From: Eljay Love-Jensen <eljay@adobe.com>
To: Purnendu/Gmail <purnendu@gmail.com>, gcc-help@gcc.gnu.org
Subject: Re: allignment in structures
Date: Thu, 26 Aug 2004 16:19:00 -0000 [thread overview]
Message-ID: <6.1.2.0.2.20040826105923.01fbd390@iplan-mn.corp.adobe.com> (raw)
In-Reply-To: <4ca029ac04082608234d62cfda@mail.gmail.com>
Hi Purnendu,
>A sizeof( struct abc) gives 3, shouldnot i expect it to be 4?
No, it should be 3 in this case.
The char data type has an alignment of 1.
#pragma pack(2) does not increase alignment requirements, it only decreases
them.
>any pointers???
Use GCC __attribute__ with aligned and pack to affect alignment and/or
packing, don't use #pragma pack.
// C++ example.
#include <cstdio>
#include <cstddef>
struct Foo
{
char a __attribute__((aligned(2)));
char b __attribute__((aligned(2)));
long c __attribute__((packed));
char d;
char e;
};
int main()
{
printf("Foo.a %d\n", offsetof(Foo, a));
printf("Foo.b %d\n", offsetof(Foo, b));
printf("Foo.c %d\n", offsetof(Foo, c));
printf("Foo.d %d\n", offsetof(Foo, d));
printf("Foo.e %d\n", offsetof(Foo, e));
}
Note: the aligned attribute has certain restrictions, depending on
platform. See the online documentation.
Often, alignment and packing are used to mimic a canonical data structure,
which populates the structure using read or fread. I strongly discourage
that practice, and encourage having a helper read routine that populates
the structure field-by-field from the byte-by-byte data source. Likewise,
the inverse for the write routines.
HTH,
--Eljay
prev parent reply other threads:[~2004-08-26 15:59 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-08-26 15:59 Purnendu/Gmail
2004-08-26 16:19 ` Eljay Love-Jensen [this message]
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