lib/malloca.c

   1 /* Safe automatic memory allocation.
   2    Copyright (C) 2003, 2006-2007, 2009-2020 Free Software Foundation, Inc.
   3    Written by Bruno Haible <bruno@clisp.org>, 2003, 2018.
   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, see <https://www.gnu.org/licenses/>.  */
  17
  18 #define _GL_USE_STDLIB_ALLOC 1
  19 #include <config.h>
  20
  21 /* Specification.  */
  22 #include "malloca.h"
  23
  24 #include "verify.h"
  25
  26 /* The speed critical point in this file is freea() applied to an alloca()
  27    result: it must be fast, to match the speed of alloca().  The speed of
  28    mmalloca() and freea() in the other case are not critical, because they
  29    are only invoked for big memory sizes.
  30    Here we use a bit in the address as an indicator, an idea by Ondřej Bílka.
  31    malloca() can return three types of pointers:
  32      - Pointers ≡ 0 mod 2*sa_alignment_max come from stack allocation.
  33      - Pointers ≡ sa_alignment_max mod 2*sa_alignment_max come from heap
  34        allocation.
  35      - NULL comes from a failed heap allocation.  */
  36
  37 /* Type for holding very small pointer differences.  */
  38 typedef unsigned char small_t;
  39 /* Verify that it is wide enough.  */
  40 verify (2 * sa_alignment_max - 1 <= (small_t) -1);
  41
  42 void *
  43 mmalloca (size_t n)
  44 {
  45 #if HAVE_ALLOCA
  46   /* Allocate one more word, used to determine the address to pass to freea(),
  47      and room for the alignment ≡ sa_alignment_max mod 2*sa_alignment_max.  */
  48   size_t nplus = n + sizeof (small_t) + 2 * sa_alignment_max - 1;
  49
  50   if (nplus >= n)
  51     {
  52       char *mem = (char *) malloc (nplus);
  53
  54       if (mem != NULL)
  55         {
  56           char *p =
  57             (char *)((((uintptr_t)mem + sizeof (small_t) + sa_alignment_max - 1)
  58                       & ~(uintptr_t)(2 * sa_alignment_max - 1))
  59                      + sa_alignment_max);
  60           /* Here p >= mem + sizeof (small_t),
  61              and p <= mem + sizeof (small_t) + 2 * sa_alignment_max - 1
  62              hence p + n <= mem + nplus.
  63              So, the memory range [p, p+n) lies in the allocated memory range
  64              [mem, mem + nplus).  */
  65           ((small_t *) p)[-1] = p - mem;
  66           /* p ≡ sa_alignment_max mod 2*sa_alignment_max.  */
  67           return p;
  68         }
  69     }
  70   /* Out of memory.  */
  71   return NULL;
  72 #else
  73 # if !MALLOC_0_IS_NONNULL
  74   if (n == 0)
  75     n = 1;
  76 # endif
  77   return malloc (n);
  78 #endif
  79 }
  80
  81 #if HAVE_ALLOCA
  82 void
  83 freea (void *p)
  84 {
  85   /* Check argument.  */
  86   if ((uintptr_t) p & (sa_alignment_max - 1))
  87     {
  88       /* p was not the result of a malloca() call.  Invalid argument.  */
  89       abort ();
  90     }
  91   /* Determine whether p was a non-NULL pointer returned by mmalloca().  */
  92   if ((uintptr_t) p & sa_alignment_max)
  93     {
  94       void *mem = (char *) p - ((small_t *) p)[-1];
  95       free (mem);
  96     }
  97 }
  98 #endif
  99
 100 /*
 101  * Hey Emacs!
 102  * Local Variables:
 103  * coding: utf-8
 104  * End:
 105  */