gl/allocsa.h

   1 /* Safe automatic memory allocation.
   2    Copyright (C) 2003-2006 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
  27 #ifdef __cplusplus
  28 extern "C" {
  29 #endif
  30
  31
  32 /* safe_alloca(N) is equivalent to alloca(N) when it is safe to call
  33    alloca(N); otherwise it returns NULL.  It either returns N bytes of
  34    memory allocated on the stack, that lasts until the function returns,
  35    or NULL.
  36    Use of safe_alloca should be avoided:
  37      - inside arguments of function calls - undefined behaviour,
  38      - in inline functions - the allocation may actually last until the
  39        calling function returns.
  40 */
  41 #if HAVE_ALLOCA
  42 /* The OS usually guarantees only one guard page at the bottom of the stack,
  43    and a page size can be as small as 4096 bytes.  So we cannot safely
  44    allocate anything larger than 4096 bytes.  Also care for the possibility
  45    of a few compiler-allocated temporary stack slots.
  46    This must be a macro, not an inline function.  */
  47 # define safe_alloca(N) ((N) < 4032 ? alloca (N) : NULL)
  48 #else
  49 # define safe_alloca(N) ((N), NULL)
  50 #endif
  51
  52 /* allocsa(N) is a safe variant of alloca(N).  It allocates N bytes of
  53    memory allocated on the stack, that must be freed using freesa() before
  54    the function returns.  Upon failure, it returns NULL.  */
  55 #if HAVE_ALLOCA
  56 # define allocsa(N) \
  57   ((N) < 4032 - sa_increment                                        \
  58    ? (void *) ((char *) alloca ((N) + sa_increment) + sa_increment) \
  59    : mallocsa (N))
  60 #else
  61 # define allocsa(N) \
  62   mallocsa (N)
  63 #endif
  64 extern void * mallocsa (size_t n);
  65
  66 /* Free a block of memory allocated through allocsa().  */
  67 #if HAVE_ALLOCA
  68 extern void freesa (void *p);
  69 #else
  70 # define freesa free
  71 #endif
  72
  73 /* Maybe we should also define a variant
  74     nallocsa (size_t n, size_t s) - behaves like allocsa (n * s)
  75    If this would be useful in your application. please speak up.  */
  76
  77
  78 #ifdef __cplusplus
  79 }
  80 #endif
  81
  82
  83 /* ------------------- Auxiliary, non-public definitions ------------------- */
  84
  85 /* Determine the alignment of a type at compile time.  */
  86 #if defined __GNUC__
  87 # define sa_alignof __alignof__
  88 #elif defined __cplusplus
  89   template <class type> struct sa_alignof_helper { char __slot1; type __slot2; };
  90 # define sa_alignof(type) offsetof (sa_alignof_helper<type>, __slot2)
  91 #elif defined __hpux
  92   /* Work around a HP-UX 10.20 cc bug with enums constants defined as offsetof
  93      values.  */
  94 # define sa_alignof(type) (sizeof (type) <= 4 ? 4 : 8)
  95 #elif defined _AIX
  96   /* Work around an AIX 3.2.5 xlc bug with enums constants defined as offsetof
  97      values.  */
  98 # define sa_alignof(type) (sizeof (type) <= 4 ? 4 : 8)
  99 #else
 100 # define sa_alignof(type) offsetof (struct { char __slot1; type __slot2; }, __slot2)
 101 #endif
 102
 103 enum
 104 {
 105 /* The desired alignment of memory allocations is the maximum alignment
 106    among all elementary types.  */
 107   sa_alignment_long = sa_alignof (long),
 108   sa_alignment_double = sa_alignof (double),
 109 #ifdef HAVE_LONG_LONG_INT
 110   sa_alignment_longlong = sa_alignof (long long),
 111 #endif
 112 #ifdef HAVE_LONG_DOUBLE
 113   sa_alignment_longdouble = sa_alignof (long double),
 114 #endif
 115   sa_alignment_max = ((sa_alignment_long - 1) | (sa_alignment_double - 1)
 116 #ifdef HAVE_LONG_LONG_INT
 117                       | (sa_alignment_longlong - 1)
 118 #endif
 119 #ifdef HAVE_LONG_DOUBLE
 120                       | (sa_alignment_longdouble - 1)
 121 #endif
 122                      ) + 1,
 123 /* The increment that guarantees room for a magic word must be >= sizeof (int)
 124    and a multiple of sa_alignment_max.  */
 125   sa_increment = ((sizeof (int) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max
 126 };
 127
 128 #endif /* _ALLOCSA_H */