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Date: Tue, 24 May 2016 18:36:02 -0400
From: Rich Felker <>
Subject: Re: sockaddr_storage and GCC 6.1

On Tue, May 24, 2016 at 03:07:35PM -0700, William Ahern wrote:
> GCC 6.1 more aggressively decomposes aggregate assignments into a series of
> scalar member assignments. This has uncovered an issue with glibc's layout
> of struct sockaddr_storage, which has a padding hole from offsets 2 to 8,
> precisely where .sin_port and .sin_addr are in struct sockaddr_in.
> musl shares this same issue. Specifically, the __ss_align member with an
> 8-byte alignment on LP64 archs. You can track the glibc resolution at
> Or not track it. Reasonable folks can disagree regarding many aspects of
> this issue, but I thought it worthwhile to bring to people's attention.

I maintain that it's a bug (violation of effective type rules) for a
program to attempt to copy sockaddr types using sockaddr_storage, but
this is a nasty application bug to track down (usually silent
breakage) that's worth avoiding since it's easy. Does the attached
patch work?

I don't think we should even consider the sorts of may_alias hacks
glibc/gcc folks are discussing, though. There's already a gcc option
for compiling broken code like that; it's called -fno-strict-aliasing.


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