gl/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2012 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 _GL_VERIFY_H
  21 # define _GL_VERIFY_H
  22
  23
  24 /* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert works as per C11.
  25    This is supported by GCC 4.6.0 and later, in C mode, and its use
  26    here generates easier-to-read diagnostics when verify (R) fails.
  27
  28    Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per C++11.
  29    This will likely be supported by future GCC versions, in C++ mode.
  30
  31    Use this only with GCC.  If we were willing to slow 'configure'
  32    down we could also use it with other compilers, but since this
  33    affects only the quality of diagnostics, why bother?  */
  34 # if (4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)) && !defined __cplusplus
  35 #  define _GL_HAVE__STATIC_ASSERT 1
  36 # endif
  37 /* The condition (99 < __GNUC__) is temporary, until we know about the
  38    first G++ release that supports static_assert.  */
  39 # if (99 < __GNUC__) && defined __cplusplus
  40 #  define _GL_HAVE_STATIC_ASSERT 1
  41 # endif
  42
  43 /* Each of these macros verifies that its argument R is nonzero.  To
  44    be portable, R should be an integer constant expression.  Unlike
  45    assert (R), there is no run-time overhead.
  46
  47    If _Static_assert works, verify (R) uses it directly.  Similarly,
  48    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
  49    that is an operand of sizeof.
  50
  51    The code below uses several ideas for C++ compilers, and for C
  52    compilers that do not support _Static_assert:
  53
  54    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  55      integral or boolean or floating-point type, this yields an
  56      expression of integral type, whose value is later verified to be
  57      constant and nonnegative.
  58
  59    * Next this expression W is wrapped in a type
  60      struct _gl_verify_type {
  61        unsigned int _gl_verify_error_if_negative: W;
  62      }.
  63      If W is negative, this yields a compile-time error.  No compiler can
  64      deal with a bit-field of negative size.
  65
  66      One might think that an array size check would have the same
  67      effect, that is, that the type struct { unsigned int dummy[W]; }
  68      would work as well.  However, inside a function, some compilers
  69      (such as C++ compilers and GNU C) allow local parameters and
  70      variables inside array size expressions.  With these compilers,
  71      an array size check would not properly diagnose this misuse of
  72      the verify macro:
  73
  74        void function (int n) { verify (n < 0); }
  75
  76    * For the verify macro, the struct _gl_verify_type will need to
  77      somehow be embedded into a declaration.  To be portable, this
  78      declaration must declare an object, a constant, a function, or a
  79      typedef name.  If the declared entity uses the type directly,
  80      such as in
  81
  82        struct dummy {...};
  83        typedef struct {...} dummy;
  84        extern struct {...} *dummy;
  85        extern void dummy (struct {...} *);
  86        extern struct {...} *dummy (void);
  87
  88      two uses of the verify macro would yield colliding declarations
  89      if the entity names are not disambiguated.  A workaround is to
  90      attach the current line number to the entity name:
  91
  92        #define _GL_CONCAT0(x, y) x##y
  93        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
  94        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
  95
  96      But this has the problem that two invocations of verify from
  97      within the same macro would collide, since the __LINE__ value
  98      would be the same for both invocations.  (The GCC __COUNTER__
  99      macro solves this problem, but is not portable.)
 100
 101      A solution is to use the sizeof operator.  It yields a number,
 102      getting rid of the identity of the type.  Declarations like
 103
 104        extern int dummy [sizeof (struct {...})];
 105        extern void dummy (int [sizeof (struct {...})]);
 106        extern int (*dummy (void)) [sizeof (struct {...})];
 107
 108      can be repeated.
 109
 110    * Should the implementation use a named struct or an unnamed struct?
 111      Which of the following alternatives can be used?
 112
 113        extern int dummy [sizeof (struct {...})];
 114        extern int dummy [sizeof (struct _gl_verify_type {...})];
 115        extern void dummy (int [sizeof (struct {...})]);
 116        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
 117        extern int (*dummy (void)) [sizeof (struct {...})];
 118        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
 119
 120      In the second and sixth case, the struct type is exported to the
 121      outer scope; two such declarations therefore collide.  GCC warns
 122      about the first, third, and fourth cases.  So the only remaining
 123      possibility is the fifth case:
 124
 125        extern int (*dummy (void)) [sizeof (struct {...})];
 126
 127    * GCC warns about duplicate declarations of the dummy function if
 128      -Wredundant-decls is used.  GCC 4.3 and later have a builtin
 129      __COUNTER__ macro that can let us generate unique identifiers for
 130      each dummy function, to suppress this warning.
 131
 132    * This implementation exploits the fact that older versions of GCC,
 133      which do not support _Static_assert, also do not warn about the
 134      last declaration mentioned above.
 135
 136    * GCC warns if -Wnested-externs is enabled and verify() is used
 137      within a function body; but inside a function, you can always
 138      arrange to use verify_expr() instead.
 139
 140    * In C++, any struct definition inside sizeof is invalid.
 141      Use a template type to work around the problem.  */
 142
 143 /* Concatenate two preprocessor tokens.  */
 144 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 145 # define _GL_CONCAT0(x, y) x##y
 146
 147 /* _GL_COUNTER is an integer, preferably one that changes each time we
 148    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 149    otherwise.  __LINE__ isn't perfect, but it's better than a
 150    constant.  */
 151 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 152 #  define _GL_COUNTER __COUNTER__
 153 # else
 154 #  define _GL_COUNTER __LINE__
 155 # endif
 156
 157 /* Generate a symbol with the given prefix, making it unique if
 158    possible.  */
 159 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 160
 161 /* Verify requirement R at compile-time, as an integer constant expression
 162    that returns 1.  If R is false, fail at compile-time, preferably
 163    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
 164
 165 # define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
 166     (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
 167
 168 # ifdef __cplusplus
 169 #  if !GNULIB_defined_struct__gl_verify_type
 170 template <int w>
 171   struct _gl_verify_type {
 172     unsigned int _gl_verify_error_if_negative: w;
 173   };
 174 #   define GNULIB_defined_struct__gl_verify_type 1
 175 #  endif
 176 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 177     _gl_verify_type<(R) ? 1 : -1>
 178 # elif defined _GL_HAVE__STATIC_ASSERT
 179 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 180      struct {                                   \
 181        _Static_assert (R, DIAGNOSTIC);          \
 182        int _gl_dummy;                          \
 183      }
 184 # else
 185 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 186      struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
 187 # endif
 188
 189 /* Verify requirement R at compile-time, as a declaration without a
 190    trailing ';'.  If R is false, fail at compile-time, preferably
 191    with a diagnostic that includes the string-literal DIAGNOSTIC.
 192
 193    Unfortunately, unlike C11, this implementation must appear as an
 194    ordinary declaration, and cannot appear inside struct { ... }.  */
 195
 196 # ifdef _GL_HAVE__STATIC_ASSERT
 197 #  define _GL_VERIFY _Static_assert
 198 # else
 199 #  define _GL_VERIFY(R, DIAGNOSTIC)                                    \
 200      extern int (*_GL_GENSYM (_gl_verify_function) (void))             \
 201        [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
 202 # endif
 203
 204 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
 205 # ifdef _GL_STATIC_ASSERT_H
 206 #  if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
 207 #   define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
 208 #  endif
 209 #  if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
 210 #   define static_assert _Static_assert /* C11 requires this #define.  */
 211 #  endif
 212 # endif
 213
 214 /* @assert.h omit start@  */
 215
 216 /* Each of these macros verifies that its argument R is nonzero.  To
 217    be portable, R should be an integer constant expression.  Unlike
 218    assert (R), there is no run-time overhead.
 219
 220    There are two macros, since no single macro can be used in all
 221    contexts in C.  verify_true (R) is for scalar contexts, including
 222    integer constant expression contexts.  verify (R) is for declaration
 223    contexts, e.g., the top level.  */
 224
 225 /* Verify requirement R at compile-time, as an integer constant expression.
 226    Return 1.  This is equivalent to verify_expr (R, 1).
 227
 228    verify_true is obsolescent; please use verify_expr instead.  */
 229
 230 # define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
 231
 232 /* Verify requirement R at compile-time.  Return the value of the
 233    expression E.  */
 234
 235 # define verify_expr(R, E) \
 236     (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
 237
 238 /* Verify requirement R at compile-time, as a declaration without a
 239    trailing ';'.  */
 240
 241 # define verify(R) _GL_VERIFY (R, "verify (" #R ")")
 242
 243 /* @assert.h omit end@  */
 244
 245 #endif