lib/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2014 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__ + (6 <= __GNUC_MINOR__) \
  35      && (201112L <= __STDC_VERSION__  || !defined __STRICT_ANSI__) \
  36      && !defined __cplusplus)
  37 # define _GL_HAVE__STATIC_ASSERT 1
  38 #endif
  39 /* The condition (99 < __GNUC__) is temporary, until we know about the
  40    first G++ release that supports static_assert.  */
  41 #if (99 < __GNUC__) && defined __cplusplus
  42 # define _GL_HAVE_STATIC_ASSERT 1
  43 #endif
  44
  45 /* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other
  46    system headers, defines a conflicting _Static_assert that is no
  47    better than ours; override it.  */
  48 #ifndef _GL_HAVE_STATIC_ASSERT
  49 # include <stddef.h>
  50 # undef _Static_assert
  51 #endif
  52
  53 /* Each of these macros verifies that its argument R is nonzero.  To
  54    be portable, R should be an integer constant expression.  Unlike
  55    assert (R), there is no run-time overhead.
  56
  57    If _Static_assert works, verify (R) uses it directly.  Similarly,
  58    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
  59    that is an operand of sizeof.
  60
  61    The code below uses several ideas for C++ compilers, and for C
  62    compilers that do not support _Static_assert:
  63
  64    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  65      integral or boolean or floating-point type, this yields an
  66      expression of integral type, whose value is later verified to be
  67      constant and nonnegative.
  68
  69    * Next this expression W is wrapped in a type
  70      struct _gl_verify_type {
  71        unsigned int _gl_verify_error_if_negative: W;
  72      }.
  73      If W is negative, this yields a compile-time error.  No compiler can
  74      deal with a bit-field of negative size.
  75
  76      One might think that an array size check would have the same
  77      effect, that is, that the type struct { unsigned int dummy[W]; }
  78      would work as well.  However, inside a function, some compilers
  79      (such as C++ compilers and GNU C) allow local parameters and
  80      variables inside array size expressions.  With these compilers,
  81      an array size check would not properly diagnose this misuse of
  82      the verify macro:
  83
  84        void function (int n) { verify (n < 0); }
  85
  86    * For the verify macro, the struct _gl_verify_type will need to
  87      somehow be embedded into a declaration.  To be portable, this
  88      declaration must declare an object, a constant, a function, or a
  89      typedef name.  If the declared entity uses the type directly,
  90      such as in
  91
  92        struct dummy {...};
  93        typedef struct {...} dummy;
  94        extern struct {...} *dummy;
  95        extern void dummy (struct {...} *);
  96        extern struct {...} *dummy (void);
  97
  98      two uses of the verify macro would yield colliding declarations
  99      if the entity names are not disambiguated.  A workaround is to
 100      attach the current line number to the entity name:
 101
 102        #define _GL_CONCAT0(x, y) x##y
 103        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 104        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
 105
 106      But this has the problem that two invocations of verify from
 107      within the same macro would collide, since the __LINE__ value
 108      would be the same for both invocations.  (The GCC __COUNTER__
 109      macro solves this problem, but is not portable.)
 110
 111      A solution is to use the sizeof operator.  It yields a number,
 112      getting rid of the identity of the type.  Declarations like
 113
 114        extern int dummy [sizeof (struct {...})];
 115        extern void dummy (int [sizeof (struct {...})]);
 116        extern int (*dummy (void)) [sizeof (struct {...})];
 117
 118      can be repeated.
 119
 120    * Should the implementation use a named struct or an unnamed struct?
 121      Which of the following alternatives can be used?
 122
 123        extern int dummy [sizeof (struct {...})];
 124        extern int dummy [sizeof (struct _gl_verify_type {...})];
 125        extern void dummy (int [sizeof (struct {...})]);
 126        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
 127        extern int (*dummy (void)) [sizeof (struct {...})];
 128        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
 129
 130      In the second and sixth case, the struct type is exported to the
 131      outer scope; two such declarations therefore collide.  GCC warns
 132      about the first, third, and fourth cases.  So the only remaining
 133      possibility is the fifth case:
 134
 135        extern int (*dummy (void)) [sizeof (struct {...})];
 136
 137    * GCC warns about duplicate declarations of the dummy function if
 138      -Wredundant-decls is used.  GCC 4.3 and later have a builtin
 139      __COUNTER__ macro that can let us generate unique identifiers for
 140      each dummy function, to suppress this warning.
 141
 142    * This implementation exploits the fact that older versions of GCC,
 143      which do not support _Static_assert, also do not warn about the
 144      last declaration mentioned above.
 145
 146    * GCC warns if -Wnested-externs is enabled and verify() is used
 147      within a function body; but inside a function, you can always
 148      arrange to use verify_expr() instead.
 149
 150    * In C++, any struct definition inside sizeof is invalid.
 151      Use a template type to work around the problem.  */
 152
 153 /* Concatenate two preprocessor tokens.  */
 154 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 155 #define _GL_CONCAT0(x, y) x##y
 156
 157 /* _GL_COUNTER is an integer, preferably one that changes each time we
 158    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 159    otherwise.  __LINE__ isn't perfect, but it's better than a
 160    constant.  */
 161 #if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 162 # define _GL_COUNTER __COUNTER__
 163 #else
 164 # define _GL_COUNTER __LINE__
 165 #endif
 166
 167 /* Generate a symbol with the given prefix, making it unique if
 168    possible.  */
 169 #define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 170
 171 /* Verify requirement R at compile-time, as an integer constant expression
 172    that returns 1.  If R is false, fail at compile-time, preferably
 173    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
 174
 175 #define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
 176    (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
 177
 178 #ifdef __cplusplus
 179 # if !GNULIB_defined_struct__gl_verify_type
 180 template <int w>
 181   struct _gl_verify_type {
 182     unsigned int _gl_verify_error_if_negative: w;
 183   };
 184 #  define GNULIB_defined_struct__gl_verify_type 1
 185 # endif
 186 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 187     _gl_verify_type<(R) ? 1 : -1>
 188 #elif defined _GL_HAVE__STATIC_ASSERT
 189 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 190     struct {                                   \
 191       _Static_assert (R, DIAGNOSTIC);          \
 192       int _gl_dummy;                          \
 193     }
 194 #else
 195 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 196     struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
 197 #endif
 198
 199 /* Verify requirement R at compile-time, as a declaration without a
 200    trailing ';'.  If R is false, fail at compile-time, preferably
 201    with a diagnostic that includes the string-literal DIAGNOSTIC.
 202
 203    Unfortunately, unlike C11, this implementation must appear as an
 204    ordinary declaration, and cannot appear inside struct { ... }.  */
 205
 206 #ifdef _GL_HAVE__STATIC_ASSERT
 207 # define _GL_VERIFY _Static_assert
 208 #else
 209 # define _GL_VERIFY(R, DIAGNOSTIC)                                     \
 210     extern int (*_GL_GENSYM (_gl_verify_function) (void))              \
 211       [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
 212 #endif
 213
 214 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
 215 #ifdef _GL_STATIC_ASSERT_H
 216 # if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
 217 #  define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
 218 # endif
 219 # if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
 220 #  define static_assert _Static_assert /* C11 requires this #define.  */
 221 # endif
 222 #endif
 223
 224 /* @assert.h omit start@  */
 225
 226 /* Each of these macros verifies that its argument R is nonzero.  To
 227    be portable, R should be an integer constant expression.  Unlike
 228    assert (R), there is no run-time overhead.
 229
 230    There are two macros, since no single macro can be used in all
 231    contexts in C.  verify_true (R) is for scalar contexts, including
 232    integer constant expression contexts.  verify (R) is for declaration
 233    contexts, e.g., the top level.  */
 234
 235 /* Verify requirement R at compile-time, as an integer constant expression.
 236    Return 1.  This is equivalent to verify_expr (R, 1).
 237
 238    verify_true is obsolescent; please use verify_expr instead.  */
 239
 240 #define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
 241
 242 /* Verify requirement R at compile-time.  Return the value of the
 243    expression E.  */
 244
 245 #define verify_expr(R, E) \
 246    (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
 247
 248 /* Verify requirement R at compile-time, as a declaration without a
 249    trailing ';'.  */
 250
 251 #define verify(R) _GL_VERIFY (R, "verify (" #R ")")
 252
 253 #ifndef __has_builtin
 254 # define __has_builtin(x) 0
 255 #endif
 256
 257 /* Assume that R always holds.  This lets the compiler optimize
 258    accordingly.  R should not have side-effects; it may or may not be
 259    evaluated.  Behavior is undefined if R is false.  */
 260
 261 #if (__has_builtin (__builtin_unreachable) \
 262      || 4 < __GNUC__ + (5 <= __GNUC_MINOR__))
 263 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
 264 #elif 1200 <= _MSC_VER
 265 # define assume(R) __assume (R)
 266 #elif (defined lint \
 267        && (__has_builtin (__builtin_trap) \
 268            || 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))))
 269   /* Doing it this way helps various packages when configured with
 270      --enable-gcc-warnings, which compiles with -Dlint.  It's nicer
 271      when 'assume' silences warnings even with older GCCs.  */
 272 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
 273 #else
 274 # define assume(R) ((void) (0 && (R)))
 275 #endif
 276
 277 /* @assert.h omit end@  */
 278
 279 #endif