* allignment in structures
@ 2004-08-26 15:59 Purnendu/Gmail
2004-08-26 16:19 ` Eljay Love-Jensen
0 siblings, 1 reply; 2+ messages in thread
From: Purnendu/Gmail @ 2004-08-26 15:59 UTC (permalink / raw)
To: gcc-help
#pragma pack(2)
struct abc {
char a;
char b;
char c;
};
A sizeof( struct abc) gives 3, shouldnot i expect it to be 4? any pointers???
i am using " gcc version 3.2.2 20030222 (Red Hat Linux 3.2.2-5)"
-purnendu
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: allignment in structures
2004-08-26 15:59 allignment in structures Purnendu/Gmail
@ 2004-08-26 16:19 ` Eljay Love-Jensen
0 siblings, 0 replies; 2+ messages in thread
From: Eljay Love-Jensen @ 2004-08-26 16:19 UTC (permalink / raw)
To: Purnendu/Gmail, gcc-help
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
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