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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.

Source Code

Download the latest version of ax_check_aligned_access_required.m4 or browse the macro’s revision history.


Copyright © 2008 Guido U. Draheim

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