spot/lib/xalloc-oversized.h

/* xalloc-oversized.h -- memory allocation size checking

   Copyright (C) 1990-2000, 2003-2004, 2006-2016 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#ifndef XALLOC_OVERSIZED_H_
#define XALLOC_OVERSIZED_H_

#include <stddef.h>

/* Default for (non-Clang) compilers that lack __has_builtin.  */
#ifndef __has_builtin
# define __has_builtin(x) 0
#endif

/* True if N * S would overflow in a size calculation.
   This expands to a constant expression if N and S are both constants.
   By gnulib convention, SIZE_MAX represents overflow in size
   calculations, so the conservative dividend to use here is
   SIZE_MAX - 1, since SIZE_MAX might represent an overflowed value.
   However, malloc (SIZE_MAX) fails on all known hosts where
   sizeof (ptrdiff_t) <= sizeof (size_t), so do not bother to test for
   exactly-SIZE_MAX allocations on such hosts; this avoids a test and
   branch when S is known to be 1.  */
#define __xalloc_oversized(n, s) \
    ((size_t) (sizeof (ptrdiff_t) <= sizeof (size_t) ? -1 : -2) / (s) < (n))


/* Return 1 if an array of N objects, each of size S, cannot exist due
   to size arithmetic overflow.  S must be positive and N must be
   nonnegative.  This is a macro, not a function, so that it
   works correctly even when SIZE_MAX < N.  */

/* GCC 7 __builtin_mul_overflow should easily compute this.  See:
   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68120  */
#if 7 <= __GNUC__
# define xalloc_oversized(n, s) __builtin_mul_overflow (n, s, (size_t *) NULL)

/* GCC 5 and Clang __builtin_mul_overflow needs a temporary, and
   should be used only for non-constant operands, so that
   xalloc_oversized is a constant expression if both arguments are.
   Do not use this if pedantic, since pedantic GCC issues a diagnostic
   for ({ ... }).  */
#elif ((5 <= __GNUC__ \
        || (__has_builtin (__builtin_mul_overflow) \
            && __has_builtin (__builtin_constant_p))) \
       && !__STRICT_ANSI__)
# define xalloc_oversized(n, s) \
   (__builtin_constant_p (n) && __builtin_constant_p (s) \
    ? __xalloc_oversized (n, s) \
    : ({ size_t __xalloc_size; __builtin_mul_overflow (n, s, &__xalloc_size); }))

/* Other compilers use integer division; this may be slower but is
   more portable.  */
#else
# define xalloc_oversized(n, s) __xalloc_oversized (n, s)
#endif

#endif /* !XALLOC_OVERSIZED_H_ */