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 3, 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, see <http://www.gnu.org/licenses/>.  */
  17
  18 #ifndef _ALLOCSA_H
  19 #define _ALLOCSA_H
  20
  21 #include <alloca.h>
  22 #include <stddef.h>
  23 #include <stdlib.h>
  24
  25
  26 #ifdef __cplusplus
  27 extern "C" {
  28 #endif
  29
  30
  31 /* safe_alloca(N) is equivalent to alloca(N) when it is safe to call
  32    alloca(N); otherwise it returns NULL.  It either returns N bytes of
  33    memory allocated on the stack, that lasts until the function returns,
  34    or NULL.
  35    Use of safe_alloca should be avoided:
  36      - inside arguments of function calls - undefined behaviour,
  37      - in inline functions - the allocation may actually last until the
  38        calling function returns.
  39 */
  40 #if HAVE_ALLOCA
  41 /* The OS usually guarantees only one guard page at the bottom of the stack,
  42    and a page size can be as small as 4096 bytes.  So we cannot safely
  43    allocate anything larger than 4096 bytes.  Also care for the possibility
  44    of a few compiler-allocated temporary stack slots.
  45    This must be a macro, not an inline function.  */
  46 # define safe_alloca(N) ((N) < 4032 ? alloca (N) : NULL)
  47 #else
  48 # define safe_alloca(N) ((N), NULL)
  49 #endif
  50
  51 /* allocsa(N) is a safe variant of alloca(N).  It allocates N bytes of
  52    memory allocated on the stack, that must be freed using freesa() before
  53    the function returns.  Upon failure, it returns NULL.  */
  54 #if HAVE_ALLOCA
  55 # define allocsa(N) \
  56   ((N) < 4032 - sa_increment                                        \
  57    ? (void *) ((char *) alloca ((N) + sa_increment) + sa_increment) \
  58    : mallocsa (N))
  59 #else
  60 # define allocsa(N) \
  61   mallocsa (N)
  62 #endif
  63 extern void * mallocsa (size_t n);
  64
  65 /* Free a block of memory allocated through allocsa().  */
  66 #if HAVE_ALLOCA
  67 extern void freesa (void *p);
  68 #else
  69 # define freesa free
  70 #endif
  71
  72 /* Maybe we should also define a variant
  73     nallocsa (size_t n, size_t s) - behaves like allocsa (n * s)
  74    If this would be useful in your application. please speak up.  */
  75
  76
  77 #ifdef __cplusplus
  78 }
  79 #endif
  80
  81
  82 /* ------------------- Auxiliary, non-public definitions ------------------- */
  83
  84 /* Determine the alignment of a type at compile time.  */
  85 #if defined __GNUC__
  86 # define sa_alignof __alignof__
  87 #elif defined __cplusplus
  88   template <class type> struct sa_alignof_helper { char __slot1; type __slot2; };
  89 # define sa_alignof(type) offsetof (sa_alignof_helper<type>, __slot2)
  90 #elif defined __hpux
  91   /* Work around a HP-UX 10.20 cc bug with enums constants defined as offsetof
  92      values.  */
  93 # define sa_alignof(type) (sizeof (type) <= 4 ? 4 : 8)
  94 #elif defined _AIX
  95   /* Work around an AIX 3.2.5 xlc bug with enums constants defined as offsetof
  96      values.  */
  97 # define sa_alignof(type) (sizeof (type) <= 4 ? 4 : 8)
  98 #else
  99 # define sa_alignof(type) offsetof (struct { char __slot1; type __slot2; }, __slot2)
 100 #endif
 101
 102 enum
 103 {
 104 /* The desired alignment of memory allocations is the maximum alignment
 105    among all elementary types.  */
 106   sa_alignment_long = sa_alignof (long),
 107   sa_alignment_double = sa_alignof (double),
 108 #ifdef HAVE_LONG_LONG
 109   sa_alignment_longlong = sa_alignof (long long),
 110 #endif
 111 #ifdef HAVE_LONG_DOUBLE
 112   sa_alignment_longdouble = sa_alignof (long double),
 113 #endif
 114   sa_alignment_max = ((sa_alignment_long - 1) | (sa_alignment_double - 1)
 115 #ifdef HAVE_LONG_LONG
 116                       | (sa_alignment_longlong - 1)
 117 #endif
 118 #ifdef HAVE_LONG_DOUBLE
 119                       | (sa_alignment_longdouble - 1)
 120 #endif
 121                      ) + 1,
 122 /* The increment that guarantees room for a magic word must be >= sizeof (int)
 123    and a multiple of sa_alignment_max.  */
 124   sa_increment = ((sizeof (int) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max
 125 };
 126
 127 #endif /* _ALLOCSA_H */