Newsgroups: comp.os.msdos.djgpp
From: dshnv AT www DOT com
Subject: relocating variables in a COFF file
Lines: 58
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NNTP-Posting-Date: Sun, 29 Jul 2001 20:01:00 EDT
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Date: Mon, 30 Jul 2001 00:01:00 GMT
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

Hello,

I've been busy writing a program to support Loadable Modules under DJGPP (I know
there are already other dynamic module loaders out there but I just wanted to
write one myself, for educational reasons). Well anyway, it seems to work quite
well. But there's one major problem I don't understand.
When processing all relocations in a section, I relocate them as
following:(exactly as the COFF docs @ http://www.delorie.com/djgpp/doc/coff
say):

Btw, ``Image'' is the pointer to where the file is loaded in memory (loaded by
read(file, Image, size) (well not exactly but you get the idea :) )

RELOC_ADDR32 relocations:

long relocation_addr;

change = (unsigned long*)(Image + Reloc.r_vaddr);

- get address of symbol referred to (relocation_addr = Image + symbol.e_value)
- add the value currently stored in the location being adjusted.
(relocation_addr += *change)
- Store the value back into the location being adjusted. (*change =
relocation_addr)

RELOC_REL32:
these are mostly imports so they just point to the functions' address in memory 
(e.g: if (!strcmp(symbol_name, "_printf") *change = (unsigned long)printf;

Okay, for the most part this works fine. Entry point in the module in this
example is 'init()'. 
So when I've got something like this:

int init()
{
printf("hello from module!\n");
printf("bye!\n");
return 42;
}

it all works ok. printf is automatically imported 'n sutff, and it works fine.
However, when I'm adding variables (especially ARRAYS!!, like:
unsigned char list_of_chars[80];) and I want to use them, the contents of these
variables is just rubbish.

things like: list_of_chars[3] = 5; printf("%d", list_of_chars[3]); will output
33734 or something. It just doesn't make any sense. (Btw, these problems really
occur a lot on bigger modules, with a lot of functions, globar vars/local vars,
etc. in it)

Did someone here had the same problem? I've already looked at other dynamic
linking packages but I don't really see what goes wrong. Is there something I'm
missing when relocating variables (I don't treat variables other than
functions), or does somebody know what I'm doing wrong here? 

Thanks.