Message-ID: <32F088E7.34CA@eik.bme.hu>
Date: Thu, 30 Jan 1997 12:41:27 +0100
From: "DR. Andras Solyom" <solyom AT eik DOT bme DOT hu>
Reply-To: solyom AT eik DOT bme DOT hu
Organization: Technical University of Budapest
MIME-Version: 1.0
To: djgpp AT delorie DOT com
Subject: Re: Help: Packing structures
References: <01bc0e37$b83989e0$1293d9c2 AT victoria DOT browns DOT co DOT uk>
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Victoria Bradburne wrote:
> 
> Please could someone help my in packing a data structure. I  am using DJGPP
> as a cross compiler to M68000 and am having trouble packing some data
> structures:
> 
> struct Packet {
>         Byte ID;
>         Word Length;
>         Byte Data[DATALEN];
> };
> 

What I always define is:

#ifndef PACKED
    #ifdef __GNUC__
        #define PACKED  __attribute__ ((packed))
    #else
        #define PACKED
    #endif
#endif

and for your case I woud do

struct Packet {
         Byte ID PACKED;
         Word Length PACKED;
         Byte Data[DATALEN] PACKED;
 };

Please take care! If you want to pack any structure you MUST:
	put this PACKED attribute after EVERY member in your struct and
	MOST NOT put two variables with one type definition and only one PACKED
attribute
            e.g. : 
			struct any {
				long l1,l2 PACKED;
				byte b: PACKED;
				int i: PACKED;
			};
            is WRONG. The correct way to do it is:
			struct any {
				long l1 PACKED;
				long l2 PACKED;
				byte b: PACKED;
				int i: PACKED;
			};
            Note that although longs should always be packed in
principle, if you do not put the
            packed attribute after them then your whole structure will
be packed.

Eli, is this covered in the FAQ?
						Andras