Re: sizeof(struct ...)

From: Erik Mouw <erik@harddisk-recovery.com>
Date: Nov 23, 2006, 16:42 UTC
Message-ID: <20061123164258.GE6581@harddisk-recovery.com>
In-Reply-To: <4565C8F4.6000606@lsrfire.ath.cx>

On Thu, Nov 23, 2006 at 05:14:44PM +0100, Ren? Scharfe wrote:

Show 27 quoted lines

> Erik Mouw schrieb:
> > On Thu, Nov 23, 2006 at 04:45:09PM +0100, Ren? Scharfe wrote:
> >>  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.
> 
> Does it add 1 byte after a and and 1 after b or two after b?
> I suspect it's the latter case -- otherwise Gerrit's patch,
> which started this thread, wouldn't help solve his problem.
> Or the pad sizing follows complicated rules that I do not
> understand at the moment.

You're right, it adds the padding after b:

#define offsetof(TYPE, MEMBER) __builtin_offsetof (TYPE, MEMBER) printf("%d %d\n", offsetof(struct foo, a), offsetof(struct foo, b));

prints "0 3" on ARM.

> Time to look for an ARM emulator, it seems.

objdump -D -S is your friend. I didn't have an ARM target ready, but at least I know enough ARM assembly that I can see what it will print :)

Erik

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