Where does C standard mention alignment requirements as written in gcc manual?

Segher Boessenkool segher@kernel.crashing.org
Tue Aug 18 16:28:00 GMT 2015


On Tue, Aug 18, 2015 at 05:57:36PM +0200, john smith wrote:
> >> "Note that the alignment of any given struct or union type is required
> >> by the ISO C standard to be at least a perfect multiple of the lowest
> >> common multiple of the alignments of all of the members of the struct
> >> or union in question."
> >>
> >> Where exactly is this requirement mentioned in C standard? I was
> >> looking for it in both C99 and C11 drafts and couldn't find it.
> >
> > C99 6.7.2.1/12 ?
> 
> It just says that "Each non-bit-field member of a structure or union
> object is aligned in an implementation-defined manner appropriate to
> its type.". It doesn't require an overall alignment of the structure
> to be "at least a perfect multiple of the lowest common multiple of
> the alignments of all of the members of the struct".

If the alignment of any member of the struct is A, the alignment of the
struct has to be a multiple of A (by the above, and the definition of
alignment).  Now consider all members together, and you get what the
GCC manual says.


Segher



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