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From: mwodrich AT cs DOT uct DOT ac DOT za (M Wodrich)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: GCC/DJGPP Weirdness?
Date: 25 Jul 1995 12:25:17 GMT
Organization: University of Cape Town
Lines: 52
References: <3v1t91$bjs AT alpha DOT epas DOT utoronto DOT ca>
Nntp-Posting-Host: cs.uct.ac.za
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Dj-Gateway: from newsgroup comp.os.msdos.djgpp

In <3v1t91$bjs AT alpha DOT epas DOT utoronto DOT ca> dscully AT blues DOT uucp (David Scully) writes:

>Hi All;

>I'm new to DJGPP and have only programmed in Borland C before now.
>I've come across some unusual behavior in GCC while attempting to compile
>a program that I had written in Borland C.  Part of my program
>involved reading a Windows BMP off of disk and I had set up a structure
>to read the BMP header into.

(Struct declaration deleted ...)

>Then I used:
>        fread(&bmp_header,54,1,in_file);
>to read the structure from the disk.  Now for some reason this doesn't
>work with DJGPP, the bytes don't end up being aligned properly after the
>read.  I discovered that  I sizeof() on the structure reveals that it is
>56 bytes long instead of 54 as declared.
>   My question is am I trying to do something non-standard that Borland
>allowed me to get away with or is this an actual DJGPP/GCC bug and if the
>latter does anyone know a work around for this.

DJGPP is a 32-bit compiler, and has a habit of aligning data on word 
boundaries (presumably this is faster to access). As a result, the bytes
you define in your struct are not stored in successive memory slots, but
could have "gaps" between them.

There are 2 options :
1) Read in each element of the struct individually (slow!).
2) Tell GCC not to pad the data.
   To do this you need to use the "attribute" option for the declaration.
   I don't have the source code to hand, but from memory the syntax is:
   
   (example struct) :
   struct test {
	char x __attribute__ ((PACKED));
	int y __attribute__ ((PACKED));
	/* etc. for all data in the struct */
   };

This ensures that all the elements of the struct are stored contiguously,
so a fread() should word OK.

Hope this long explanation helps! (and isn't wrong!)
Adios
Mark.

--

-------------------------------------------------------------------------
Mark Wodrich					/\/\.\/\/.
Electrical Engineering Student,			"The truth is out there"