From: zidharta AT geocities DOT com
Newsgroups: comp.os.msdos.djgpp
Subject: Re: what is _attribute_((packed)) ?? why DJGPP use it?
Date: Tue, 03 Aug 1999 12:14:33 GMT
Organization: Deja.com - Share what you know. Learn what you don't.
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References: <7o5doq$gj8$1 AT nnrp1 DOT deja DOT com> <7o6831$4un$1 AT taliesin DOT netcom DOT net DOT uk>
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Im going to read the FAQ ... so if you think I can find this easily on
the FAQ just ignore this ...
as far as I know I need __attribute_((packed)) for reading bmp file ...
is it always needed for reading file ... ?
or is it only needed when you want to transfer file info into struct
like when you read BMP file??
thanks alot for your reply

Sid

In article <7o6831$4un$1 AT taliesin DOT netcom DOT net DOT uk>,
  "Michael Stewart" <mike AT reggin DOT freeserve DOT co DOT uk> wrote:
> zidharta AT geocities DOT com wrote in message
<7o5doq$gj8$1 AT nnrp1 DOT deja DOT com>...
> >I was looking at this bitmap loader source code ... and I noticed
that
> >he used __attribute_((packed)) ... and I dont understand what is
it...
> >I ve been looking at my C library help file and I couldnt find
anything
> >about it ...
>
> >
> >if anyone can explain to me what that is and why DJGPP need it?.. is
it
> >has something to do with protected mode?..
>
> Its all in the FAQ, heres an extract:
> GCC generates 32-bit code, and in that mode, there is a significant
penalty
> (in terms of run-time performance) for unaligned accesses, like
accessing a
> 16-bit short which isn't aligned on a word boundary, or accessing a
32-bit
> int which isn't aligned on a dword boundary.  To produce faster code,
GCC
> pads struct fields so that each field can be accessed without delays;
this
> sometimes produces struct size which is larger than the sum of the
sizes of
> its fields.
> ...
> If the layout of the structure cannot be changed (e.g., when it must
match
> some external specification, like a block of data returned by some
system
> call), you can use the __attribute__((packed)) extension of GCC (See
the Gcc
> docs.) to prevent GCC from padding the structure fields; this will
make
> accesses to some of the fields slower.
>
>


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