-Wcast-align and struct sockaddr_in / struct sockaddr: why the warning?

[Evan Jones]

When compiling the following code with GCC 4.4.0 on Solaris 10/Sparc 
with the -Wcast-align flag, I get a warning:


#include <stdio.h>
#include <netinet/in.h>

int main() {
     struct sockaddr wtf;
     struct sockaddr* w = &wtf;
     struct sockaddr_in* w2 = (struct sockaddr_in*) w;
     printf("%p\n", w2);
     return 0;
}


test.cc: In function ‘int main()’:
test.cc:7: warning: cast from ‘sockaddr*’ to ‘sockaddr_in*’ increases 
required alignment of target type

It is easy enough to work around this warning by assigning to void* 
first. However, why does GCC think that struct sockaddr and struct 
sockaddr_in have different alignment requirements? They are both 16 
bytes long. If you create an array of the two types, the elements are 
packed without any padding. Thus, it seems like GCC thinks they have the 
*same* alignment requirements in arrays, but different requirements in 
pointers? I don't get it. Is this a bug?

Thanks for your help,

Evan Jones

-- 
Evan Jones
http://evanjones.ca/