Hi, I am clearly missing something here … can someone point out where it is?
https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Variable-Attributes.html#Variable%20Attributes in the discussion of applying this to structure fields: "The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well." Consider: struct odd { int * __attribute__((aligned(2))) a; char c; }; I would expect, given reading of the information on the aligned attribute, that the under-alignment of a would be ignored (since there is no packed attribute on either the field or the struct). However, on x86_64, powerpc64 linux and x86_64, powerpc Darwin, I see that the size of the struct is sizeof(pointer) + 2 and the alignment is 2. OTOH: struct OK { int __attribute__((aligned(2))) a; char c; }; behaves as expected (the under-alignment is ignored, silently). as does this… struct maybe { int *a __attribute__((aligned(2))); char c; }; * the type of the pointer does not seem to be relevant (i.e. AFAICT the behaviour is the same for char * etc.) Is there some special rule about pointers that I have not found ? [it’s making an ABI mismatch with clang, which treats the int * as expected from the documentation quoted above] cheers Iain
