[lug] More packing troubles...

[David Trowbridge]

After digging through some gcc code (ewwww) I figured it out.

the form that works, instead of this:
#pragma pack (push, pre_spi)
#pragma pack (2)
...
#pragma pack (pop, pre_spi)

is this:
#pragma pack (push, pre_spi, 2)
...
#pragma pack (pop)

I'll wait until 3 is out for a little while, then probably file a bug
report. Since #pragma pack(push[, id], <n>) is intended for MS VC(++)
compatibility, I think it would be best if it used the same
syntax/specification!

Thanks for all your comments,
-David

-------------------
David Trowbridge
jupiter at flatirons.org
http://jupiter.babylonia.flatirons.org

"Base 8 is just like base 10 really...if you're missing two fingers"
	-Tom Lehrer

On 11 Jun 2001, Tom Tromey wrote:

> >>>>> "David" == David Trowbridge <jupiter at flatirons.org> writes:
>
> David> I just now ran into the same problem with porting a network app and
> David> #pragma pack. My problem is this: in the app, before a network command
> David> structure, there are these two lines:
> David> #pragma pack (push, pre_spi)
> David> #pragma pack (2)
> David> and ending with this after the struct:
> David> #pragma pack (pop, pre_spi)
>
> David> I've defined HAVE_PRAGMA_PACK_PUSH_POP to 1, but I get the
> David> following error
>
> HAVE_PRAGMA_PACK_PUSH_POP is a gcc back-end macro.  It is only used when
> building gcc itself.  There's no point in defining it in your code.
>
> HAVE_PRAGMA_PACK_PUSH_POP is defined for all Linux targets.  So this
> feature should be available in your Linux gcc.  You could try
> compiling with `-Wunknown-pragmas' to verify this.
>
> David> and can't find any docs on the exact format gcc wants:
> David> spi.h:35: warning: malformed '#pragma pack(push[,id],<n>)'
> David> spi.h:35: warning: ignoring pragma:
>
> I looked at the manual and I also didn't see anything.  Well, there's
> some stuff in tm.texi, that's lame since that document is describing
> the back end and not what users ought to do.  Could you submit a gcc
> bug report for this?  It is very easy to do:
>
>     http://gcc.gnu.org/bugs.html
>
>
> The text in tm.texi, which is part of gcc 3.0, says (I just extract
> the Texinfo):
>
>     The pack(push,<n>) pragma specifies the maximum alignment
>     (in bytes) of fields within a structure, in much the same way as the
>     @samp{__aligned__} and @samp{__packed__} @code{__attribute__}s do.  A
>     pack value of zero resets the behaviour to the default.  Successive
>     invocations of this pragma cause the previous values to be stacked, so
>     that invocations of @samp{#pragma pack(pop)} will return to the previous
>     value.
>
>
> I'd advise not using these pragmas if you possibly can avoid it.  I
> imagine (though I'm not positive -- the back end isn't really my area)
> that the resulting code it will be slow.
>
> Tom
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