AC_CHECK_ALIGNED_ACCESS_REQUIRED
, 

2006-08-17

Guido U. Draheim <guidod@gmx.de>

GPLWithACException
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation. As a special exception, the respective Autoconf Macro's copyright owner gives unlimited permission to copy, distribute and modify the configure scripts that are the output of Autoconf when processing the Macro. You need not follow the terms of the GNU General Public License when using or distributing such scripts

guidod's C Support (released)

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
])