gl/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2010 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */
  19
  20 #ifndef VERIFY_H
  21 # define VERIFY_H 1
  22
  23 /* Each of these macros verifies that its argument R is nonzero.  To
  24    be portable, R should be an integer constant expression.  Unlike
  25    assert (R), there is no run-time overhead.
  26
  27    There are two macros, since no single macro can be used in all
  28    contexts in C.  verify_true (R) is for scalar contexts, including
  29    integer constant expression contexts.  verify (R) is for declaration
  30    contexts, e.g., the top level.
  31
  32    Symbols ending in "__" are private to this header.
  33
  34    The code below uses several ideas.
  35
  36    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  37      integral or boolean or floating-point type, this yields an
  38      expression of integral type, whose value is later verified to be
  39      constant and nonnegative.
  40
  41    * Next this expression W is wrapped in a type
  42      struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
  43      If W is negative, this yields a compile-time error.  No compiler can
  44      deal with a bit-field of negative size.
  45
  46      One might think that an array size check would have the same
  47      effect, that is, that the type struct { unsigned int dummy[W]; }
  48      would work as well.  However, inside a function, some compilers
  49      (such as C++ compilers and GNU C) allow local parameters and
  50      variables inside array size expressions.  With these compilers,
  51      an array size check would not properly diagnose this misuse of
  52      the verify macro:
  53
  54        void function (int n) { verify (n < 0); }
  55
  56    * For the verify macro, the struct verify_type__ will need to
  57      somehow be embedded into a declaration.  To be portable, this
  58      declaration must declare an object, a constant, a function, or a
  59      typedef name.  If the declared entity uses the type directly,
  60      such as in
  61
  62        struct dummy {...};
  63        typedef struct {...} dummy;
  64        extern struct {...} *dummy;
  65        extern void dummy (struct {...} *);
  66        extern struct {...} *dummy (void);
  67
  68      two uses of the verify macro would yield colliding declarations
  69      if the entity names are not disambiguated.  A workaround is to
  70      attach the current line number to the entity name:
  71
  72        #define _GL_CONCAT0(x, y) x##y
  73        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
  74        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
  75
  76      But this has the problem that two invocations of verify from
  77      within the same macro would collide, since the __LINE__ value
  78      would be the same for both invocations.  (The GCC __COUNTER__
  79      macro solves this problem, but is not portable.)
  80
  81      A solution is to use the sizeof operator.  It yields a number,
  82      getting rid of the identity of the type.  Declarations like
  83
  84        extern int dummy [sizeof (struct {...})];
  85        extern void dummy (int [sizeof (struct {...})]);
  86        extern int (*dummy (void)) [sizeof (struct {...})];
  87
  88      can be repeated.
  89
  90    * Should the implementation use a named struct or an unnamed struct?
  91      Which of the following alternatives can be used?
  92
  93        extern int dummy [sizeof (struct {...})];
  94        extern int dummy [sizeof (struct verify_type__ {...})];
  95        extern void dummy (int [sizeof (struct {...})]);
  96        extern void dummy (int [sizeof (struct verify_type__ {...})]);
  97        extern int (*dummy (void)) [sizeof (struct {...})];
  98        extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];
  99
 100      In the second and sixth case, the struct type is exported to the
 101      outer scope; two such declarations therefore collide.  GCC warns
 102      about the first, third, and fourth cases.  So the only remaining
 103      possibility is the fifth case:
 104
 105        extern int (*dummy (void)) [sizeof (struct {...})];
 106
 107    * GCC warns about duplicate declarations of the dummy function if
 108      -Wredundant_decls is used.  GCC 4.3 and later have a builtin
 109      __COUNTER__ macro that can let us generate unique identifiers for
 110      each dummy function, to suppress this warning.
 111
 112    * This implementation exploits the fact that GCC does not warn about
 113      the last declaration mentioned above.  If a future version of GCC
 114      introduces a warning for this, the problem could be worked around
 115      by using code specialized to GCC, just as __COUNTER__ is already
 116      being used if available.
 117
 118        #if 4 <= __GNUC__
 119        # define verify(R) [another version to keep GCC happy]
 120        #endif
 121
 122    * In C++, any struct definition inside sizeof is invalid.
 123      Use a template type to work around the problem.  */
 124
 125 /* Concatenate two preprocessor tokens.  */
 126 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 127 # define _GL_CONCAT0(x, y) x##y
 128
 129 /* _GL_COUNTER is an integer, preferably one that changes each time we
 130    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 131    otherwise.  __LINE__ isn't perfect, but it's better than a
 132    constant.  */
 133 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 134 #  define _GL_COUNTER __COUNTER__
 135 # else
 136 #  define _GL_COUNTER __LINE__
 137 # endif
 138
 139 /* Generate a symbol with the given prefix, making it unique if
 140    possible.  */
 141 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 142
 143 /* Verify requirement R at compile-time, as an integer constant expression.
 144    Return 1.  */
 145
 146 # ifdef __cplusplus
 147 template <int w>
 148   struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
 149 #  define verify_true(R) \
 150      (!!sizeof (verify_type__<(R) ? 1 : -1>))
 151 # else
 152 #  define verify_true(R) \
 153      (!!sizeof \
 154       (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
 155 # endif
 156
 157 /* Verify requirement R at compile-time, as a declaration without a
 158    trailing ';'.  */
 159
 160 # define verify(R) \
 161     extern int (* _GL_GENSYM (verify_function) (void)) [verify_true (R)]
 162
 163 #endif