[lwip-devel] [patch #5846] struct sockaddr_in has different alignment than struct sockaddr

[Jonathan Larmour]

Follow-up Comment #7, patch #5846 (project lwip):

The reason I was concerned about unnecessary overhead (extra code and/or
extra footprint) was because you had said earlier in the comment to which I
replied: "In other words, I think if the one is packed, the other should be
packed, also."

I was also extrapolating a bit - in your patch you, for example, align the
struct sockaddr to 64-bits for MEM_ALIGNMENT==8, but MEM_ALIGNMENT has
nothing to do with the alignment requirements of a struct in_addr which might
only need alignment to 1, 2, or 4 bytes. So to guarantee they'd match to avoid
any warning when the cast is made, since it happens *both* ways, you'd also
need to add some alignment component to struct sockaddr_in. With that patch's
approach anyway. The patch as it stands isn't right.

Anyway, the warning is irrelevant - as you say yourself, a struct sockaddr is
never deferenced in itself. There is not, and will not be, any problem here -
people don't create struct sockaddrs.

Although if/when someone finishes off ipv6, there'll be an AF_INET6 class to
distinguish and a struct sockaddr_in6 will probably have different alignment
constraints to a struct sockaddr_in.

Other stacks don't bother with this anyway: warnings, especially those
generated by obscure warning flags, are guides, not problems to be fixed. If
others think this really is a "problem", then the solution will have to
involve unions.


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