glibc/malloc/malloc-internal.h

/* Internal declarations for malloc, for use within libc.
   Copyright (C) 2016-2017 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation; either version 2.1 of the
   License, or (at your option) any later version.

   The GNU C Library 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; see the file COPYING.LIB.  If
   not, see <http://www.gnu.org/licenses/>.  */

#ifndef _MALLOC_INTERNAL_H
#define _MALLOC_INTERNAL_H

#include <malloc-machine.h>
#include <malloc-sysdep.h>

/* INTERNAL_SIZE_T is the word-size used for internal bookkeeping of
   chunk sizes.

   The default version is the same as size_t.

   While not strictly necessary, it is best to define this as an
   unsigned type, even if size_t is a signed type. This may avoid some
   artificial size limitations on some systems.

   On a 64-bit machine, you may be able to reduce malloc overhead by
   defining INTERNAL_SIZE_T to be a 32 bit `unsigned int' at the
   expense of not being able to handle more than 2^32 of malloced
   space. If this limitation is acceptable, you are encouraged to set
   this unless you are on a platform requiring 16byte alignments. In
   this case the alignment requirements turn out to negate any
   potential advantages of decreasing size_t word size.

   Implementors: Beware of the possible combinations of:
     - INTERNAL_SIZE_T might be signed or unsigned, might be 32 or 64 bits,
       and might be the same width as int or as long
     - size_t might have different width and signedness as INTERNAL_SIZE_T
     - int and long might be 32 or 64 bits, and might be the same width

   To deal with this, most comparisons and difference computations
   among INTERNAL_SIZE_Ts should cast them to unsigned long, being
   aware of the fact that casting an unsigned int to a wider long does
   not sign-extend. (This also makes checking for negative numbers
   awkward.) Some of these casts result in harmless compiler warnings
   on some systems.  */
#ifndef INTERNAL_SIZE_T
# define INTERNAL_SIZE_T size_t
#endif

/* The corresponding word size.  */
#define SIZE_SZ (sizeof (INTERNAL_SIZE_T))

/* MALLOC_ALIGNMENT is the minimum alignment for malloc'ed chunks.  It
   must be a power of two at least 2 * SIZE_SZ, even on machines for
   which smaller alignments would suffice. It may be defined as larger
   than this though. Note however that code and data structures are
   optimized for the case of 8-byte alignment.  */
#ifndef MALLOC_ALIGNMENT
# define MALLOC_ALIGNMENT (2 * SIZE_SZ < __alignof__ (long double) \
                           ? __alignof__ (long double) : 2 * SIZE_SZ)
#endif

/* The corresponding bit mask value.  */
#define MALLOC_ALIGN_MASK (MALLOC_ALIGNMENT - 1)


/* Called in the parent process before a fork.  */
void __malloc_fork_lock_parent (void) internal_function attribute_hidden;

/* Called in the parent process after a fork.  */
void __malloc_fork_unlock_parent (void) internal_function attribute_hidden;

/* Called in the child process after a fork.  */
void __malloc_fork_unlock_child (void) internal_function attribute_hidden;

/* Set *RESULT to LEFT * RIGHT.  Return true if the multiplication
   overflowed.  */
static inline bool
check_mul_overflow_size_t (size_t left, size_t right, size_t *result)
{
#if __GNUC__ >= 5
  return __builtin_mul_overflow (left, right, result);
#else
  /* size_t is unsigned so the behavior on overflow is defined.  */
  *result = left * right;
  size_t half_size_t = ((size_t) 1) << (8 * sizeof (size_t) / 2);
  if (__glibc_unlikely ((left | right) >= half_size_t))
    {
      if (__glibc_unlikely (right != 0 && *result / right != left))
        return true;
    }
  return false;
#endif
}

#endif /* _MALLOC_INTERNAL_H */