C Programming help .....wtf ?!?!

[Rick Bowers]

Then again, one way to decrease the size of structures such as this is to 
order them so that the larger elements are at the top. This will eliminate 
much of the padding the compiler inserts.

typedef struct _dummymenu
         {
         int d_sumnumber ;
         char d_char ;
         } DummyMenu ;

~Rick

At 02:43 AM 1/26/2005, you wrote:
>On Tue, 2005-01-25 at 21:30, Vu Pham wrote:
> > > -----Original Message-----
> > > From: linux-users-bounces at linux-sxs.org
> > > [mailto:linux-users-bounces at linux-sxs.org] On Behalf Of Ben Duncan
> > > Sent: Tuesday, January 25, 2005 2:16 PM
> > > To: Linux tips and tricks
> > > Subject: Re: C Programming help .....wtf ?!?!
> > >
> > > Why does the sizeof( struct _dummymenu) on the below return 8 ?
> > >
> > >
> > >    typedef struct _dummymenu
> > >         {
> > >         char d_char ;
> > >         int d_sumnumber ;
> > >         } DummyMenu ;
> > >
> > > Yet if I do one a structure like this:
> > >
> > > typedef struct _dummymenu
> > >         {
> > >         char a_char ;
> > >         char b_char ;
> > >         char c_char ;
> > >         } DummyMenu ;
> > >
> > > it returns 3 ......
> > >
> >
> > I think because of the memory aligmnet option of the compiler. You can try
> > #pragma pack(n) to change the default alignment, like
>
>Of course. I go for this as well (over my eariler pre-coffee dribble) .
>I forgot. We use this for data structures that we send over the net to
>machines running other OS. Like OS/9. Keep in mind that you should only
>pack when needed as the default behavior results in faster access to the
>parts. Especially on platforms that do, say, 32-bit access no matter.
>Then the compiler has to mask and shift the data as well. This is
>another reason to use the sizeof() operator and not guess the size.
>
> >
> > #pragma pack(1)       // set the alignment to 1, all vars are adjacent 
> to each
> > other
> > typedef struct _dummymenu
> > {
> >         char d_char ;
> >          int d_sumnumber ;
> > } DummyMenu ;
> >
> >
> > Vu
> >
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