lib/verify.h

   1 /* Compile-time assert-like macros.
   2
   3    Copyright (C) 2005-2006, 2009-2013 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 /* Each of these macros verifies that its argument R is nonzero.  To
  46    be portable, R should be an integer constant expression.  Unlike
  47    assert (R), there is no run-time overhead.
  48
  49    If _Static_assert works, verify (R) uses it directly.  Similarly,
  50    _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
  51    that is an operand of sizeof.
  52
  53    The code below uses several ideas for C++ compilers, and for C
  54    compilers that do not support _Static_assert:
  55
  56    * The first step is ((R) ? 1 : -1).  Given an expression R, of
  57      integral or boolean or floating-point type, this yields an
  58      expression of integral type, whose value is later verified to be
  59      constant and nonnegative.
  60
  61    * Next this expression W is wrapped in a type
  62      struct _gl_verify_type {
  63        unsigned int _gl_verify_error_if_negative: W;
  64      }.
  65      If W is negative, this yields a compile-time error.  No compiler can
  66      deal with a bit-field of negative size.
  67
  68      One might think that an array size check would have the same
  69      effect, that is, that the type struct { unsigned int dummy[W]; }
  70      would work as well.  However, inside a function, some compilers
  71      (such as C++ compilers and GNU C) allow local parameters and
  72      variables inside array size expressions.  With these compilers,
  73      an array size check would not properly diagnose this misuse of
  74      the verify macro:
  75
  76        void function (int n) { verify (n < 0); }
  77
  78    * For the verify macro, the struct _gl_verify_type will need to
  79      somehow be embedded into a declaration.  To be portable, this
  80      declaration must declare an object, a constant, a function, or a
  81      typedef name.  If the declared entity uses the type directly,
  82      such as in
  83
  84        struct dummy {...};
  85        typedef struct {...} dummy;
  86        extern struct {...} *dummy;
  87        extern void dummy (struct {...} *);
  88        extern struct {...} *dummy (void);
  89
  90      two uses of the verify macro would yield colliding declarations
  91      if the entity names are not disambiguated.  A workaround is to
  92      attach the current line number to the entity name:
  93
  94        #define _GL_CONCAT0(x, y) x##y
  95        #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
  96        extern struct {...} * _GL_CONCAT (dummy, __LINE__);
  97
  98      But this has the problem that two invocations of verify from
  99      within the same macro would collide, since the __LINE__ value
 100      would be the same for both invocations.  (The GCC __COUNTER__
 101      macro solves this problem, but is not portable.)
 102
 103      A solution is to use the sizeof operator.  It yields a number,
 104      getting rid of the identity of the type.  Declarations like
 105
 106        extern int dummy [sizeof (struct {...})];
 107        extern void dummy (int [sizeof (struct {...})]);
 108        extern int (*dummy (void)) [sizeof (struct {...})];
 109
 110      can be repeated.
 111
 112    * Should the implementation use a named struct or an unnamed struct?
 113      Which of the following alternatives can be used?
 114
 115        extern int dummy [sizeof (struct {...})];
 116        extern int dummy [sizeof (struct _gl_verify_type {...})];
 117        extern void dummy (int [sizeof (struct {...})]);
 118        extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
 119        extern int (*dummy (void)) [sizeof (struct {...})];
 120        extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
 121
 122      In the second and sixth case, the struct type is exported to the
 123      outer scope; two such declarations therefore collide.  GCC warns
 124      about the first, third, and fourth cases.  So the only remaining
 125      possibility is the fifth case:
 126
 127        extern int (*dummy (void)) [sizeof (struct {...})];
 128
 129    * GCC warns about duplicate declarations of the dummy function if
 130      -Wredundant-decls is used.  GCC 4.3 and later have a builtin
 131      __COUNTER__ macro that can let us generate unique identifiers for
 132      each dummy function, to suppress this warning.
 133
 134    * This implementation exploits the fact that older versions of GCC,
 135      which do not support _Static_assert, also do not warn about the
 136      last declaration mentioned above.
 137
 138    * GCC warns if -Wnested-externs is enabled and verify() is used
 139      within a function body; but inside a function, you can always
 140      arrange to use verify_expr() instead.
 141
 142    * In C++, any struct definition inside sizeof is invalid.
 143      Use a template type to work around the problem.  */
 144
 145 /* Concatenate two preprocessor tokens.  */
 146 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
 147 # define _GL_CONCAT0(x, y) x##y
 148
 149 /* _GL_COUNTER is an integer, preferably one that changes each time we
 150    use it.  Use __COUNTER__ if it works, falling back on __LINE__
 151    otherwise.  __LINE__ isn't perfect, but it's better than a
 152    constant.  */
 153 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__
 154 #  define _GL_COUNTER __COUNTER__
 155 # else
 156 #  define _GL_COUNTER __LINE__
 157 # endif
 158
 159 /* Generate a symbol with the given prefix, making it unique if
 160    possible.  */
 161 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
 162
 163 /* Verify requirement R at compile-time, as an integer constant expression
 164    that returns 1.  If R is false, fail at compile-time, preferably
 165    with a diagnostic that includes the string-literal DIAGNOSTIC.  */
 166
 167 # define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
 168     (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
 169
 170 # ifdef __cplusplus
 171 #  if !GNULIB_defined_struct__gl_verify_type
 172 template <int w>
 173   struct _gl_verify_type {
 174     unsigned int _gl_verify_error_if_negative: w;
 175   };
 176 #   define GNULIB_defined_struct__gl_verify_type 1
 177 #  endif
 178 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 179     _gl_verify_type<(R) ? 1 : -1>
 180 # elif defined _GL_HAVE__STATIC_ASSERT
 181 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 182      struct {                                   \
 183        _Static_assert (R, DIAGNOSTIC);          \
 184        int _gl_dummy;                          \
 185      }
 186 # else
 187 #  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
 188      struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
 189 # endif
 190
 191 /* Verify requirement R at compile-time, as a declaration without a
 192    trailing ';'.  If R is false, fail at compile-time, preferably
 193    with a diagnostic that includes the string-literal DIAGNOSTIC.
 194
 195    Unfortunately, unlike C11, this implementation must appear as an
 196    ordinary declaration, and cannot appear inside struct { ... }.  */
 197
 198 # ifdef _GL_HAVE__STATIC_ASSERT
 199 #  define _GL_VERIFY _Static_assert
 200 # else
 201 #  define _GL_VERIFY(R, DIAGNOSTIC)                                    \
 202      extern int (*_GL_GENSYM (_gl_verify_function) (void))             \
 203        [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
 204 # endif
 205
 206 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
 207 # ifdef _GL_STATIC_ASSERT_H
 208 #  if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
 209 #   define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
 210 #  endif
 211 #  if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
 212 #   define static_assert _Static_assert /* C11 requires this #define.  */
 213 #  endif
 214 # endif
 215
 216 /* @assert.h omit start@  */
 217
 218 /* Each of these macros verifies that its argument R is nonzero.  To
 219    be portable, R should be an integer constant expression.  Unlike
 220    assert (R), there is no run-time overhead.
 221
 222    There are two macros, since no single macro can be used in all
 223    contexts in C.  verify_true (R) is for scalar contexts, including
 224    integer constant expression contexts.  verify (R) is for declaration
 225    contexts, e.g., the top level.  */
 226
 227 /* Verify requirement R at compile-time, as an integer constant expression.
 228    Return 1.  This is equivalent to verify_expr (R, 1).
 229
 230    verify_true is obsolescent; please use verify_expr instead.  */
 231
 232 # define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
 233
 234 /* Verify requirement R at compile-time.  Return the value of the
 235    expression E.  */
 236
 237 # define verify_expr(R, E) \
 238     (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
 239
 240 /* Verify requirement R at compile-time, as a declaration without a
 241    trailing ';'.  */
 242
 243 # define verify(R) _GL_VERIFY (R, "verify (" #R ")")
 244
 245 /* @assert.h omit end@  */
 246
 247 #endif