On 10/30/2013 11:04 AM, Norbert van Bolhuis wrote:
> I don't understand the -Wpacked description from the GCC manual page, it reads:
>
> -Wpacked
> Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be
> mis-aligned for little benefit. For instance, in this
> code, the variable "f.x" in "struct bar" will be misaligned even though "struct bar" does not itself have the packed attribute:
>
> struct foo {
> int x;
> char a, b, c, d;
> } __attribute__((packed));
> struct bar {
> char z;
> struct foo f;
> };
>
>
> why does the "__attribute__((packed))" for struct foo cause misalignment for struct bar ?
__attribute__((packed)) is applied piecewise to each member of a packed
struct.
> I would expect misalignment only if struct bar is defined with __attribute__((packed))
> or its member f.
>
> I thought __attribute__((packed)) only works for (the members of) the struct, but apparently
> it tries to avoid 3 bytes extra padding in struct bar. However, sizeof(struct bar) = 12 !?
> so there still is 3 bytes padding *and* misalignment. Why would anyone ever want this gcc behaviour ?
Try an array of struct bar.
Andrew.
