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Re: Problem with sizeof?

To: Muthukumar Ratty <muthu5@sbcglobal.net>
Subject: Re: Problem with sizeof?
From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Mon, 8 Jul 2002 10:31:45 +0200 (MEST)
Cc: Venkatesh M R <venkatesh@multitech.co.in>, Linux/MIPS Development <linux-mips@oss.sgi.com>
In-reply-to: <Pine.LNX.4.33.0207080100100.30691-100000@Muruga.localdomain>
Sender: owner-linux-mips@oss.sgi.com
On Mon, 8 Jul 2002, Muthukumar Ratty wrote:
> > I am  not sure, but can this be memory allignment problem ?
> > you can check this by adding one more (dummy ) element of __u32 length (MIPS
> > should print 40 insted of 44 and pc shuld print 40 as expected )
> 
> You are right. when I add a __u32 dummy  to the tc_stats then it still
> prints the size as 40. Is it get padded at the end? My main concern is,
> is this a normal behavior or do I need to upgrade my MIPS build
> environment?

The alignment rule for 64-bit value is 8 bytes on MIPS, and only 4 bytes on
ia32.

Since sizeof(struct ...) must return a value that's a multiple of the alignment
rule for the largest element in the struct, it returns 40 on MIPS.

> Thanks a lot,
> Muthu.
> 
> >
> > if  it works then you can get rid of the above problem by changing the
> > compilar settings.
> >
> > Regards,
> > venkatesh M R
> >
> >
> >
> >
> > Muthukumar Ratty wrote:
> >
> > > Hi,
> > > In my MIPS system sizeof(struct tc_stats) returns 40 but it returns 36 in
> > > i386 PC.
> > >
> > > tc_stats is defined in include/linux/pkt_sched.c as
> > > struct tc_stats
> > > {
> > >         __u64   bytes;                  /* NUmber of enqueues bytes */
> > >         __u32   packets;                /* Number of enqueued packets   */
> > >         __u32   drops;                  /* Packets dropped because of lack
> > > of resources */
> > >         __u32   overlimits;             /* Number of throttle events when
> > > this
> > >                                          * flow goes out of allocated
> > > bandwidth*/
> > >         __u32   bps;                    /* Current flow byte rate */
> > >         __u32   pps;                    /* Current flow packet rate */
> > >         __u32   qlen;
> > >         __u32   backlog;
> > > #ifdef __KERNEL__
> > >         spinlock_t *lock;
> > > #endif
> > > };
> > > I printed the offsets of individual fields and they are same in i386 and
> > > MIPS. If the "spinlock_t" is defined then this may be the case but I
> > > am compiling an application, so it shouldnt be there? Right now I
> > > think the problem could be with the "sizeof" operator.
> > > Could someone please let me know if this is the case or I am doing
> > > something wrong. Also any solution.

Gr{oetje,eeting}s,

                                                Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                                            -- Linus Torvalds


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