterns,  Up: FAQ

Is flex GNU or not?
===================


     To: Cameron MacKinnon <mackin@interlog.com>
     Subject: Re: Flex documentation bug
     In-reply-to: Your message of Mon, 02 Dec 1996 00:07:08 PST.
     Date: Sun, 01 Dec 1996 22:29:39 PST
     From: Vern Paxson <vern>

     > I'm not sure how or where to submit bug reports (documentation or
     > otherwise) for the GNU project stuff ...

     Well, strictly speaking flex isn't part of the GNU project.  They just
     distribute it because no one's written a decent GPL'd lex replacement.
     So you should send bugs directly to me.  Those sent to the GNU folks
     sometimes find there way to me, but some may drop between the cracks.

     > In GNU Info, under the section 'Start Conditions', and also in the man
     > page (mine's dated April '95) is a nice little snippet showing how to
     > parse C quoted strings into a buffer, defined to be MAX_STR_CONST in
     > size. Unfortunately, no overflow checking is ever done ...

     This is already mentioned in the manual:

     Finally, here's an example of how to  match  C-style  quoted
     strings using exclusive start conditions, including expanded
     escape sequences (but not including checking  for  a  string
     that's too long):

     The reason for not doing the overflow checking is that it will needlessly
     clutter up an example whose main purpose is just to demonstrate how to
     use flex.

     The latest release is 2.5.4, by the way, available from ftp.ee.lbl.gov.

                Vern

\x1f
File: flex.info,  Node: ERASEME53,  Next: I need to scan if-then-else blocks and while loops,  Prev: Is flex GNU or not?,  Up: FAQ

ERASEME53
=========


     To: tsv@cs.UManitoba.CA
     Subject: Re: Flex (reg)..
     In-reply-to: Your message of Thu, 06 Mar 1997 23:50:16 PST.
     Date: Thu, 06 Mar 1997 15:54:19 PST
     From: Vern Paxson <vern>

     > [:alpha:] ([:alnum:] | \\_)*

     If your rule really has embedded blanks as shown above, then it won't
     work, as the first blank delimits the rule from the action.  (It wouldn't
     even compile ...)  You need instead:

     [:alpha:]([:alnum:]|\\_)*

     and that should work fine - there's no restriction on what can go inside
     of ()'s except for the trailing context operator, '/'.

                Vern

\x1f
File: flex.info,  Node: I need to scan if-then-else blocks and while loops,  Next: ERASEME55,  Prev: ERASEME53,  Up: FAQ

I need to scan if-then-else blocks and while loops
==================================================


     To: "Mike Stolnicki" <mstolnic@ford.com>
     Subject: Re: FLEX help
     In-reply-to: Your message of Fri, 30 May 1997 13:33:27 PDT.
     Date: Fri, 30 May 1997 10:46:35 PDT
     From: Vern Paxson <vern>

     > We'd like to add "if-then-else", "while", and "for" statements to our
     > language ...
     > We've investigated many possible solutions.  The one solution that seems
     > the most reasonable involves knowing the position of a TOKEN in yyin.

     I strongly advise you to instead build a parse tree (abstract syntax tree)
     and loop over that instead.  You'll find this has major benefits in keeping
     your interpreter simple and extensible.

     That said, the functionality you mention for get_position and set_position
     have been on the to-do list for a while.  As flex is a purely spare-time
     project for me, no guarantees when this will be added (in particular, it
     for sure won't be for many months to come).

                Vern

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ERASEME55
=========


     To: Colin Paul Adams <colin@colina.demon.co.uk>
     Subject: Re: Flex C++ classes and Bison
     In-reply-to: Your message of 09 Aug 1997 17:11:41 PDT.
     Date: Fri, 15 Aug 1997 10:48:19 PDT
     From: Vern Paxson <vern>

     > #define YY_DECL   int yylex (YYSTYPE *lvalp, struct parser_control
     > *parm)
     >
     > I have been trying  to get this to work as a C++ scanner, but it does
     > not appear to be possible (warning that it matches no declarations in
     > yyFlexLexer, or something like that).
     >
     > Is this supposed to be possible, or is it being worked on (I DID
     > notice the comment that scanner classes are still experimental, so I'm
     > not too hopeful)?

     What you need to do is derive a subclass from yyFlexLexer that provides
     the above yylex() method, squirrels away lvalp and parm into member
     variables, and then invokes yyFlexLexer::yylex() to do the regular scanning.

                Vern

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ERASEME56
=========


     To: Mikael.Latvala@lmf.ericsson.se
     Subject: Re: Possible mistake in Flex v2.5 document
     In-reply-to: Your message of Fri, 05 Sep 1997 16:07:24 PDT.
     Date: Fri, 05 Sep 1997 10:01:54 PDT
     From: Vern Paxson <vern>

     > In that example you show how to count comment lines when using
     > C style /* ... */ comments. My question is, shouldn't you take into
     > account a scenario where end of a comment marker occurs inside
     > character or string literals?

     The scanner certainly needs to also scan character and string literals.
     However it does that (there's an example in the man page for strings), the
     lexer will recognize the beginning of the literal before it runs across the
     embedded "/*".  Consequently, it will finish scanning the literal before it
     even considers the possibility of matching "/*".

     Example:

        '([^']*|{ESCAPE_SEQUENCE})'

     will match all the text between the ''s (inclusive).  So the lexer
     considers this as a token beginning at the first ', and doesn't even
     attempt to match other tokens inside it.

     I thinnk this subtlety is not worth putting in the manual, as I suspect
     it would confuse more people than it would enlighten.

                Vern

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ERASEME57
=========


     To: "Marty Leisner" <leisner@sdsp.mc.xerox.com>
     Subject: Re: flex limitations
     In-reply-to: Your message of Sat, 06 Sep 1997 11:27:21 PDT.
     Date: Mon, 08 Sep 1997 11:38:08 PDT
     From: Vern Paxson <vern>

     > %%
     > [a-zA-Z]+       /* skip a line */
     >                 {  printf("got %s\n", yytext); }
     > %%

     What version of flex are you using?  If I feed this to 2.5.4, it complains:

        "bug.l", line 5: EOF encountered inside an action
        "bug.l", line 5: unrecognized rule
        "bug.l", line 5: fatal parse error

     Not the world's greatest error message, but it manages to flag the problem.

     (With the introduction of start condition scopes, flex can't accommodate
     an action on a separate line, since it's ambiguous with an indented rule.)

     You can get 2.5.4 from ftp.ee.lbl.gov.

                Vern

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File: flex.info,  Node: Is there a repository for flex scanners?,  Next: How can I conditionally compile or preprocess my flex input file?,  Prev: ERASEME57,  Up: FAQ

Is there a repository for flex scanners?
========================================

Not that we know of. You might try asking on comp.compilers.

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File: flex.info,  Node: How can I conditionally compile or preprocess my flex input file?,  Next: Where can I find grammars for lex and yacc?,  Prev: Is there a repository for flex scanners?,  Up: FAQ

How can I conditionally compile or preprocess my flex input file?
=================================================================

Flex doesn't have a preprocessor like C does.  You might try using m4,
or the C preprocessor plus a sed script to clean up the result.

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File: flex.info,  Node: Where can I find grammars for lex and yacc?,  Next: I get an end-of-buffer message for each character scanned.,  Prev: How can I conditionally compile or preprocess my flex input file?,  Up: FAQ

Where can I find grammars for lex and yacc?
===========================================

In the sources for flex and bison.

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File: flex.info,  Node: I get an end-of-buffer message for each character scanned.,  Next: unnamed-faq-62,  Prev: Where can I find grammars for lex and yacc?,  Up: FAQ

I get an end-of-buffer message for each character scanned.
==========================================================

This will happen if your LexerInput() function returns only one
character at a time, which can happen either if you're scanner is
"interactive", or if the streams library on your platform always
returns 1 for yyin->gcount().

   Solution: override LexerInput() with a version that returns whole
buffers.

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unnamed-faq-62
==============


     To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
     Subject: Re: Flex maximums
     In-reply-to: Your message of Mon, 17 Nov 1997 17:16:06 PST.
     Date: Mon, 17 Nov 1997 17:16:15 PST
     From: Vern Paxson <vern>

     > I took a quick look into the flex-sources and altered some #defines in
     > flexdefs.h:
     >
     >  #define INITIAL_MNS 64000
     >  #define MNS_INCREMENT 1024000
     >  #define MAXIMUM_MNS 64000

     The things to fix are to add a couple of zeroes to:

     #define JAMSTATE -32766 /* marks a reference to the state that always jams */
     #define MAXIMUM_MNS 31999
     #define BAD_SUBSCRIPT -32767
     #define MAX_SHORT 32700

     and, if you get complaints about too many rules, make the following change too:

        #define YY_TRAILING_MASK 0x200000
        #define YY_TRAILING_HEAD_MASK 0x400000

     - Vern

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unnamed-faq-63
==============


     To: jimmey@lexis-nexis.com (Jimmey Todd)
     Subject: Re: FLEX question regarding istream vs ifstream
     In-reply-to: Your message of Mon, 08 Dec 1997 15:54:15 PST.
     Date: Mon, 15 Dec 1997 13:21:35 PST
     From: Vern Paxson <vern>

     >         stdin_handle = YY_CURRENT_BUFFER;
     >         ifstream fin( "aFile" );
     >         yy_switch_to_buffer( yy_create_buffer( fin, YY_BUF_SIZE ) );
     >
     > What I'm wanting to do, is pass the contents of a file thru one set
     > of rules and then pass stdin thru another set... It works great if, I
     > don't use the C++ classes. But since everything else that I'm doing is
     > in C++, I thought I'd be consistent.
     >
     > The problem is that 'yy_create_buffer' is expecting an istream* as it's
     > first argument (as stated in the man page). However, fin is a ifstream
     > object. Any ideas on what I might be doing wrong? Any help would be
     > appreciated. Thanks!!

     You need to pass &fin, to turn it into an ifstream* instead of an ifstream.
     Then its type will be compatible with the expected istream*, because ifstream
     is derived from istream.

                Vern

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unnamed-faq-64
==============


     To: Enda Fadian <fadiane@piercom.ie>
     Subject: Re: Question related to Flex man page?
     In-reply-to: Your message of Tue, 16 Dec 1997 15:17:34 PST.
     Date: Tue, 16 Dec 1997 14:17:09 PST
     From: Vern Paxson <vern>

     > Can you explain to me what is ment by a long-jump in relation to flex?

     Using the longjmp() function while inside yylex() or a routine called by it.

     > what is the flex activation frame.

     Just yylex()'s stack frame.

     > As far as I can see yyrestart will bring me back to the sart of the input
     > file and using flex++ isnot really an option!

     No, yyrestart() doesn't imply a rewind, even though its name might sound
     like it does.  It tells the scanner to flush its internal buffers and
     start reading from the given file at its present location.

                Vern

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unnamed-faq-65
==============


     To: hassan@larc.info.uqam.ca (Hassan Alaoui)
     Subject: Re: Need urgent Help
     In-reply-to: Your message of Sat, 20 Dec 1997 19:38:19 PST.
     Date: Sun, 21 Dec 1997 21:30:46 PST
     From: Vern Paxson <vern>

     > /usr/lib/yaccpar: In function `int yyparse()':
     > /usr/lib/yaccpar:184: warning: implicit declaration of function `int yylex(...)'
     >
     > ld: Undefined symbol
     >    _yylex
     >    _yyparse
     >    _yyin

     This is a known problem with Solaris C++ (and/or Solaris yacc).  I believe
     the fix is to explicitly insert some 'extern "C"' statements for the
     corresponding routines/symbols.

                Vern

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unnamed-faq-66
==============


     To: mc0307@mclink.it
     Cc: gnu@prep.ai.mit.edu
     Subject: Re: [mc0307@mclink.it: Help request]
     In-reply-to: Your message of Fri, 12 Dec 1997 17:57:29 PST.
     Date: Sun, 21 Dec 1997 22:33:37 PST
     From: Vern Paxson <vern>

     > This is my definition for float and integer types:
     > . . .
     > NZD          [1-9]
     > ...
     > I've tested my program on other lex version (on UNIX Sun Solaris an HP
     > UNIX) and it work well, so I think that my definitions are correct.
     > There are any differences between Lex and Flex?

     There are indeed differences, as discussed in the man page.  The one
     you are probably running into is that when flex expands a name definition,
     it puts parentheses around the expansion, while lex does not.  There's
     an example in the man page of how this can lead to different matching.
     Flex's behavior complies with the POSIX standard (or at least with the
     last POSIX draft I saw).

                Vern

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unnamed-faq-67
==============


     To: hassan@larc.info.uqam.ca (Hassan Alaoui)
     Subject: Re: Thanks
     In-reply-to: Your message of Mon, 22 Dec 1997 16:06:35 PST.
     Date: Mon, 22 Dec 1997 14:35:05 PST
     From: Vern Paxson <vern>

     > Thank you very much for your help. I compile and link well with C++ while
     > declaring 'yylex ...' extern, But a little problem remains. I get a
     > segmentation default when executing ( I linked with lfl library) while it
     > works well when using LEX instead of flex. Do you have some ideas about the
     > reason for this ?

     The one possible reason for this that comes to mind is if you've defined
     yytext as "extern char yytext[]" (which is what lex uses) instead of
     "extern char *yytext" (which is what flex uses).  If it's not that, then
     I'm afraid I don't know what the problem might be.

                Vern

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unnamed-faq-68
==============


     To: "Bart Niswonger" <NISWONGR@almaden.ibm.com>
     Subject: Re: flex 2.5: c++ scanners & start conditions
     In-reply-to: Your message of Tue, 06 Jan 1998 10:34:21 PST.
     Date: Tue, 06 Jan 1998 19:19:30 PST
     From: Vern Paxson <vern>

     > The problem is that when I do this (using %option c++) start
     > conditions seem to not apply.

     The BEGIN macro modifies the yy_start variable.  For C scanners, this
     is a static with scope visible through the whole file.  For C++ scanners,
     it's a member variable, so it only has visible scope within a member
     function.  Your lexbegin() routine is not a member function when you
     build a C++ scanner, so it's not modifying the correct yy_start.  The
     diagnostic that indicates this is that you found you needed to add
     a declaration of yy_start in order to get your scanner to compile when
     using C++; instead, the correct fix is to make lexbegin() a member
     function (by deriving from yyFlexLexer).

                Vern

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unnamed-faq-69
==============


     To: "Boris Zinin" <boris@ippe.rssi.ru>
     Subject: Re: current position in flex buffer
     In-reply-to: Your message of Mon, 12 Jan 1998 18:58:23 PST.
     Date: Mon, 12 Jan 1998 12:03:15 PST
     From: Vern Paxson <vern>

     > The problem is how to determine the current position in flex active
     > buffer when a rule is matched....

     You will need to keep track of this explicitly, such as by redefining
     YY_USER_ACTION to count the number of characters matched.

     The latest flex release, by the way, is 2.5.4, available from ftp.ee.lbl.gov.

                Vern

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unnamed-faq-70
==============


     To: Bik.Dhaliwal@bis.org
     Subject: Re: Flex question
     In-reply-to: Your message of Mon, 26 Jan 1998 13:05:35 PST.
     Date: Tue, 27 Jan 1998 22:41:52 PST
     From: Vern Paxson <vern>

     > That requirement involves knowing
     > the character position at which a particular token was matched
     > in the lexer.

     The way you have to do this is by explicitly keeping track of where
     you are in the file, by counting the number of characters scanned
     for each token (available in yyleng).  It may prove convenient to
     do this by redefining YY_USER_ACTION, as described in the manual.

                Vern

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unnamed-faq-71
==============


     To: Vladimir Alexiev <vladimir@cs.ualberta.ca>
     Subject: Re: flex: how to control start condition from parser?
     In-reply-to: Your message of Mon, 26 Jan 1998 05:50:16 PST.
     Date: Tue, 27 Jan 1998 22:45:37 PST
     From: Vern Paxson <vern>

     > It seems useful for the parser to be able to tell the lexer about such
     > context dependencies, because then they don't have to be limited to
     > local or sequential context.

     One way to do this is to have the parser call a stub routine that's
     included in the scanner's .l file, and consequently that has access ot
     BEGIN.  The only ugliness is that the parser can't pass in the state
     it wants, because those aren't visible - but if you don't have many
     such states, then using a different set of names doesn't seem like
     to much of a burden.

     While generating a .h file like you suggests is certainly cleaner,
     flex development has come to a virtual stand-still :-(, so a workaround
     like the above is much more pragmatic than waiting for a new feature.

                Vern

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unnamed-faq-72
==============


     To: Barbara Denny <denny@3com.com>
     Subject: Re: freebsd flex bug?
     In-reply-to: Your message of Fri, 30 Jan 1998 12:00:43 PST.
     Date: Fri, 30 Jan 1998 12:42:32 PST
     From: Vern Paxson <vern>

     > lex.yy.c:1996: parse error before `='

     This is the key, identifying this error.  (It may help to pinpoint
     it by using flex -L, so it doesn't generate #line directives in its
     output.)  I will bet you heavy money that you have a start condition
     name that is also a variable name, or something like that; flex spits
     out #define's for each start condition name, mapping them to a number,
     so you can wind up with:

        %x foo
        %%
                ...
        %%
        void bar()
                {
                int foo = 3;
                }

     and the penultimate will turn into "int 1 = 3" after C preprocessing,
     since flex will put "#define foo 1" in the generated scanner.

                Vern

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unnamed-faq-73
==============


     To: Maurice Petrie <mpetrie@infoscigroup.com>
     Subject: Re: Lost flex .l file
     In-reply-to: Your message of Mon, 02 Feb 1998 14:10:01 PST.
     Date: Mon, 02 Feb 1998 11:15:12 PST
     From: Vern Paxson <vern>

     > I am curious as to
     > whether there is a simple way to backtrack from the generated source to
     > reproduce the lost list of tokens we are searching on.

     In theory, it's straight-forward to go from the DFA representation
     back to a regular-expression representation - the two are isomorphic.
     In practice, a huge headache, because you have to unpack all the tables
     back into a single DFA representation, and then write a program to munch
     on that and translate it into an RE.

     Sorry for the less-than-happy news ...

                Vern

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unnamed-faq-74
==============


     To: jimmey@lexis-nexis.com (Jimmey Todd)
     Subject: Re: Flex performance question
     In-reply-to: Your message of Thu, 19 Feb 1998 11:01:17 PST.
     Date: Thu, 19 Feb 1998 08:48:51 PST
     From: Vern Paxson <vern>

     > What I have found, is that the smaller the data chunk, the faster the
     > program executes. This is the opposite of what I expected. Should this be
     > happening this way?

     This is exactly what will happen if your input file has embedded NULs.
     From the man page:

     A final note: flex is slow when matching NUL's, particularly
     when  a  token  contains multiple NUL's.  It's best to write
     rules which match short amounts of text if it's  anticipated
     that the text will often include NUL's.

     So that's the first thing to look for.

                Vern

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unnamed-faq-75
==============


     To: jimmey@lexis-nexis.com (Jimmey Todd)
     Subject: Re: Flex performance question
     In-reply-to: Your message of Thu, 19 Feb 1998 11:01:17 PST.
     Date: Thu, 19 Feb 1998 15:42:25 PST
     From: Vern Paxson <vern>

     So there are several problems.

     First, to go fast, you want to match as much text as possible, which
     your scanners don't in the case that what they're scanning is *not*
     a <RN> tag.  So you want a rule like:

        [^<]+

     Second, C++ scanners are particularly slow if they're interactive,
     which they are by default.  Using -B speeds it up by a factor of 3-4
     on my workstation.

     Third, C++ scanners that use the istream interface are slow, because
     of how poorly implemented istream's are.  I built two versions of
     the following scanner:

        %%
        .*\n
        .*
        %%

     and the C version inhales a 2.5MB file on my workstation in 0.8 seconds.
     The C++ istream version, using -B, takes 3.8 seconds.

                Vern

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unnamed-faq-76
==============


     To: "Frescatore, David (CRD, TAD)" <frescatore@exc01crdge.crd.ge.com>
     Subject: Re: FLEX 2.5 & THE YEAR 2000
     In-reply-to: Your message of Wed, 03 Jun 1998 11:26:22 PDT.
     Date: Wed, 03 Jun 1998 10:22:26 PDT
     From: Vern Paxson <vern>

     > I am researching the Y2K problem with General Electric R&D
     > and need to know if there are any known issues concerning
     > the above mentioned software and Y2K regardless of version.

     There shouldn't be, all it ever does with the date is ask the system
     for it and then print it out.

                Vern

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unnamed-faq-77
==============


     To: "Hans Dermot Doran" <htd@ibhdoran.com>
     Subject: Re: flex problem
     In-reply-to: Your message of Wed, 15 Jul 1998 21:30:13 PDT.
     Date: Tue, 21 Jul 1998 14:23:34 PDT
     From: Vern Paxson <vern>

     > To overcome this, I gets() the stdin into a string and lex the string. The
     > string is lexed OK except that the end of string isn't lexed properly
     > (yy_scan_string()), that is the lexer dosn't recognise the end of string.

     Flex doesn't contain mechanisms for recognizing buffer endpoints.  But if
     you use fgets instead (which you should anyway, to protect against buffer
     overflows), then the final \n will be preserved in the string, and you can
     scan that in order to find the end of the string.

                Vern

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unnamed-faq-78
==============


     To: soumen@almaden.ibm.com
     Subject: Re: Flex++ 2.5.3 instance member vs. static member
     In-reply-to: Your message of Mon, 27 Jul 1998 02:10:04 PDT.
     Date: Tue, 28 Jul 1998 01:10:34 PDT
     From: Vern Paxson <vern>

     > %{
     > int mylineno = 0;
     > %}
     > ws      [ \t]+
     > alpha   [A-Za-z]
     > dig     [0-9]
     > %%
     >
     > Now you'd expect mylineno to be a member of each instance of class
     > yyFlexLexer, but is this the case?  A look at the lex.yy.cc file seems to
     > indicate otherwise; unless I am missing something the declaration of
     > mylineno seems to be outside any class scope.
     >
     > How will this work if I want to run a multi-threaded application with each
     > thread creating a FlexLexer instance?

     Derive your own subclass and make mylineno a member variable of it.

                Vern

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unnamed-faq-79
==============


     To: Adoram Rogel <adoram@hybridge.com>
     Subject: Re: More than 32K states change hangs
     In-reply-to: Your message of Tue, 04 Aug 1998 16:55:39 PDT.
     Date: Tue, 04 Aug 1998 22:28:45 PDT
     From: Vern Paxson <vern>

     > Vern Paxson,
     >
     > I followed your advice, posted on Usenet bu you, and emailed to me
     > personally by you, on how to overcome the 32K states limit. I'm running
     > on Linux machines.
     > I took the full source of version 2.5.4 and did the following changes in
     > flexdef.h:
     > #define JAMSTATE -327660
     > #define MAXIMUM_MNS 319990
     > #define BAD_SUBSCRIPT -327670
     > #define MAX_SHORT 327000
     >
     > and compiled.
     > All looked fine, including check and bigcheck, so I installed.

     Hmmm, you shouldn't increase MAX_SHORT, though looking through my email
     archives I see that I did indeed recommend doing so.  Try setting it back
     to 32700; that should suffice that you no longer need -Ca.  If it still
     hangs, then the interesting question is - where?

     > Compiling the same hanged program with a out-of-the-box (RedHat 4.2
     > distribution of Linux)
     > flex 2.5.4 binary works.

     Since Linux comes with source code, you should diff it against what
     you have to see what problems they missed.

     > Should I always compile with the -Ca option now ? even short and simple
     > filters ?

     No, definitely not.  It's meant to be for those situations where you
     absolutely must squeeze every last cycle out of your scanner.

                Vern

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unnamed-faq-80
==============


     To: "Schmackpfeffer, Craig" <Craig.Schmackpfeffer@usa.xerox.com>
     Subject: Re: flex output for static code portion
     In-reply-to: Your message of Tue, 11 Aug 1998 11:55:30 PDT.
     Date: Mon, 17 Aug 1998 23:57:42 PDT
     From: Vern Paxson <vern>

     > I would like to use flex under the hood to generate a binary file
     > containing the data structures that control the parse.

     This has been on the wish-list for a long time.  In principle it's
     straight-forward - you redirect mkdata() et al's I/O to another file,
     and modify the skeleton to have a start-up function that slurps these
     into dynamic arrays.  The concerns are (1) the scanner generation code
     is hairy and full of corner cases, so it's easy to get surprised when
     going down this path :-( ; and (2) being careful about buffering so
     that when the tables change you make sure the scanner starts in the
     correct state and reading at the right point in the input file.

     > I was wondering if you know of anyone who has used flex in this way.

     I don't - but it seems like a reasonable project to undertake (unlike
     numerous other flex tweaks :-).

                Vern

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==============


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        by ee.lbl.gov (8.9.1/8.9.1) with ESMTP id AAA03838
        for <vern@ee.lbl.gov>; Thu, 20 Aug 1998 00:47:57 -0700 (PDT)
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        by deimos.rz.uni-osnabrueck.de (8.8.7/8.8.8) with ESMTP id JAA34694
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     Received: (from georg@localhost) by hal.cl-ki.uni-osnabrueck.de (8.6.12/8.6.12) id JAA34834 for vern@ee.lbl.gov; Thu, 20 Aug 1998 09:47:54 +0200
     From: Georg Rehm <georg@hal.cl-ki.uni-osnabrueck.de>
     Message-Id: <199808200747.JAA34834@hal.cl-ki.uni-osnabrueck.de>
     Subject: "flex scanner push-back overflow"
     To: vern@ee.lbl.gov
     Date: Thu, 20 Aug 1998 09:47:54 +0200 (MEST)
     Reply-To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
     X-NoJunk: Do NOT send commercial mail, spam or ads to this address!
     X-URL: http://www.cl-ki.uni-osnabrueck.de/~georg/
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     Hi Vern,

     Yesterday, I encountered a strange problem: I use the macro processor m4
     to include some lengthy lists into a .l file. Following is a flex macro
     definition that causes some serious pain in my neck:

     AUTHOR           ("A. Boucard / L. Boucard"|"A. Dastarac / M. Levent"|"A.Boucaud / L.Boucaud"|"Abderrahim Lamchichi"|"Achmat Dangor"|"Adeline Toullier"|"Adewale Maja-Pearce"|"Ahmed Ziri"|"Akram Ellyas"|"Alain Bihr"|"Alain Gresh"|"Alain Guillemoles"|"Alain Joxe"|"Alain Morice"|"Alain Renon"|"Alain Zecchini"|"Albert Memmi"|"Alberto Manguel"|"Alex De Waal"|"Alfonso Artico"| [...])

     The complete list contains about 10kB. When I try to "flex" this file
     (on a Solaris 2.6 machine, using a modified flex 2.5.4 (I only increased
     some of the predefined values in flexdefs.h) I get the error:

     myflex/flex -8  sentag.tmp.l
     flex scanner push-back overflow

     When I remove the slashes in the macro definition everything works fine.
     As I understand it, the double quotes escape the slash-character so it
     really means "/" and not "trailing context". Furthermore, I tried to
     escape the slashes with backslashes, but with no use, the same error message
     appeared when flexing the code.

     Do you have an idea what's going on here?

     Greetings from Germany,
        Georg
     --
     Georg Rehm                                     georg@cl-ki.uni-osnabrueck.de
     Institute for Semantic Information Processing, University of Osnabrueck, FRG

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unnamed-faq-82
==============


     To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
     Subject: Re: "flex scanner push-back overflow"
     In-reply-to: Your message of Thu, 20 Aug 1998 09:47:54 PDT.
     Date: Thu, 20 Aug 1998 07:05:35 PDT
     From: Vern Paxson <vern>

     > myflex/flex -8  sentag.tmp.l
     > flex scanner push-back overflow

     Flex itself uses a flex scanner.  That scanner is running out of buffer
     space when it tries to unput() the humongous macro you've defined.  When
     you remove the '/'s, you make it small enough so that it fits in the buffer;
     removing spaces would do the same thing.

     The fix is to either rethink how come you're using such a big macro and
     perhaps there's another/better way to do it; or to rebuild flex's own
     scan.c with a larger value for

        #define YY_BUF_SIZE 16384

     - Vern

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unnamed-faq-83
==============


     To: Jan Kort <jan@research.techforce.nl>
     Subject: Re: Flex
     In-reply-to: Your message of Fri, 04 Sep 1998 12:18:43 +0200.
     Date: Sat, 05 Sep 1998 00:59:49 PDT
     From: Vern Paxson <vern>

     > %%
     >
     > "TEST1\n"       { fprintf(stderr, "TEST1\n"); yyless(5); }
     > ^\n             { fprintf(stderr, "empty line\n"); }
     > .               { }
     > \n              { fprintf(stderr, "new line\n"); }
     >
     > %%
     > -- input ---------------------------------------
     > TEST1
     > -- output --------------------------------------
     > TEST1
     > empty line
     > ------------------------------------------------

     IMHO, it's not clear whether or not this is in fact a bug.  It depends
     on whether you view yyless() as backing up in the input stream, or as
     pushing new characters onto the beginning of the input stream.  Flex
     interprets it as the latter (for implementation convenience, I'll admit),
     and so considers the newline as in fact matching at the beginning of a
     line, as after all the last token scanned an entire line and so the
     scanner is now at the beginning of a new line.

     I agree that this is counter-intuitive for yyless(), given its
     functional description (it's less so for unput(), depending on whether
     you're unput()'ing new text or scanned text).  But I don't plan to
     change it any time soon, as it's a pain to do so.  Consequently,
     you do indeed need to use yy_set_bol() and YY_AT_BOL() to tweak
     your scanner into the behavior you desire.

     Sorry for the less-than-completely-satisfactory answer.

                Vern

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unnamed-faq-84
==============


     To: Patrick Krusenotto <krusenot@mac-info-link.de>
     Subject: Re: Problems with restarting flex-2.5.2-generated scanner
     In-reply-to: Your message of Thu, 24 Sep 1998 10:14:07 PDT.
     Date: Thu, 24 Sep 1998 23:28:43 PDT
     From: Vern Paxson <vern>

     > I am using flex-2.5.2 and bison 1.25 for Solaris and I am desperately
     > trying to make my scanner restart with a new file after my parser stops
     > with a parse error. When my compiler restarts, the parser always
     > receives the token after the token (in the old file!) that caused the
     > parser error.

     I suspect the problem is that your parser has read ahead in order
     to attempt to resolve an ambiguity, and when it's restarted it picks
     up with that token rather than reading a fresh one.  If you're using
     yacc, then the special "error" production can sometimes be used to
     consume tokens in an attempt to get the parser into a consistent state.

                Vern

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unnamed-faq-85
==============


     To: Henric Jungheim <junghelh@pe-nelson.com>
     Subject: Re: flex 2.5.4a
     In-reply-to: Your message of Tue, 27 Oct 1998 16:41:42 PST.
     Date: Tue, 27 Oct 1998 16:50:14 PST
     From: Vern Paxson <vern>

     > This brings up a feature request:  How about a command line
     > option to specify the filename when reading from stdin?  That way one
     > doesn't need to create a temporary file in order to get the "#line"
     > directives to make sense.

     Use -o combined with -t (per the man page description of -o).

     > P.S., Is there any simple way to use non-blocking IO to parse multiple
     > streams?

     Simple, no.

     One approach might be to return a magic character on EWOULDBLOCK and
     have a rule

        .*<magic-character>     // put back .*, eat magic character

     This is off the top of my head, not sure it'll work.

                Vern

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unnamed-faq-86
==============


     To: "Repko, Billy D" <billy.d.repko@intel.com>
     Subject: Re: Compiling scanners
     In-reply-to: Your message of Wed, 13 Jan 1999 10:52:47 PST.
     Date: Thu, 14 Jan 1999 00:25:30 PST
     From: Vern Paxson <vern>

     > It appears that maybe it cannot find the lfl library.

     The Makefile in the distribution builds it, so you should have it.
     It's exceedingly trivial, just a main() that calls yylex() and
     a yyrap() that always returns 1.

     > %%
     >       \n      ++num_lines; ++num_chars;
     >       .       ++num_chars;

     You can't indent your rules like this - that's where the errors are coming
     from.  Flex copies indented text to the output file, it's how you do things
     like

        int num_lines_seen = 0;

     to declare local variables.

                Vern

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unnamed-faq-87
==============


     To: Erick Branderhorst <Erick.Branderhorst@asml.nl>
     Subject: Re: flex input buffer
     In-reply-to: Your message of Tue, 09 Feb 1999 13:53:46 PST.
     Date: Tue, 09 Feb 1999 21:03:37 PST
     From: Vern Paxson <vern>

     > In the flex.skl file the size of the default input buffers is set.  Can you
     > explain why this size is set and why it is such a high number.

     It's large to optimize performance when scanning large files.  You can
     safely make it a lot lower if needed.

                Vern

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unnamed-faq-88
==============


     To: "Guido Minnen" <guidomi@cogs.susx.ac.uk>
     Subject: Re: Flex error message
     In-reply-to: Your message of Wed, 24 Feb 1999 15:31:46 PST.
     Date: Thu, 25 Feb 1999 00:11:31 PST
     From: Vern Paxson <vern>

     > I'm extending a larger scanner written in Flex and I keep running into
     > problems. More specifically, I get the error message:
     > "flex: input rules are too complicated (>= 32000 NFA states)"

     Increase the definitions in flexdef.h for:

     #define JAMSTATE -32766 /* marks a reference to the state that always j
     ams */
     #define MAXIMUM_MNS 31999
     #define BAD_SUBSCRIPT -32767

     recompile everything, and it should all work.

                Vern

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unnamed-faq-90
==============


     To: "Dmitriy Goldobin" <gold@ems.chel.su>
     Subject: Re: FLEX trouble
     In-reply-to: Your message of Mon, 31 May 1999 18:44:49 PDT.
     Date: Tue, 01 Jun 1999 00:15:07 PDT
     From: Vern Paxson <vern>

     >   I have a trouble with FLEX. Why rule "/*".*"*/" work properly,=20
     > but rule "/*"(.|\n)*"*/" don't work ?

     The second of these will have to scan the entire input stream (because
     "(.|\n)*" matches an arbitrary amount of any text) in order to see if
     it ends with "*/", terminating the comment.  That potentially will overflow
     the input buffer.

     >   More complex rule "/*"([^*]|(\*/[^/]))*"*/ give an error
     > 'unrecognized rule'.

     You can't use the '/' operator inside parentheses.  It's not clear
     what "(a/b)*" actually means.

     >   I now use workaround with state <comment>, but single-rule is
     > better, i think.

     Single-rule is nice but will always have the problem of either setting
     restrictions on comments (like not allowing multi-line comments) and/or
     running the risk of consuming the entire input stream, as noted above.

                Vern

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unnamed-faq-91
==============


     Received: from mc-qout4.whowhere.com (mc-qout4.whowhere.com [209.185.123.18])
        by ee.lbl.gov (8.9.3/8.9.3) with SMTP id IAA05100
        for <vern@ee.lbl.gov>; Tue, 15 Jun 1999 08:56:06 -0700 (PDT)
     Received: from Unknown/Local ([?.?.?.?]) by my-deja.com; Tue Jun 15 08:55:43 1999
     To: vern@ee.lbl.gov
     Date: Tue, 15 Jun 1999 08:55:43 -0700
     From: "Aki Niimura" <neko@my-deja.com>
     Message-ID: <KNONDOHDOBGAEAAA@my-deja.com>
     Mime-Version: 1.0
     Cc:
     X-Sent-Mail: on
     Reply-To:
     X-Mailer: MailCity Service
     Subject: A question on flex C++ scanner
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     Organization: My Deja Email  (http://www.my-deja.com:80)
     Content-Type: text/plain; charset=us-ascii
     Content-Transfer-Encoding: 7bit

     Dear Dr. Paxon,

     I have been using flex for years.
     It works very well on many projects.
     Most case, I used it to generate a scanner on C language.
     However, one project I needed to generate  a scanner
     on C++ lanuage. Thanks to your enhancement, flex did
     the job.

     Currently, I'm working on enhancing my previous project.
     I need to deal with multiple input streams (recursive
     inclusion) in this scanner (C++).
     I did similar thing for another scanner (C) as you
     explained in your documentation.

     The generated scanner (C++) has necessary methods:
     - switch_to_buffer(struct yy_buffer_state *b)
     - yy_create_buffer(istream *is, int sz)
     - yy_delete_buffer(struct yy_buffer_state *b)

     However, I couldn't figure out how to access current
     buffer (yy_current_buffer).

     yy_current_buffer is a protected member of yyFlexLexer.
     I can't access it directly.
     Then, I thought yy_create_buffer() with is = 0 might
     return current stream buffer. But it seems not as far
     as I checked the source. (flex 2.5.4)

     I went through the Web in addition to Flex documentation.
     However, it hasn't been successful, so far.

     It is not my intention to bother you, but, can you
     comment about how to obtain the current stream buffer?

     Your response would be highly appreciated.

     Best regards,
     Aki Niimura

     --== Sent via Deja.com http://www.deja.com/ ==--
     Share what you know. Learn what you don't.

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unnamed-faq-92
==============


     To: neko@my-deja.com
     Subject: Re: A question on flex C++ scanner
     In-reply-to: Your message of Tue, 15 Jun 1999 08:55:43 PDT.
     Date: Tue, 15 Jun 1999 09:04:24 PDT
     From: Vern Paxson <vern>

     > However, I couldn't figure out how to access current
     > buffer (yy_current_buffer).

     Derive your own subclass from yyFlexLexer.

                Vern

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unnamed-faq-93
==============


     To: "Stones, Darren" <Darren.Stones@nectech.co.uk>
     Subject: Re: You're the man to see?
     In-reply-to: Your message of Wed, 23 Jun 1999 11:10:29 PDT.
     Date: Wed, 23 Jun 1999 09:01:40 PDT
     From: Vern Paxson <vern>

     > I hope you can help me.  I am using Flex and Bison to produce an interpreted
     > language.  However all goes well until I try to implement an IF statement or
     > a WHILE.  I cannot get this to work as the parser parses all the conditions
     > eg. the TRUE and FALSE conditons to check for a rule match.  So I cannot
     > make a decision!!

     You need to use the parser to build a parse tree (= abstract syntax trwee),
     and when that's all done you recursively evaluate the tree, binding variables
     to values at that time.

                Vern

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unnamed-faq-94
==============


     To: Petr Danecek <petr@ics.cas.cz>
     Subject: Re: flex - question
     In-reply-to: Your message of Mon, 28 Jun 1999 19:21:41 PDT.
     Date: Fri, 02 Jul 1999 16:52:13 PDT
     From: Vern Paxson <vern>

     > file, it takes an enormous amount of time. It is funny, because the
     > source code has only 12 rules!!! I think it looks like an exponencial
     > growth.

     Right, that's the problem - some patterns (those with a lot of
     ambiguity, where yours has because at any given time the scanner can
     be in the middle of all sorts of combinations of the different
     rules) blow up exponentially.

     For your rules, there is an easy fix.  Change the ".*" that comes fater
     the directory name to "[^ ]*".  With that in place, the rules are no
     longer nearly so ambiguous, because then once one of the directories
     has been matched, no other can be matched (since they all require a
     leading blank).

     If that's not an acceptable solution, then you can enter a start state
     to pick up the .*\n after each directory is matched.

     Also note that for speed, you'll want to add a ".*" rule at the end,
     otherwise rules that don't match any of the patterns will be matched
     very slowly, a character at a time.

                Vern

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unnamed-faq-95
==============


     To: Tielman Koekemoer <tielman@spi.co.za>
     Subject: Re: Please help.
     In-reply-to: Your message of Thu, 08 Jul 1999 13:20:37 PDT.
     Date: Thu, 08 Jul 1999 08:20:39 PDT
     From: Vern Paxson <vern>

     > I was hoping you could help me with my problem.
     >
     > I tried compiling (gnu)flex on a Solaris 2.4 machine
     > but when I ran make (after configure) I got an error.
     >
     > --------------------------------------------------------------
     > gcc -c -I. -I. -g -O parse.c
     > ./flex -t -p  ./scan.l >scan.c
     > sh: ./flex: not found
     > *** Error code 1
     > make: Fatal error: Command failed for target `scan.c'
     > -------------------------------------------------------------
     >
     > What's strange to me is that I'm only
     > trying to install flex now. I then edited the Makefile to
     > and changed where it says "FLEX = flex" to "FLEX = lex"
     > ( lex: the native Solaris one ) but then it complains about
     > the "-p" option. Is there any way I can compile flex without
     > using flex or lex?
     >
     > Thanks so much for your time.

     You managed to step on the bootstrap sequence, which first copies
     initscan.c to scan.c in order to build flex.  Try fetching a fresh
     distribution from ftp.ee.lbl.gov.  (Or you can first try removing
     ".bootstrap" and doing a make again.)

                Vern

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unnamed-faq-96
==============


     To: Tielman Koekemoer <tielman@spi.co.za>
     Subject: Re: Please help.
     In-reply-to: Your message of Fri, 09 Jul 1999 09:16:14 PDT.
     Date: Fri, 09 Jul 1999 00:27:20 PDT
     From: Vern Paxson <vern>

     > First I removed .bootstrap (and ran make) - no luck. I downloaded the
     > software but I still have the same problem. Is there anything else I
     > could try.

     Try:

        cp initscan.c scan.c
        touch scan.c
        make scan.o

     If this last tries to first build scan.c from scan.l using ./flex, then
     your "make" is broken, in which case compile scan.c to scan.o by hand.

                Vern

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unnamed-faq-97
==============


     To: Sumanth Kamenani <skamenan@crl.nmsu.edu>
     Subject: Re: Error
     In-reply-to: Your message of Mon, 19 Jul 1999 23:08:41 PDT.
     Date: Tue, 20 Jul 1999 00:18:26 PDT
     From: Vern Paxson <vern>

     > I am getting a compilation error. The error is given as "unknown symbol- yylex".

     The parser relies on calling yylex(), but you're instead using the C++ scanning
     class, so you need to supply a yylex() "glue" function that calls an instance
     scanner of the scanner (e.g., "scanner->yylex()").

                Vern

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unnamed-faq-98
==============


     To: daniel@synchrods.synchrods.COM (Daniel Senderowicz)
     Subject: Re: lex
     In-reply-to: Your message of Mon, 22 Nov 1999 11:19:04 PST.
     Date: Tue, 23 Nov 1999 15:54:30 PST
     From: Vern Paxson <vern>

     Well, your problem is the

     switch (yybgin-yysvec-1) {      /* witchcraft */

     at the beginning of lex rules.  "witchcraft" == "non-portable".  It's
     assuming knowledge of the AT&T lex's internal variables.

     For flex, you can probably do the equivalent using a switch on YYSTATE.

                Vern

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unnamed-faq-99
==============


     To: archow@hss.hns.com
     Subject: Re: Regarding distribution of flex and yacc based grammars
     In-reply-to: Your message of Sun, 19 Dec 1999 17:50:24 +0530.
     Date: Wed, 22 Dec 1999 01:56:24 PST
     From: Vern Paxson <vern>

     > When we provide the customer with an object code distribution, is it
     > necessary for us to provide source
     > for the generated C files from flex and bison since they are generated by
     > flex and bison ?

     For flex, no.  I don't know what the current state of this is for bison.

     > Also, is there any requrirement for us to neccessarily  provide source for
     > the grammar files which are fed into flex and bison ?

     Again, for flex, no.

     See the file "COPYING" in the flex distribution for the legalese.

                Vern

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unnamed-faq-100
===============


     To: Martin Gallwey <gallweym@hyperion.moe.ul.ie>
     Subject: Re: Flex, and self referencing rules
     In-reply-to: Your message of Sun, 20 Feb 2000 01:01:21 PST.
     Date: Sat, 19 Feb 2000 18:33:16 PST
     From: Vern Paxson <vern>

     > However, I do not use unput anywhere. I do use self-referencing
     > rules like this:
     >
     > UnaryExpr               ({UnionExpr})|("-"{UnaryExpr})

     You can't do this - flex is *not* a parser like yacc (which does indeed
     allow recursion), it is a scanner that's confined to regular expressions.

                Vern

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unnamed-faq-101
===============


     To: slg3@lehigh.edu (SAMUEL L. GULDEN)
     Subject: Re: Flex problem
     In-reply-to: Your message of Thu, 02 Mar 2000 12:29:04 PST.
     Date: Thu, 02 Mar 2000 23:00:46 PST
     From: Vern Paxson <vern>

     If this is exactly your program:

     > digit [0-9]
     > digits {digit}+
     > whitespace [ \t\n]+
     >
     > %%
     > "[" { printf("open_brac\n");}
     > "]" { printf("close_brac\n");}
     > "+" { printf("addop\n");}
     > "*" { printf("multop\n");}
     > {digits} { printf("NUMBER = %s\n", yytext);}
     > whitespace ;

     then the problem is that the last rule needs to be "{whitespace}" !

                Vern

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File: flex.info,  Node: What is the difference between YYLEX_PARAM and YY_DECL?,  Next: Why do I get "conflicting types for yylex" error?,  Prev: unnamed-faq-101,  Up: FAQ

What is the difference between YYLEX_PARAM and YY_DECL?
=======================================================

YYLEX_PARAM is not a flex symbol. It is for Bison. It tells Bison to
pass extra params when it calls yylex() from the parser.

   YY_DECL is the Flex declaration of yylex. The default is similar to
this:


     #define int yy_lex ()

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File: flex.info,  Node: Why do I get "conflicting types for yylex" error?,  Next: How do I access the values set in a Flex action from within a Bison action?,  Prev: What is the difference between YYLEX_PARAM and YY_DECL?,  Up: FAQ

Why do I get "conflicting types for yylex" error?
=================================================

This is a compiler error regarding a generated Bison parser, not a Flex
scanner.  It means you need a prototype of yylex() in the top of the
Bison file.  Be sure the prototype matches YY_DECL.

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File: flex.info,  Node: How do I access the values set in a Flex action from within a Bison action?,  Prev: Why do I get "conflicting types for yylex" error?,  Up: FAQ

How do I access the values set in a Flex action from within a Bison action?
===========================================================================

With $1, $2, $3, etc. These are called "Semantic Values" in the Bison
manual.  See *Note Top: (bison)Top.

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File: flex.info,  Node: Appendices,  Next: Indices,  Prev: FAQ,  Up: Top

Appendix A Appendices
*********************

* Menu:

* Makefiles and Flex::
* Bison Bridge::
* M4 Dependency::
* Common Patterns::

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File: flex.info,  Node: Makefiles and Flex,  Next: Bison Bridge,  Prev: Appendices,  Up: Appendices

A.1 Makefiles and Flex
======================

In this appendix, we provide tips for writing Makefiles to build your
scanners.

   In a traditional build environment, we say that the `.c' files are
the sources, and the `.o' files are the intermediate files. When using
`flex', however, the `.l' files are the sources, and the generated `.c'
files (along with the `.o' files) are the intermediate files.  This
requires you to carefully plan your Makefile.

   Modern `make' programs understand that `foo.l' is intended to
generate `lex.yy.c' or `foo.c', and will behave accordingly(1)(2).  The
following Makefile does not explicitly instruct `make' how to build
`foo.c' from `foo.l'. Instead, it relies on the implicit rules of the
`make' program to build the intermediate file, `scan.c':


         # Basic Makefile -- relies on implicit rules
         # Creates "myprogram" from "scan.l" and "myprogram.c"
         #
         LEX=flex
         myprogram: scan.o myprogram.o
         scan.o: scan.l

   For simple cases, the above may be sufficient. For other cases, you
may have to explicitly instruct `make' how to build your scanner.  The
following is an example of a Makefile containing explicit rules:


         # Basic Makefile -- provides explicit rules
         # Creates "myprogram" from "scan.l" and "myprogram.c"
         #
         LEX=flex
         myprogram: scan.o myprogram.o
                 $(CC) -o $@  $(LDFLAGS) $^

         myprogram.o: myprogram.c
                 $(CC) $(CPPFLAGS) $(CFLAGS) -o $@ -c $^

         scan.o: scan.c
                 $(CC) $(CPPFLAGS) $(CFLAGS) -o $@ -c $^

         scan.c: scan.l
                 $(LEX) $(LFLAGS) -o $@ $^

         clean:
                 $(RM) *.o scan.c

   Notice in the above example that `scan.c' is in the `clean' target.
This is because we consider the file `scan.c' to be an intermediate
file.

   Finally, we provide a realistic example of a `flex' scanner used
with a `bison' parser(3).  There is a tricky problem we have to deal
with. Since a `flex' scanner will typically include a header file
(e.g., `y.tab.h') generated by the parser, we need to be sure that the
header file is generated BEFORE the scanner is compiled. We handle this
case in the following example:


         # Makefile example -- scanner and parser.
         # Creates "myprogram" from "scan.l", "parse.y", and "myprogram.c"
         #
         LEX     = flex
         YACC    = bison -y
         YFLAGS  = -d
         objects = scan.o parse.o myprogram.o

         myprogram: $(objects)
         scan.o: scan.l parse.c
         parse.o: parse.y
         myprogram.o: myprogram.c

   In the above example, notice the line,


         scan.o: scan.l parse.c

   , which lists the file `parse.c' (the generated parser) as a
dependency of `scan.o'. We want to ensure that the parser is created
before the scanner is compiled, and the above line seems to do the
trick. Feel free to experiment with your specific implementation of
`make'.

   For more details on writing Makefiles, see *Note Top: (make)Top.

   ---------- Footnotes ----------

   (1) GNU `make' and GNU `automake' are two such programs that provide
implicit rules for flex-generated scanners.

   (2) GNU `automake' may generate code to execute flex in
lex-compatible mode, or to stdout. If this is not what you want, then
you should provide an explicit rule in your Makefile.am

   (3) This example also applies to yacc parsers.

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File: flex.info,  Node: Bison Bridge,  Next: M4 Dependency,  Prev: Makefiles and Flex,  Up: Appendices

A.2 C Scanners with Bison Parsers
=================================

This section describes the `flex' features useful when integrating
`flex' with `GNU bison'(1).  Skip this section if you are not using
`bison' with your scanner.  Here we discuss only the `flex' half of the
`flex' and `bison' pair.  We do not discuss `bison' in any detail.  For
more information about generating `bison' parsers, see *Note Top:
(bison)Top.

   A compatible `bison' scanner is generated by declaring `%option
bison-bridge' or by supplying `--bison-bridge' when invoking `flex'
from the command line.  This instructs `flex' that the macro `yylval'
may be used. The data type for `yylval', `YYSTYPE', is typically
defined in a header file, included in section 1 of the `flex' input
file.  For a list of functions and macros available, *Note
bison-functions::.

   The declaration of yylex becomes,


           int yylex ( YYSTYPE * lvalp, yyscan_t scanner );

   If `%option bison-locations' is specified, then the declaration
becomes,


           int yylex ( YYSTYPE * lvalp, YYLTYPE * llocp, yyscan_t scanner );

   Note that the macros `yylval' and `yylloc' evaluate to pointers.
Support for `yylloc' is optional in `bison', so it is optional in
`flex' as well. The following is an example of a `flex' scanner that is
compatible with `bison'.


         /* Scanner for "C" assignment statements... sort of. */
         %{
         #include "y.tab.h"  /* Generated by bison. */
         %}

         %option bison-bridge bison-locations
         %

         [[:digit:]]+  { yylval->num = atoi(yytext);   return NUMBER;}
         [[:alnum:]]+  { yylval->str = strdup(yytext); return STRING;}
         "="|";"       { return yytext[0];}
         .  {}
         %

   As you can see, there really is no magic here. We just use `yylval'
as we would any other variable. The data type of `yylval' is generated
by `bison', and included in the file `y.tab.h'. Here is the
corresponding `bison' parser:


         /* Parser to convert "C" assignments to lisp. */
         %{
         /* Pass the argument to yyparse through to yylex. */
         #define YYPARSE_PARAM scanner
         #define YYLEX_PARAM   scanner
         %}
         %locations
         %pure_parser
         %union {
             int num;
             char* str;
         }
         %token <str> STRING
         %token <num> NUMBER
         %%
         assignment:
             STRING '=' NUMBER ';' {
                 printf( "(setf %s %d)", $1, $3 );
            }
         ;

   ---------- Footnotes ----------

   (1) The features described here are purely optional, and are by no
means the only way to use flex with bison.  We merely provide some glue
to ease development of your parser-scanner pair.

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File: flex.info,  Node: M4 Dependency,  Next: Common Patterns,  Prev: Bison Bridge,  Up: Appendices

A.3 M4 Dependency
=================

The macro processor `m4'(1) must be installed wherever flex is
installed.  `flex' invokes `m4', found by searching the directories in
the `PATH' environment variable. Any code you place in section 1 or in
the actions will be sent through m4. Please follow these rules to
protect your code from unwanted `m4' processing.

   * Do not use symbols that begin with, `m4_', such as, `m4_define',
     or `m4_include', since those are reserved for `m4' macro names. If
     for some reason you need m4_ as a prefix, use a preprocessor
     #define to get your symbol past m4 unmangled.

   * Do not use the strings `[[' or `]]' anywhere in your code. The
     former is not valid in C, except within comments and strings, but
     the latter is valid in code such as `x[y[z]]'. The solution is
     simple. To get the literal string `"]]"', use `"]""]"'. To get the
     array notation `x[y[z]]', use `x[y[z] ]'. Flex will attempt to
     detect these sequences in user code, and escape them. However,
     it's best to avoid this complexity where possible, by removing
     such sequences from your code.


   `m4' is only required at the time you run `flex'. The generated
scanner is ordinary C or C++, and does _not_ require `m4'.

   ---------- Footnotes ----------

   (1) The use of m4 is subject to change in future revisions of flex.
It is not part of the public API of flex. Do not depend on it.

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File: flex.info,  Node: Common Patterns,  Prev: M4 Dependency,  Up: Appendices

A.4 Common Patterns
===================

This appendix provides examples of common regular expressions you might
use in your scanner.

* Menu:

* Numbers::
* Identifiers::
* Quoted Constructs::
* Addresses::

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File: flex.info,  Node: Numbers,  Next: Identifiers,  Up: Common Patterns

A.4.1 Numbers
-------------

C99 decimal constant
     `([[:digit:]]{-}[0])[[:digit:]]*'

C99 hexadecimal constant
     `0[xX][[:xdigit:]]+'

C99 octal constant
     `0[0123456]*'

C99 floating point constant

      {dseq}      ([[:digit:]]+)
      {dseq_opt}  ([[:digit:]]*)
      {frac}      (({dseq_opt}"."{dseq})|{dseq}".")
      {exp}       ([eE][+-]?{dseq})
      {exp_opt}   ({exp}?)
      {fsuff}     [flFL]
      {fsuff_opt} ({fsuff}?)
      {hpref}     (0[xX])
      {hdseq}     ([[:xdigit:]]+)
      {hdseq_opt} ([[:xdigit:]]*)
      {hfrac}     (({hdseq_opt}"."{hdseq})|({hdseq}"."))
      {bexp}      ([pP][+-]?{dseq})
      {dfc}       (({frac}{exp_opt}{fsuff_opt})|({dseq}{exp}{fsuff_opt}))
      {hfc}       (({hpref}{hfrac}{bexp}{fsuff_opt})|({hpref}{hdseq}{bexp}{fsuff_opt}))

      {c99_floating_point_constant}  ({dfc}|{hfc})

     See C99 section 6.4.4.2 for the gory details.


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File: flex.info,  Node: Identifiers,  Next: Quoted Constructs,  Prev: Numbers,  Up: Common Patterns

A.4.2 Identifiers
-----------------

C99 Identifier

     ucn        ((\\u([[:xdigit:]]{4}))|(\\U([[:xdigit:]]{8})))
     nondigit    [_[:alpha:]]
     c99_id     ([_[:alpha:]]|{ucn})([_[:alnum:]]|{ucn})*

     Technically, the above pattern does not encompass all possible C99
     identifiers, since C99 allows for "implementation-defined"
     characters. In practice, C compilers follow the above pattern,
     with the addition of the `$' character.

UTF-8 Encoded Unicode Code Point

     [\x09\x0A\x0D\x20-\x7E]|[\xC2-\xDF][\x80-\xBF]|\xE0[\xA0-\xBF][\x80-\xBF]|[\xE1-\xEC\xEE\xEF]([\x80-\xBF]{2})|\xED[\x80-\x9F][\x80-\xBF]|\xF0[\x90-\xBF]([\x80-\xBF]{2})|[\xF1-\xF3]([\x80-\xBF]{3})|\xF4[\x80-\x8F]([\x80-\xBF]{2})


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File: flex.info,  Node: Quoted Constructs,  Next: Addresses,  Prev: Identifiers,  Up: Common Patterns

A.4.3 Quoted Constructs
-----------------------

C99 String Literal
     `L?\"([^\"\\\n]|(\\['\"?\\abfnrtv])|(\\([0123456]{1,3}))|(\\x[[:xdigit:]]+)|(\\u([[:xdigit:]]{4}))|(\\U([[:xdigit:]]{8})))*\"'

C99 Comment
     `("/*"([^*]|"*"[^/])*"*/")|("/"(\\\n)*"/"[^\n]*)'

     Note that in C99, a `//'-style comment may be split across lines,
     and, contrary to popular belief, does not include the trailing
     `\n' character.

     A better way to scan `/* */' comments is by line, rather than
     matching possibly huge comments all at once. This will allow you
     to scan comments of unlimited length, as long as line breaks
     appear at sane intervals. This is also more efficient when used
     with automatic line number processing. *Note option-yylineno::.


     <INITIAL>{
         "/*"      BEGIN(COMMENT);
     }
     <COMMENT>{
         "*/"      BEGIN(0);
         [^*\n]+   ;
         "*"[^/]   ;
         \n        ;
     }


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File: flex.info,  Node: Addresses,  Prev: Quoted Constructs,  Up: Common Patterns

A.4.4 Addresses
---------------

IPv4 Address
     `(([[:digit:]]{1,3}"."){3}([[:digit:]]{1,3}))'

IPv6 Address

     hex4         ([[:xdigit:]]{1,4})
     hexseq       ({hex4}(:{hex4}*))
     hexpart      ({hexseq}|({hexseq}::({hexseq}?))|::{hexseq})
     IPv6address  ({hexpart}(":"{IPv4address})?)

     See RFC2373 for details.

URI
     `(([^:/?#]+):)?("//"([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?'

     This pattern is nearly useless, since it allows just about any
     character to appear in a URI, including spaces and control
     characters.  See RFC2396 for details.


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File: flex.info,  Node: Indices,  Prev: Appendices,  Up: Top

Indices
*******

* Menu:

* Concept Index::
* Index of Functions and Macros::
* Index of Variables::
* Index of Data Types::
* Index of Hooks::
* Index of Scanner Options::