malloc/malloc-internal.h

   1 /* Internal declarations for malloc, for use within libc.
   2    Copyright (C) 2016-2018 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public License as
   7    published by the Free Software Foundation; either version 2.1 of the
   8    License, or (at your option) any later version.
   9
  10    The GNU C Library 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 GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; see the file COPYING.LIB.  If
  17    not, see <http://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _MALLOC_INTERNAL_H
  20 #define _MALLOC_INTERNAL_H
  21
  22 #include <malloc-machine.h>
  23 #include <malloc-sysdep.h>
  24
  25 /* INTERNAL_SIZE_T is the word-size used for internal bookkeeping of
  26    chunk sizes.
  27
  28    The default version is the same as size_t.
  29
  30    While not strictly necessary, it is best to define this as an
  31    unsigned type, even if size_t is a signed type. This may avoid some
  32    artificial size limitations on some systems.
  33
  34    On a 64-bit machine, you may be able to reduce malloc overhead by
  35    defining INTERNAL_SIZE_T to be a 32 bit `unsigned int' at the
  36    expense of not being able to handle more than 2^32 of malloced
  37    space. If this limitation is acceptable, you are encouraged to set
  38    this unless you are on a platform requiring 16byte alignments. In
  39    this case the alignment requirements turn out to negate any
  40    potential advantages of decreasing size_t word size.
  41
  42    Implementors: Beware of the possible combinations of:
  43      - INTERNAL_SIZE_T might be signed or unsigned, might be 32 or 64 bits,
  44        and might be the same width as int or as long
  45      - size_t might have different width and signedness as INTERNAL_SIZE_T
  46      - int and long might be 32 or 64 bits, and might be the same width
  47
  48    To deal with this, most comparisons and difference computations
  49    among INTERNAL_SIZE_Ts should cast them to unsigned long, being
  50    aware of the fact that casting an unsigned int to a wider long does
  51    not sign-extend. (This also makes checking for negative numbers
  52    awkward.) Some of these casts result in harmless compiler warnings
  53    on some systems.  */
  54 #ifndef INTERNAL_SIZE_T
  55 # define INTERNAL_SIZE_T size_t
  56 #endif
  57
  58 /* The corresponding word size.  */
  59 #define SIZE_SZ (sizeof (INTERNAL_SIZE_T))
  60
  61 /* The corresponding bit mask value.  */
  62 #define MALLOC_ALIGN_MASK (MALLOC_ALIGNMENT - 1)
  63
  64
  65 /* Called in the parent process before a fork.  */
  66 void __malloc_fork_lock_parent (void) attribute_hidden;
  67
  68 /* Called in the parent process after a fork.  */
  69 void __malloc_fork_unlock_parent (void) attribute_hidden;
  70
  71 /* Called in the child process after a fork.  */
  72 void __malloc_fork_unlock_child (void) attribute_hidden;
  73
  74 /* Set *RESULT to LEFT * RIGHT.  Return true if the multiplication
  75    overflowed.  */
  76 static inline bool
  77 check_mul_overflow_size_t (size_t left, size_t right, size_t *result)
  78 {
  79 #if __GNUC__ >= 5
  80   return __builtin_mul_overflow (left, right, result);
  81 #else
  82   /* size_t is unsigned so the behavior on overflow is defined.  */
  83   *result = left * right;
  84   size_t half_size_t = ((size_t) 1) << (8 * sizeof (size_t) / 2);
  85   if (__glibc_unlikely ((left | right) >= half_size_t))
  86     {
  87       if (__glibc_unlikely (right != 0 && *result / right != left))
  88         return true;
  89     }
  90   return false;
  91 #endif
  92 }
  93
  94 #endif /* _MALLOC_INTERNAL_H */