From: Paul Koning <pkoning@equallogic.com>
To: dave.korn@artimi.com
Cc: sjackman@gmail.com, gcc@sources.redhat.com
Subject: RE: memcpy to an unaligned address
Date: Tue, 02 Aug 2005 19:48:00 -0000 [thread overview]
Message-ID: <17135.52757.353000.216332@gargle.gargle.HOWL> (raw)
In-Reply-To: <SERRANObDBRArVc4Iu1000000e0@SERRANO.CAM.ARTIMI.COM>
>>>>> "Dave" == Dave Korn <dave.korn@artimi.com> writes:
Dave> ----Original Message----
>> From: Shaun Jackman Sent: 02 August 2005 20:26
>> On 8/2/05, Paul Koning <pkoning@equallogic.com> wrote:
>>> One of the things that continues to baffle me (and my colleagues)
>>> is the bizarre way in which attributes such as "packed" work when
>>> applied to structs.
>>>
>>> It would be natural to assume, as Shaun did, that marking a
>>> struct "packed" (or, for that matter, "packed,aligned(2)") would
>>> apply that attribute to the fields of the struct.
>> This is exactly the behaviour suggested by the info docs:
>>
>> $ info gcc 'C Ext' 'Type Attr' ... Specifying this attribute for
>> `struct' and `union' types is equivalent to specifying the
>> `packed' attribute on each of the structure or union members.
>>
Dave> There are two separate issues here:
Dave> 1) Is the base of the struct aligned to the natural alignment,
Dave> or can the struct be based at any address
Dave> 2) Is there padding between the struct members to maintain
Dave> their natural alignments (on the assumption that the struct's
Dave> base address is aligned.)
Sure. But in Shaun's case it looks like (2) has been applied, except
that the compiler doesn't adjust the generated code correctly. I
would argue that "packed" applied to a whole struct should produce
BOTH effects 1 and 2.
There's a third case for which there appears to be no notation:
3) A pointer to a T that doesn't have the normal alignment of
the type T.
For example, as far as I can tell, GCC offers no way to say "pointer
to unaligned int" -- short of creating a one-member struct.
paul
next prev parent reply other threads:[~2005-08-02 19:48 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 [this message]
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
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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