From: Segher Boessenkool <segher@kernel.crashing.org>
To: john smith <wempwer@gmail.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Where does C standard mention alignment requirements as written in gcc manual?
Date: Tue, 18 Aug 2015 16:28:00 -0000 [thread overview]
Message-ID: <20150818162835.GD30482@gate.crashing.org> (raw)
In-Reply-To: <CAKmQUfYdG9b45tpxh86z-JTTgKt=d7FEknb7HE_jMvjQg3W+nQ@mail.gmail.com>
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
prev parent reply other threads:[~2015-08-18 16:28 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-18 15:04 john smith
2015-08-18 15:50 ` Segher Boessenkool
2015-08-18 15:57 ` john smith
2015-08-18 16:25 ` Nicholas Mc Guire
2015-08-18 16:28 ` Segher Boessenkool [this message]
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