sysdeps/generic/malloc-size.h

   1 /* Define INTERNAL_SIZE_T, SIZE_SZ, MALLOC_ALIGNMENT and MALLOC_ALIGN_MASK
   2    for malloc.
   3    Copyright (C) 2021-2024 Free Software Foundation, Inc.
   4    This file is part of the GNU C Library.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <https://www.gnu.org/licenses/>.  */
  19
  20 #ifndef _GENERIC_MALLOC_SIZE_H
  21 #define _GENERIC_MALLOC_SIZE_H
  22
  23 /* INTERNAL_SIZE_T is the word-size used for internal bookkeeping of
  24    chunk sizes.
  25
  26    The default version is the same as size_t.
  27
  28    While not strictly necessary, it is best to define this as an
  29    unsigned type, even if size_t is a signed type. This may avoid some
  30    artificial size limitations on some systems.
  31
  32    On a 64-bit machine, you may be able to reduce malloc overhead by
  33    defining INTERNAL_SIZE_T to be a 32 bit `unsigned int' at the
  34    expense of not being able to handle more than 2^32 of malloced
  35    space. If this limitation is acceptable, you are encouraged to set
  36    this unless you are on a platform requiring 16byte alignments. In
  37    this case the alignment requirements turn out to negate any
  38    potential advantages of decreasing size_t word size.
  39
  40    Implementors: Beware of the possible combinations of:
  41      - INTERNAL_SIZE_T might be signed or unsigned, might be 32 or 64 bits,
  42        and might be the same width as int or as long
  43      - size_t might have different width and signedness as INTERNAL_SIZE_T
  44      - int and long might be 32 or 64 bits, and might be the same width
  45
  46    To deal with this, most comparisons and difference computations
  47    among INTERNAL_SIZE_Ts should cast them to unsigned long, being
  48    aware of the fact that casting an unsigned int to a wider long does
  49    not sign-extend. (This also makes checking for negative numbers
  50    awkward.) Some of these casts result in harmless compiler warnings
  51    on some systems.  */
  52 #ifndef INTERNAL_SIZE_T
  53 # define INTERNAL_SIZE_T size_t
  54 #endif
  55
  56 /* The corresponding word size.  */
  57 #define SIZE_SZ (sizeof (INTERNAL_SIZE_T))
  58
  59 #include <malloc-alignment.h>
  60
  61 /* The corresponding bit mask value.  */
  62 #define MALLOC_ALIGN_MASK (MALLOC_ALIGNMENT - 1)
  63
  64 #endif /* _GENERIC_MALLOC_SIZE_H */