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

[Roger Oberholtzer]

As a related point, X programmers use a macro called (IIRC) OFFSET(x)
that tells where item 'x' is relative to the start of the structure. It
is most often used when initializing a number of items in a data
structure from some list of initializers - something X does more than
just about any environment I have come across. So, if you are doing lots
of initializing, this could be a useful method to add to your toolbox. I
personally have stayed away from it. But that is just me. Just check
some X code (maybe Xt) for examples.

On Wed, 2005-01-26 at 09:22 -0500, Rick Bowers wrote:
> 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
> > >
> > > _______________________________________________
> > > Linux-users mailing list
> > > Linux-users at linux-sxs.org
> > > http://mail.linux-sxs.org/cgi-bin/mailman/listinfo/linux-users
> > >
> > > Need to chat further on this subject? Check out #linux-users on 
> > irc.linux-sxs.org !
> >+????????????????????????????+???????????????????????????????+
> >? Roger Oberholtzer          ?   E-mail: roger at opq.se        ?
> >? OPQ Systems AB             ?      WWW: http://www.opq.se/  ?
> >? Nybrogatan 66 nb           ?    Phone: Int + 46 8   314223 ?
> >? 114 41 Stockholm           ?   Mobile: Int + 46 733 621657 ?
> >? Sweden                     ?      Fax: Int + 46 8   314223 ?
> >+????????????????????????????+???????????????????????????????+
> >
> >_______________________________________________
> >Linux-users mailing list
> >Linux-users at linux-sxs.org
> >http://mail.linux-sxs.org/cgi-bin/mailman/listinfo/linux-users
> >
> >Need to chat further on this subject? Check out #linux-users on 
> >irc.linux-sxs.org !
> 
> _______________________________________________
> Linux-users mailing list
> Linux-users at linux-sxs.org
> http://mail.linux-sxs.org/cgi-bin/mailman/listinfo/linux-users
> 
> Need to chat further on this subject? Check out #linux-users on irc.linux-sxs.org !
>