On Thu, Nov 23, 2006 at 05:14:44PM +0100, Ren? Scharfe wrote:
> 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
--
+-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 --
| Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands
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