u-tools/apps/tar/gnu/verify.h

   1 /* -*- buffer-read-only: t -*- vi: set ro: */
   2 /* DO NOT EDIT! GENERATED AUTOMATICALLY! */
   3 /* Compile-time assert-like macros.
   4
   5    Copyright (C) 2005-2006, 2009-2011 Free Software Foundation, Inc.
   6
   7    This program is free software: you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */
  21
  22 #ifndef VERIFY_H
  23 # define VERIFY_H 1
  24
  25 /* Each of these macros verifies that its argument R is nonzero.  To
  26    be portable, R should be an integer constant expression.  Unlike
  27    assert (R), there is no run-time overhead.
  28
  29    There are two macros, since no single macro can be used in all
  30    contexts in C.  verify_true (R) is for scalar contexts, including
  31    integer constant expression contexts.  verify (R) is for declaration
  32    contexts, e.g., the top level.
  33
  34    Symbols ending in "__" are private to this header.
  35
  36    The code below uses several ideas.
  37
  38    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  39      integral or boolean or floating-point type, this yields an
  40      expression of integral type, whose value is later verified to be
  41      constant and nonnegative.
  42
  43    * Next this expression W is wrapped in a type
  44      struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
  45      If W is negative, this yields a compile-time error.  No compiler can
  46      deal with a bit-field of negative size.
  47
  48      One might think that an array size check would have the same
  49      effect, that is, that the type struct { unsigned int dummy[W]; }
  50      would work as well.  However, inside a function, some compilers
  51      (such as C++ compilers and GNU C) allow local parameters and
  52      variables inside array size expressions.  With these compilers,
  53      an array size check would not properly diagnose this misuse of
  54      the verify macro:
  55
  56        void function (int n) { verify (n < 0); }
  57
  58    * For the verify macro, the struct verify_type__ will need to
  59      somehow be embedded into a declaration.  To be portable, this
  60      declaration must declare an object, a constant, a function, or a
  61      typedef name.  If the declared entity uses the type directly,
  62      such as in
  63
  64        struct dummy {...};
  65        typedef struct {...} dummy;
  66        extern struct {...} *dummy;
  67        extern void dummy (struct {...} *);
  68        extern struct {...} *dummy (void);
  69
  70      two uses of the verify macro would yield colliding declarations
  71      if the entity names are not disambiguated.  A workaround is to
  72      attach the current line number to the entity name:
  73
  74        #define _GL_CONCAT0(x, y) x##y
  75        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
  76        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
  77
  78      But this has the problem that two invocations of verify from
  79      within the same macro would collide, since the __LINE__ value
  80      would be the same for both invocations.  (The GCC __COUNTER__
  81      macro solves this problem, but is not portable.)
  82
  83      A solution is to use the sizeof operator.  It yields a number,
  84      getting rid of the identity of the type.  Declarations like
  85
  86        extern int dummy [sizeof (struct {...})];
  87        extern void dummy (int [sizeof (struct {...})]);
  88        extern int (*dummy (void)) [sizeof (struct {...})];
  89
  90      can be repeated.
  91
  92    * Should the implementation use a named struct or an unnamed struct?
  93      Which of the following alternatives can be used?
  94
  95        extern int dummy [sizeof (struct {...})];
  96        extern int dummy [sizeof (struct verify_type__ {...})];
  97        extern void dummy (int [sizeof (struct {...})]);
  98        extern void dummy (int [sizeof (struct verify_type__ {...})]);
  99        extern int (*dummy (void)) [sizeof (struct {...})];
 100        extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];
 101
 102      In the second and sixth case, the struct type is exported to the
 103      outer scope; two such declarations therefore collide.  GCC warns
 104      about the first, third, and fourth cases.  So the only remaining
 105      possibility is the fifth case:
 106
 107        extern int (*dummy (void)) [sizeof (struct {...})];
 108
 109    * GCC warns about duplicate declarations of the dummy function if
 110      -Wredundant_decls is used.  GCC 4.3 and later have a builtin
 111      __COUNTER__ macro that can let us generate unique identifiers for
 112      each dummy function, to suppress this warning.
 113
 114    * This implementation exploits the fact that GCC does not warn about
 115      the last declaration mentioned above.  If a future version of GCC
 116      introduces a warning for this, the problem could be worked around
 117      by using code specialized to GCC, just as __COUNTER__ is already
 118      being used if available.
 119
 120        #if 4 <= __GNUC__
 121        # define verify(R) [another version to keep GCC happy]
 122        #endif
 123
 124    * In C++, any struct definition inside sizeof is invalid.
 125      Use a template type to work around the problem.  */
 126
 127 /* Concatenate two preprocessor tokens.  */
 128 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 129 # define _GL_CONCAT0(x, y) x##y
 130
 131 /* _GL_COUNTER is an integer, preferably one that changes each time we
 132    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 133    otherwise.  __LINE__ isn't perfect, but it's better than a
 134    constant.  */
 135 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 136 #  define _GL_COUNTER __COUNTER__
 137 # else
 138 #  define _GL_COUNTER __LINE__
 139 # endif
 140
 141 /* Generate a symbol with the given prefix, making it unique if
 142    possible.  */
 143 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 144
 145 /* Verify requirement R at compile-time, as an integer constant expression.
 146    Return 1.  */
 147
 148 # ifdef __cplusplus
 149 template <int w>
 150   struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
 151 #  define verify_true(R) \
 152      (!!sizeof (verify_type__<(R) ? 1 : -1>))
 153 # else
 154 #  define verify_true(R) \
 155      (!!sizeof \
 156       (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
 157 # endif
 158
 159 /* Verify requirement R at compile-time, as a declaration without a
 160    trailing ';'.  */
 161
 162 # define verify(R) \
 163     extern int (* _GL_GENSYM (verify_function) (void)) [verify_true (R)]
 164
 165 #endif