C/the.ansi.c.programming.language/c.programming.notes/sx14e.html

   1 <!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
   2 <!-- This collection of hypertext pages is Copyright 1995, 1996 by Steve Summit. -->
   3 <!-- This material may be freely redistributed and used -->
   4 <!-- but may not be republished or sold without permission. -->
   5 <html>
   6 <head>
   7 <link rev="owner" href="mailto:scs@eskimo.com">
   8 <link rev="made" href="mailto:scs@eskimo.com">
   9 <title>C Preprocessor</title>
  10 <link href="sx14d.html" rev=precedes>
  11 <link href="sx14f.html" rel=precedes>
  12 <link href="sx14.html" rev=subdocument>
  13 </head>
  14 <body>
  15 <H2>C Preprocessor</H2>
  16
  17 <p>If you're careful,
  18 it's possible
  19 (and
  20 can be
  21 useful)
  22 to use <TT>#include</TT> within a header file,
  23 so that you end up with ``nested header files.''
  24 </p><p>It's possible to use <TT>#define</TT>
  25 to define ``function-like'' macros
  26 that accept arguments;
  27 the expansion of the macro can therefore depend on the
  28 arguments it's ``invoked'' with.
  29 </p><p>Two special preprocessing operators <TT>#</TT> and <TT>##</TT>
  30 let you control the expansion of macro arguments
  31 in fancier ways.
  32 </p><p>The preprocessor directive <TT>#if</TT> lets you conditionally
  33 include (or, with <TT>#else</TT>, conditionally not include)
  34 a section of code depending on some arbitrary compile-time expression.
  35 (<TT>#if</TT> can also do the same macro-definedness tests
  36 as <TT>#ifdef</TT> and <TT>#ifndef</TT>,
  37 because the expression can
  38 use
  39 a <TT>defined()</TT> operator.)
  40 </p><p>Other preprocessing directives are
  41 <TT>#elif</TT>, <TT>#error</TT>,
  42 <TT>#line</TT>, and <TT>#pragma</TT>.
  43 </p><p>There are a few predefined preprocessor macros,
  44 some required by the C standard,
  45 others perhaps defined by
  46 particular compilation environments.
  47 These are useful for conditional compilation
  48 (<TT>#ifdef</TT>, <TT>#ifndef</TT>).
  49 </p><hr>
  50 <p>
  51 Read sequentially:
  52 <a href="sx14d.html" rev=precedes>prev</a>
  53 <a href="sx14f.html" rel=precedes>next</a>
  54 <a href="sx14.html" rev=subdocument>up</a>
  55 <a href="top.html">top</a>
  56 </p>
  57 <p>
  58 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
  59 // <a href="copyright.html">Copyright</a> 1995, 1996
  60 // <a href="mailto:scs@eskimo.com">mail feedback</a>
  61 </p>
  62 </body>
  63 </html>