From: "John M. Aldrich" <fighteer AT cs DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Problem with Packed structs
Date: Mon, 07 Apr 1997 11:52:18 -0700
Organization: Two pounds of chaos and a pinch of salt
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To: djgpp AT delorie DOT com
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Mark T Logan wrote:
> 
> Since DJGPP packs structs, the structs are of a different size than you
> might find when using a 16 bit compiler.  You can't read the file in unless
> the structs that you are reading into are the same as the structs that were
> written.
> 
> What if I want to read a file I make with say, a Visual Basic 16 bit
> program?

Your terminology is backwards.  DJGPP _aligns_ struct fields, padding
them with sufficient space to align them on 4 byte boundaries.  It also
adds padding to the end of each structure.  In order to prevent it from
doing this, you must use the following syntax:

struct foo
{
   char bar __attribute__((packed));
   short baz __attribute__((packed));
   long xyz[3] __attribute__((packed));
};

Without packing, this struct would be (1 + 3) + (2 + 2) + (4 * 3 + 0) =
20 bytes.  With packing, it becomes 15 bytes, which is the same as it
would be under Turbo C.  Note that as long as you're using C and not
C++, it is sufficient to append the __attribute__((packed)) to the end
of the struct definition, like so:

struct foo
{
   char bar;
   short baz;
   long xyz[3];
} __attribute__((packed));

Due to a bug in GCC, this syntax does not work when compiling C++
programs.

You should also be aware that DJGPP uses 4 byte integers, not the 2 byte
integers that you are probably used to.  Therefore, if you want to read
2 byte integers, you must declare the struct field as 'short'.  In fact,
for portable code you should never use 'int', only 'short' and 'long'.

BTW, the FAQ goes into great detail on the reasons for alignment and
packing in chapters 22.9 and 22.10.

-- 
John M. Aldrich <fighteer AT cs DOT com>                      

* Anything that happens, happens.
* Anything that, in happening, causes something else to happen,
  causes something else to happen.
* Anything that, in happening, causes itself to happen again, happens
  again.
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