From:	"John M. Aldrich" <fighteer AT cs DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: attribute packed??
Date:	Thu, 19 Sep 1996 23:11:56 -0700
Organization:	Three pounds of chaos and a pinch of salt
Lines:	34
Message-ID:	<324235AC.2CB3@cs.com>
References:	<Pine DOT VUL DOT 3 DOT 91 DOT 960919114251 DOT 2058C-100000 AT zeus DOT adv-boeblingen DOT de>
Reply-To:	fighteer AT cs DOT com
NNTP-Posting-Host:	ppp202.cs.com
Mime-Version:	1.0
To:	Lennart Steinke <steinke AT zeus DOT adv-boeblingen DOT de>
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Lennart Steinke wrote:
> 
> Hi!
> 
> Could someone tell me what __attribute__, and
> esp. the packed option of it does, and why it's
> neccassary to use them?

I recommend that you download the DJGPP Frequently Asked Questions
List from the following URL:

ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2/faq201b.zip

The part relevant to __attribute__ and why it is needed is section 22.9.

In brief:  DJGPP is a 32-bit compiler.  In a 32-bit operating system
it is inefficient to access memory regions that are not "aligned", i.e.,
that start on a 2- or 4-byte boundary.  Thus, all variables and
structures in gcc (and thus DJGPP) are aligned automatically by default.
__attribute__((packed)) is a GNU C extension to ANSI C which allows you
to "turn off" this automatic alignment and squeeze structures into the
minimum space possible.  Further documentation can be found in the info
files for gcc (type: info gcc "C Extensions" "Variable Attributes").
Note that punctuation and capitalization are important!

-- 
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| John M. Aldrich, aka Fighteer I    |         fighteer AT cs DOT com          
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