gl/allocsa.h

   1 /* Safe automatic memory allocation.
   2    Copyright (C) 2003-2004 Free Software Foundation, Inc.
   3    Written by Bruno Haible <bruno@clisp.org>, 2003.
   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 2, or (at your option)
   8    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, write to the Free Software Foundation,
  17    Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  18
  19 #ifndef _ALLOCSA_H
  20 #define _ALLOCSA_H
  21
  22 #include <alloca.h>
  23 #include <stddef.h>
  24 #include <stdlib.h>
  25
  26 /* safe_alloca(N) is equivalent to alloca(N) when it is safe to call
  27    alloca(N); otherwise it returns NULL.  It either returns N bytes of
  28    memory allocated on the stack, that lasts until the function returns,
  29    or NULL.
  30    Use of safe_alloca should be avoided:
  31      - inside arguments of function calls - undefined behaviour,
  32      - in inline functions - the allocation may actually last until the
  33        calling function returns.
  34 */
  35 #if HAVE_ALLOCA
  36 /* The OS usually guarantees only one guard page at the bottom of the stack,
  37    and a page size can be as small as 4096 bytes.  So we cannot safely
  38    allocate anything larger than 4096 bytes.  Also care for the possibility
  39    of a few compiler-allocated temporary stack slots.
  40    This must be a macro, not an inline function.  */
  41 # define safe_alloca(N) ((N) < 4032 ? alloca (N) : NULL)
  42 #else
  43 # define safe_alloca(N) ((N), NULL)
  44 #endif
  45
  46 /* allocsa(N) is a safe variant of alloca(N).  It allocates N bytes of
  47    memory allocated on the stack, that must be freed using freesa() before
  48    the function returns.  Upon failure, it returns NULL.  */
  49 #if HAVE_ALLOCA
  50 # define allocsa(N) \
  51   ((N) < 4032 - sa_increment                                        \
  52    ? (void *) ((char *) alloca ((N) + sa_increment) + sa_increment) \
  53    : mallocsa (N))
  54 #else
  55 # define allocsa(N) \
  56   mallocsa (N)
  57 #endif
  58 extern void * mallocsa (size_t n);
  59
  60 /* Free a block of memory allocated through allocsa().  */
  61 #if HAVE_ALLOCA
  62 extern void freesa (void *p);
  63 #else
  64 # define freesa free
  65 #endif
  66
  67 /* Maybe we should also define a variant
  68     nallocsa (size_t n, size_t s) - behaves like allocsa (n * s)
  69    If this would be useful in your application. please speak up.  */
  70
  71
  72 /* ------------------- Auxiliary, non-public definitions ------------------- */
  73
  74 /* Determine the alignment of a type at compile time.  */
  75 #if defined __GNUC__
  76 # define sa_alignof __alignof__
  77 #elif defined __cplusplus
  78   template <class type> struct sa_alignof_helper { char __slot1; type __slot2; };
  79 # define sa_alignof(type) offsetof (sa_alignof_helper<type>, __slot2)
  80 #elif defined __hpux
  81   /* Work around a HP-UX 10.20 cc bug with enums constants defined as offsetof
  82      values.  */
  83 # define sa_alignof(type) (sizeof (type) <= 4 ? 4 : 8)
  84 #else
  85 # define sa_alignof(type) offsetof (struct { char __slot1; type __slot2; }, __slot2)
  86 #endif
  87
  88 enum
  89 {
  90 /* The desired alignment of memory allocations is the maximum alignment
  91    among all elementary types.  */
  92   sa_alignment_long = sa_alignof (long),
  93   sa_alignment_double = sa_alignof (double),
  94 #ifdef HAVE_LONG_LONG
  95   sa_alignment_longlong = sa_alignof (long long),
  96 #endif
  97 #ifdef HAVE_LONG_DOUBLE
  98   sa_alignment_longdouble = sa_alignof (long double),
  99 #endif
 100   sa_alignment_max = ((sa_alignment_long - 1) | (sa_alignment_double - 1)
 101 #ifdef HAVE_LONG_LONG
 102                       | (sa_alignment_longlong - 1)
 103 #endif
 104 #ifdef HAVE_LONG_DOUBLE
 105                       | (sa_alignment_longdouble - 1)
 106 #endif
 107                      ) + 1,
 108 /* The increment that guarantees room for a magic word must be >= sizeof (int)
 109    and a multiple of sa_alignment_max.  */
 110   sa_increment = ((sizeof (int) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max
 111 };
 112
 113 #endif /* _ALLOCSA_H */