allignment in structures

allignment in structures

[Purnendu/Gmail]

#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

Re: allignment in structures

[Eljay Love-Jensen]

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