lib/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2019 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 <https://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 is supported by GCC 6.1.0 and later, 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 #if (6 <= __GNUC__) && defined __cplusplus
  40 # define _GL_HAVE_STATIC_ASSERT 1
  41 #endif
  42
  43 /* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other
  44    system headers, defines a conflicting _Static_assert that is no
  45    better than ours; override it.  */
  46 #ifndef _GL_HAVE_STATIC_ASSERT
  47 # include <stddef.h>
  48 # undef _Static_assert
  49 #endif
  50
  51 /* Each of these macros verifies that its argument R is nonzero.  To
  52    be portable, R should be an integer constant expression.  Unlike
  53    assert (R), there is no run-time overhead.
  54
  55    If _Static_assert works, verify (R) uses it directly.  Similarly,
  56    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
  57    that is an operand of sizeof.
  58
  59    The code below uses several ideas for C++ compilers, and for C
  60    compilers that do not support _Static_assert:
  61
  62    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  63      integral or boolean or floating-point type, this yields an
  64      expression of integral type, whose value is later verified to be
  65      constant and nonnegative.
  66
  67    * Next this expression W is wrapped in a type
  68      struct _gl_verify_type {
  69        unsigned int _gl_verify_error_if_negative: W;
  70      }.
  71      If W is negative, this yields a compile-time error.  No compiler can
  72      deal with a bit-field of negative size.
  73
  74      One might think that an array size check would have the same
  75      effect, that is, that the type struct { unsigned int dummy[W]; }
  76      would work as well.  However, inside a function, some compilers
  77      (such as C++ compilers and GNU C) allow local parameters and
  78      variables inside array size expressions.  With these compilers,
  79      an array size check would not properly diagnose this misuse of
  80      the verify macro:
  81
  82        void function (int n) { verify (n < 0); }
  83
  84    * For the verify macro, the struct _gl_verify_type will need to
  85      somehow be embedded into a declaration.  To be portable, this
  86      declaration must declare an object, a constant, a function, or a
  87      typedef name.  If the declared entity uses the type directly,
  88      such as in
  89
  90        struct dummy {...};
  91        typedef struct {...} dummy;
  92        extern struct {...} *dummy;
  93        extern void dummy (struct {...} *);
  94        extern struct {...} *dummy (void);
  95
  96      two uses of the verify macro would yield colliding declarations
  97      if the entity names are not disambiguated.  A workaround is to
  98      attach the current line number to the entity name:
  99
 100        #define _GL_CONCAT0(x, y) x##y
 101        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 102        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
 103
 104      But this has the problem that two invocations of verify from
 105      within the same macro would collide, since the __LINE__ value
 106      would be the same for both invocations.  (The GCC __COUNTER__
 107      macro solves this problem, but is not portable.)
 108
 109      A solution is to use the sizeof operator.  It yields a number,
 110      getting rid of the identity of the type.  Declarations like
 111
 112        extern int dummy [sizeof (struct {...})];
 113        extern void dummy (int [sizeof (struct {...})]);
 114        extern int (*dummy (void)) [sizeof (struct {...})];
 115
 116      can be repeated.
 117
 118    * Should the implementation use a named struct or an unnamed struct?
 119      Which of the following alternatives can be used?
 120
 121        extern int dummy [sizeof (struct {...})];
 122        extern int dummy [sizeof (struct _gl_verify_type {...})];
 123        extern void dummy (int [sizeof (struct {...})]);
 124        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
 125        extern int (*dummy (void)) [sizeof (struct {...})];
 126        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
 127
 128      In the second and sixth case, the struct type is exported to the
 129      outer scope; two such declarations therefore collide.  GCC warns
 130      about the first, third, and fourth cases.  So the only remaining
 131      possibility is the fifth case:
 132
 133        extern int (*dummy (void)) [sizeof (struct {...})];
 134
 135    * GCC warns about duplicate declarations of the dummy function if
 136      -Wredundant-decls is used.  GCC 4.3 and later have a builtin
 137      __COUNTER__ macro that can let us generate unique identifiers for
 138      each dummy function, to suppress this warning.
 139
 140    * This implementation exploits the fact that older versions of GCC,
 141      which do not support _Static_assert, also do not warn about the
 142      last declaration mentioned above.
 143
 144    * GCC warns if -Wnested-externs is enabled and verify() is used
 145      within a function body; but inside a function, you can always
 146      arrange to use verify_expr() instead.
 147
 148    * In C++, any struct definition inside sizeof is invalid.
 149      Use a template type to work around the problem.  */
 150
 151 /* Concatenate two preprocessor tokens.  */
 152 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 153 #define _GL_CONCAT0(x, y) x##y
 154
 155 /* _GL_COUNTER is an integer, preferably one that changes each time we
 156    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 157    otherwise.  __LINE__ isn't perfect, but it's better than a
 158    constant.  */
 159 #if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 160 # define _GL_COUNTER __COUNTER__
 161 #else
 162 # define _GL_COUNTER __LINE__
 163 #endif
 164
 165 /* Generate a symbol with the given prefix, making it unique if
 166    possible.  */
 167 #define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 168
 169 /* Verify requirement R at compile-time, as an integer constant expression
 170    that returns 1.  If R is false, fail at compile-time, preferably
 171    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
 172
 173 #define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
 174    (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
 175
 176 #ifdef __cplusplus
 177 # if !GNULIB_defined_struct__gl_verify_type
 178 template <int w>
 179   struct _gl_verify_type {
 180     unsigned int _gl_verify_error_if_negative: w;
 181   };
 182 #  define GNULIB_defined_struct__gl_verify_type 1
 183 # endif
 184 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 185     _gl_verify_type<(R) ? 1 : -1>
 186 #elif defined _GL_HAVE__STATIC_ASSERT
 187 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 188     struct {                                   \
 189       _Static_assert (R, DIAGNOSTIC);          \
 190       int _gl_dummy;                          \
 191     }
 192 #else
 193 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 194     struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
 195 #endif
 196
 197 /* Verify requirement R at compile-time, as a declaration without a
 198    trailing ';'.  If R is false, fail at compile-time, preferably
 199    with a diagnostic that includes the string-literal DIAGNOSTIC.
 200
 201    Unfortunately, unlike C11, this implementation must appear as an
 202    ordinary declaration, and cannot appear inside struct { ... }.  */
 203
 204 #ifdef _GL_HAVE__STATIC_ASSERT
 205 # define _GL_VERIFY _Static_assert
 206 #else
 207 # define _GL_VERIFY(R, DIAGNOSTIC)                                     \
 208     extern int (*_GL_GENSYM (_gl_verify_function) (void))              \
 209       [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
 210 #endif
 211
 212 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
 213 #ifdef _GL_STATIC_ASSERT_H
 214 # if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
 215 #  define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
 216 # endif
 217 # if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
 218 #  define static_assert _Static_assert /* C11 requires this #define.  */
 219 # endif
 220 #endif
 221
 222 /* @assert.h omit start@  */
 223
 224 /* Each of these macros verifies that its argument R is nonzero.  To
 225    be portable, R should be an integer constant expression.  Unlike
 226    assert (R), there is no run-time overhead.
 227
 228    There are two macros, since no single macro can be used in all
 229    contexts in C.  verify_true (R) is for scalar contexts, including
 230    integer constant expression contexts.  verify (R) is for declaration
 231    contexts, e.g., the top level.  */
 232
 233 /* Verify requirement R at compile-time, as an integer constant expression.
 234    Return 1.  This is equivalent to verify_expr (R, 1).
 235
 236    verify_true is obsolescent; please use verify_expr instead.  */
 237
 238 #define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
 239
 240 /* Verify requirement R at compile-time.  Return the value of the
 241    expression E.  */
 242
 243 #define verify_expr(R, E) \
 244    (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
 245
 246 /* Verify requirement R at compile-time, as a declaration without a
 247    trailing ';'.  */
 248
 249 #ifdef __GNUC__
 250 # define verify(R) _GL_VERIFY (R, "verify (" #R ")")
 251 #else
 252 /* PGI barfs if R is long.  Play it safe.  */
 253 # define verify(R) _GL_VERIFY (R, "verify (...)")
 254 #endif
 255
 256 #ifndef __has_builtin
 257 # define __has_builtin(x) 0
 258 #endif
 259
 260 /* Assume that R always holds.  This lets the compiler optimize
 261    accordingly.  R should not have side-effects; it may or may not be
 262    evaluated.  Behavior is undefined if R is false.  */
 263
 264 #if (__has_builtin (__builtin_unreachable) \
 265      || 4 < __GNUC__ + (5 <= __GNUC_MINOR__))
 266 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
 267 #elif 1200 <= _MSC_VER
 268 # define assume(R) __assume (R)
 269 #elif ((defined GCC_LINT || defined lint) \
 270        && (__has_builtin (__builtin_trap) \
 271            || 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))))
 272   /* Doing it this way helps various packages when configured with
 273      --enable-gcc-warnings, which compiles with -Dlint.  It's nicer
 274      when 'assume' silences warnings even with older GCCs.  */
 275 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
 276 #else
 277   /* Some tools grok NOTREACHED, e.g., Oracle Studio 12.6.  */
 278 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
 279 #endif
 280
 281 /* @assert.h omit end@  */
 282
 283 #endif