------- Comment #8 from mcvick_e at iname dot com 2006-08-22 16:42 -------
To try to be more helpful here, after doing a large amount of investigation into the signature of this problem, it's been observed that the GNU compiler simply defines (or appears to define) a bitfield (regardless of it's type (char, short, int)) as nothing more than a byte aligned entity. Now if the structure has a larger constituent primitive type (short, int) then the structure will align to the largest primitive type. For example. One of the tests that I performed was simply to define two structures. struct foo1 { unsigned int bar1 : 10; unsigned int bar2 : 10; unsigned int bar3 : 12; unsigned short bar4; unsigned char bar5; }; struct foo2 { unsigned int bar1 : 10; unsigned int bar2 : 10; unsigned int bar3 : 12; unsigned short bar4; unsigned char bar5; unsigned char bar6; }; foo1 will report the size as being 0x8 (which is correct), where as foo2 will report the size as 0xA which is incorrect. It's correct if the structure has short alignment, however according to the ABI the structure should align to a 32-bit alignment because of the unsigned int bitfield. Taking the same example above, if you substitute an unsigned int for the unsigned short, then you get the alignment that you expect across all situations. Merely because an integer primitive type is embedded within the structure. I hope this helps some. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28763