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Alignment

Symptoms

When

Why

Catching

You can instruct the kernel to set up an alignment trap so you can see when its occuring (you're not always lucky enough to get a signal from the kernel, so you can force it this way). To get a warning + signal

echo 5 > /proc/cpu/alignment

Other integer values trigger different levels of warning/signals/none.

Observations

Notes

This is a nice page discussing some of the issues on gumstix. Pertinent comment at the bottom (exactly valid or not?):

> Thanks for the input. After reading that I tend to agree that the code I > am using should be fixed to not try and access unaligned data. In fact I > have done this already with a bunch of memcpy's to make it run correctly.

Yeah - that sounds like the right way (read - most portable).

> The actual place where it is being used is in a network protocol reader > where there is a binary stream which gets copied into a memory buffer and > then it contains floats, ints, doubles, etc. referring to different bits > of data. It depends on where in the buffer the stream happens to be as to > whether there will be an alignment issue or not.

Yeah - so copying the data using mempcy from the unaligned buffer into an aligned buffer is the best way of dealing with this type of data.


2022-08-13 12:37