ax_check_aligned_access_required

While the x86 CPUs allow access to memory objects to be unaligned it happens that most of the modern designs require objects to be aligned - or they will fail with a buserror. That mode is quite known by big-endian machines (sparc, etc) however the alpha cpu is little- endian.

The following function will test for aligned access to be required and set a config.h define HAVE_ALIGNED_ACCESS_REQUIRED (name derived by standard usage). Structures loaded from a file (or mmapped to memory) should be accessed per-byte in that case to avoid segfault type errors.

AC_DEFUN([AX_CHECK_ALIGNED_ACCESS_REQUIRED],
[AC_CACHE_CHECK([if pointers to integers require aligned access],
  [ax_cv_have_aligned_access_required],
  [AC_TRY_RUN([
#include <stdio.h>
#include <stdlib.h>

int main()
{
  char* string = malloc(40);
  int i;
  for (i=0; i < 40; i++) string[[i]] = i;
  {
     void* s = string;
     int* p = s+1;
     int* q = s+2;

     if (*p == *q) { return 1; }
  }
  return 0;
}
              ],
     [ax_cv_have_aligned_access_required=yes],
     [ax_cv_have_aligned_access_required=no],
     [ax_cv_have_aligned_access_required=no])
  ])
if test "$ax_cv_have_aligned_access_required" = yes ; then
  AC_DEFINE([HAVE_ALIGNED_ACCESS_REQUIRED], [1],
    [Define if pointers to integers require aligned access])
fi
])

Copyright © 2006 Guido U. Draheim <guidod@gmx.de>

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