lib/xalloc-oversized.h

   1 /* xalloc-oversized.h -- memory allocation size checking
   2
   3    Copyright (C) 1990-2000, 2003-2004, 2006-2018 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 #ifndef XALLOC_OVERSIZED_H_
  19 #define XALLOC_OVERSIZED_H_
  20
  21 #include <stddef.h>
  22 #include <stdint.h>
  23
  24 /* True if N * S would overflow in a size_t calculation,
  25    or would generate a value larger than PTRDIFF_MAX.
  26    This expands to a constant expression if N and S are both constants.
  27    By gnulib convention, SIZE_MAX represents overflow in size
  28    calculations, so the conservative size_t-based dividend to use here
  29    is SIZE_MAX - 1.  */
  30 #define __xalloc_oversized(n, s) \
  31   ((size_t) (PTRDIFF_MAX < SIZE_MAX ? PTRDIFF_MAX : SIZE_MAX - 1) / (s) < (n))
  32
  33 #if PTRDIFF_MAX < SIZE_MAX
  34 typedef ptrdiff_t __xalloc_count_type;
  35 #else
  36 typedef size_t __xalloc_count_type;
  37 #endif
  38
  39 /* Return 1 if an array of N objects, each of size S, cannot exist
  40    reliably due to size or ptrdiff_t arithmetic overflow.  S must be
  41    positive and N must be nonnegative.  This is a macro, not a
  42    function, so that it works correctly even when SIZE_MAX < N.  */
  43
  44 #if 7 <= __GNUC__
  45 # define xalloc_oversized(n, s) \
  46    __builtin_mul_overflow_p (n, s, (__xalloc_count_type) 1)
  47 #elif 5 <= __GNUC__ && !defined __ICC && !__STRICT_ANSI__
  48 # define xalloc_oversized(n, s) \
  49    (__builtin_constant_p (n) && __builtin_constant_p (s) \
  50     ? __xalloc_oversized (n, s) \
  51     : ({ __xalloc_count_type __xalloc_count; \
  52          __builtin_mul_overflow (n, s, &__xalloc_count); }))
  53
  54 /* Other compilers use integer division; this may be slower but is
  55    more portable.  */
  56 #else
  57 # define xalloc_oversized(n, s) __xalloc_oversized (n, s)
  58 #endif
  59
  60 #endif /* !XALLOC_OVERSIZED_H_ */