From: Andrew Pinski <pinskia@physics.uc.edu>
To: mrs@apple.com (Mike Stump)
Cc: dave.korn@artimi.com (Dave Korn),
pkoning@equallogic.com (Paul Koning),
gcc@sources.redhat.com, sjackman@gmail.com (Shaun Jackman)
Subject: Re: memcpy to an unaligned address
Date: Tue, 02 Aug 2005 20:38:00 -0000 [thread overview]
Message-ID: <200508022037.j72Kbr4T012558@earth.phy.uc.edu> (raw)
In-Reply-To: <78966CDA-DC9F-46F5-AE63-563051D9EF3C@apple.com>
>
> On Aug 2, 2005, at 1:15 PM, Shaun Jackman wrote:
> > There is no padding. The structure is defined as
> > __attribute__((packed)) to explicitly remove the padding. The result
> > is that gcc knows the unaligned four byte member is at an offset of
> > two bytes from the base of the struct, but uses a four byte load at
> > the unaligned address of base+2. I don't expect...
> > p->unaligned = n;
> > ... to work,
>
> Actually, that works just fine, with:
>
> typedef struct {
> unsigned short int a;
> unsigned int b;
> } __attribute__((packed)) st;
>
> void foo(st *s, int n)
> {
> s->b = n;
> }
>
> Ah, I was having trouble getting it to fail for me... Now I can:
>
> #include <memory.h>
>
> typedef struct {
> unsigned short int a;
> unsigned int b;
> } __attribute__((packed)) st;
>
> void foo(st *s, int n)
> {
> memcpy(&s->b, &n, sizeof n);
> }
>
> Yes, this is a compiler bug in the expansion of memcpy, please file a
> bug report. The solution is for the compiler to notice the memory
> alignment of the destination and `do-the-right-thing' when it isn't
> aligned.
No it is not, once you take the address (which should be rejected), it
is of type "unsigned int *" and not unaligned variable, passing it to
memcpy assumes the type alignment is the natural alignment.
-- Pinski
next prev parent reply other threads:[~2005-08-02 20:38 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-08-02 17:32 Shaun Jackman
2005-08-02 17:43 ` Dave Korn
2005-08-02 19:13 ` Shaun Jackman
2005-08-02 19:19 ` Paul Koning
2005-08-02 19:26 ` Shaun Jackman
2005-08-02 19:40 ` Dave Korn
2005-08-02 19:48 ` Paul Koning
2005-08-02 20:15 ` Shaun Jackman
2005-08-02 20:29 ` Paul Koning
2005-08-02 20:29 ` Mike Stump
2005-08-02 20:38 ` Andrew Pinski [this message]
2005-08-02 20:45 ` Ian Lance Taylor
2005-08-02 21:30 ` Mike Stump
2005-08-02 21:34 ` Joe Buck
2005-08-03 18:00 ` Richard Henderson
2005-08-03 18:15 ` Shaun Jackman
2005-08-03 18:19 ` Dave Korn
2005-08-03 21:26 ` Richard Henderson
2005-08-04 4:42 ` Ian Lance Taylor
2005-08-04 12:40 ` Paul Koning
2005-08-02 20:46 ` Paul Koning
2005-08-02 22:17 ` Shaun Jackman
2005-08-03 17:16 ` Paul Koning
2005-08-02 22:26 ` Shaun Jackman
2005-08-02 22:29 ` Shaun Jackman
2005-08-02 21:05 ` Mike Stump
2005-08-02 21:11 ` Joe Buck
2005-08-02 22:15 ` Shaun Jackman
2005-08-02 22:12 ` Joe Buck
2005-08-02 17:48 ` Falk Hueffner
2005-08-02 18:03 ` Mike Stump
[not found] <345be691050804025955c0b4ab@mail.gmail.com>
2005-08-04 15:06 ` Shaun Jackman
2005-08-04 15:09 ` Christian Joensson
2005-08-05 8:41 ` Carl Whitwell
2005-08-05 16:09 ` Shaun Jackman
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