Re: sizeof(struct ...)

From: Erik Mouw <erik@harddisk-recovery.com>
Date: Nov 23, 2006, 15:54 UTC
Message-ID: <20061123155431.GD6581@harddisk-recovery.com>
In-Reply-To: <4565C205.8050300@lsrfire.ath.cx>

On Thu, Nov 23, 2006 at 04:45:09PM +0100, Ren? Scharfe wrote:

Show 11 quoted lines

> Andy Whitcroft schrieb:
> > You see packing attributes applied to similar things in the kernel.
> > Perhaps they are relevant here?
> > Is there not some kind of attribute thing we can apply to this structure
> > to prevent the padding?  You see that in the kernel from time to time.
> > 
> > struct foo {
> > } __attribute__((packed));
> 
> Yes, that would be nice, but unfortunately __attribute__ is no standard
> C.

There is no standard C way to pack structures. Some compilers use #pragma's, gcc uses __attribute__((packed)).

>  Is there really a compiler that inserts padding between arrays of
> unsigned chars?

Yes, that compiler is called "gcc".

#include <stdio.h>

struct foo {
        unsigned char a[3];
        unsigned char b[3];
};

int main(void)
{
        printf("%d\n", sizeof(struct foo));
        return 0;
}

On i386 that prints 6, on ARM it prints 8.

Erik

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