@node ax_check_aligned_access_required
@unnumberedsec ax_check_aligned_access_required

@majorheading Synopsis

@smallexample
AX_CHECK_ALIGNED_ACCESS_REQUIRED
@end smallexample

@majorheading Description

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.

The function checks if unaligned access would ignore the lowest bit of
the address. If that happens or if the test binary crashes, aligned
access is required.

If cross-compiling, assume that aligned access is needed to be safe. Set
ax_cv_have_aligned_access_required=no to override that assumption.

@majorheading Source Code

Download the
@uref{http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_check_aligned_access_required.m4,latest
version of @file{ax_check_aligned_access_required.m4}} or browse
@uref{http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=history;f=m4/ax_check_aligned_access_required.m4,the
macro's revision history}.

@majorheading License

@w{Copyright @copyright{} 2008 Guido U. Draheim @email{guidod@@gmx.de}}

Copying and distribution of this file, with or without modification, are
permitted in any medium without royalty provided the copyright notice
and this notice are preserved.  This file is offered as-is, without any
warranty.