Re: [Nasm-devel] nasm externs and macho

[Frank K. <fbk...@co...>]

pnk...@gm... wrote:
> Hello all-
> 
> I'm trying to understand some weird behavior I'm seeing with nasm on an
> Intel Mac.

Nice to see someone using the new code! I don't know much about "macho". 
I'll cc this to the nasm-devel list - probably the author of the code 
will have some insight. Thanks for the feedback!!!

Best,
Frank


> I have two assembly files, bar.asm and baz.asm.  bar references labels
> defined by baz.  But it doesn't seem like the object files are being
> linked together properly, unless I am misinterpreting what gdb is
> printing and what my C code is doing.  In particular, the references in
> the data of bar do not correspond to the actual addresses of the data
> in baz.
> 
> Perhaps I have misunderstood something about the macho format or about
> the Mac OS X ABI.  Does dynamic linking get in the way of doing this
> sort of resolution of addresses in the data section?
> 
> I have replicated a simpler version of the problem with four files (the
> two assembly files, some C code to print out the bad values, and a
> Makefile to build it all and print the output to a text file).
> I have copied the four files below, along with the output that is
> printed to problem.txt.  The key problem is that &baz is determined by
> the C code to be 0x00002024, but the entry in the bar array
> corresponding to the label for baz is the number 0x00001cdf.
> 
> Does anyone have an idea why this might be happening, and whether the
> problem is in the OS X linker (unlikely), Nasm's mach-o support, or in
> my mental model of how the two assembly files should be linking
> together?
> 
> ;;; baz.asm
> 	section .data
> 	align 8
> 	global _baz
> _baz:
> 	dd	0xdeadbead
> ;;; end of baz.asm
> 
> ;;; bar.asm
> 	section .data
> 	align 8
> 	global _bar
> 	extern	_foo
> 	extern  _foil
> 	extern	_baz
> _bar:
> 	dd	0x12345678
> 	dd	_foo
> 	dd	_foil
> 	dd	_baz
> 	dd	_bar
> 
> ;;; end of bar.asm
> 
> /* foo.c */
> #include <stdio.h>
> 
> int foiled(int x) {  return 4*x + 7; }
> 
> int foil(int x) {  return 3*x + 2; }
> 
> int foo(int x) {  return 2*x + 1; }
> 
> extern int *bar;
> extern int *baz;
> 
> int main() {
>   printf("  &foil:               0x%08x\n",   &foil);
>   printf("  &foo:                0x%08x\n",   &foo);
>   printf("   bar:                0x%08x\n",    bar);
>   printf("  &bar:                0x%08x\n",   &bar);
>   printf("   baz:                0x%08x\n",    baz);
>   printf("  &baz:                0x%08x\n",   &baz);
>   printf(" (&bar)[0] 0x12345678: 0x%08x\n",  (&bar)[0]);
>   printf(" (&bar)[1] _foo      : 0x%08x\n",  (&bar)[1]);
>   printf(" (&bar)[2] _foil     : 0x%08x\n",  (&bar)[2]);
>   printf(" (&bar)[3] _baz      : 0x%08x\n",  (&bar)[3]);
>   printf(" (&bar)[4] _bar      : 0x%08x\n",  (&bar)[4]);
>   return 0;
> }
> /* end of foo.c */
> 
> # Makefile
> problem.txt: foo
> 	echo problem.txt > problem.txt
> 	nasm -v >> problem.txt
> 	./foo >> problem.txt
> 	echo end of problem.txt >> problem.txt
> 
> foo: foo.c bar.o baz.o
> 	gcc -o foo foo.c bar.o baz.o
> 
> bar.o: bar.asm
> 	nasm -f macho bar.asm
> 
> baz.o: baz.asm
> 	nasm -f macho baz.asm
> # End of Makefile
> 
> problem.txt
> NASM version 0.98.40 (Apple Computer, Inc. build 9) compiled on Apr  6
> 2006
>   &foil:               0x00001cdf
>   &foo:                0x00001cf3
>    bar:                0x12345678
>   &bar:                0x00002010
>    baz:                0xdeadbead
>   &baz:                0x00002024
>  (&bar)[0] 0x12345678: 0x12345678
>  (&bar)[1] _foo      : 0x00001cf3
>  (&bar)[2] _foil     : 0x00001cdf
>  (&bar)[3] _baz      : 0x00001cdf
>  (&bar)[4] _bar      : 0x00002010
> end of problem.txt
>