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Unaligned memory access in rs_block_sig_init #135

wRAR opened this issue Nov 25, 2017 · 2 comments

Unaligned memory access in rs_block_sig_init #135

wRAR opened this issue Nov 25, 2017 · 2 comments


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@wRAR wRAR commented Nov 25, 2017

On the Debian SPARC64 port the sumset test fails with SIGBUS with the following circumstances:

0x0000010000002344 in rs_block_sig_init (sig=0x1000010815a, weak_sum=463471464, strong_sum=0x7fefffff0e8, strong_len=6) at ./src/sumset.c:41
41          sig->weak_sum = weak_sum;

As you can see, sig is not aligned on 4 while sig->weak_sum has size 4. Looks like it's because block_sigs is an unpadded array of structures of size 6.

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@dbaarda dbaarda commented Nov 27, 2017

My bad, I should have realized that would be a problem when I did the packed rs_block_sigs to save memory.

I'll try to fix it in the next week or so. What is the best practice for avoiding alignment problems like this? Would aligning it to sizeof(int) be good enough? Or should I align to sizeof(rs_weak_sum_t) be better (since that's the size of the first element of the rs_block_sig_t.

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@jrtc27 jrtc27 commented Nov 27, 2017

You can use __alignof__(rs_block_sig_t) to find out the minimum alignment requirements. That's non-standard (though both GCC and Clang implement it), and it's always safe to align to sizeof(largest_member). The structure alignment is always defined recursively as the maximum alignment over each of its members, so no, just looking at the first member isn't correct (though may happen to be if it also has the strictest alignment requirements).

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