Calling NASM procedure from gcc code

[crackers]

I have problem with doing it
asm code

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[BITS 32]
[GLOBAL do_something]
[EXTERN _k_main]
call _k_main
hlt

do_something:
  mov eax, 4
  ret

c code

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extern void do_something(void);
void k_main()
{
  do_something();
}

I'm getting linker error - "undefined reference to '_do_something'".
So I've changed asm file by adding _ before both do_something but I'm still getting the same error. Anybody know what I'm doing wrong?

[pkd]

You need to use Name Mangling in your asm code.

for elf32/64 (LINUX not sure for windows)

name your function _Z12Do_Somethingv

Check your C/C++ executable with a Hex Editor to see the mangled name that it is trying to find

the _Z is standard
the number is how many chars in Name
the v stands for void (Parameter)
i for int
c for char
etc.


and call form c/c++

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   Do_Something(void);

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[Bits 32]
global _Z12Do_Somethingv

_Z12Do_Somethingv

...code here

  ret

Hope this helps if not google for name mangling

bye
pkd

[zloba]

try this

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extern "C" void do_something(void);

this tells the C++ compiler not to use mangling when looking for do_something

[crackers]

You need to use Name Mangling in your asm code.

for elf32/64 (LINUX not sure for windows)

name your function _Z12Do_Somethingv

Check your C/C++ executable with a Hex Editor to see the mangled name that it is trying to find

Well I checked my compiled file (but not assembled) and I only see _do_something.

try this

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extern "C" void do_something(void);

this tells the C++ compiler not to use mangling when looking for do_something

Well I'm using C compiler and when I try to do something like that I'm getting "syntax error before string constant". The funniest thing is that under linux it's working just fine. Only under windows (djgpp) I'm having such problems.

[Solar]

Did you add the underscore both to the "GLOBAL" statement and the function label?

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[BITS 32]
[GLOBAL _do_something]
[EXTERN _k_main]
call _k_main
hlt

_do_something:
  mov eax, 4
  ret

To avoid the whole issue, use your original code, and force the C compiler not to use leading underscores: [tt]gcc -fno-leading-underscore ...[/tt]

[crackers]

Did you add the underscore both to the "GLOBAL" statement and the function label?

Yes I did.

To avoid the whole issue, use your original code, and force the C compiler not to use leading underscores: [tt]gcc -fno-leading-underscore ...[/tt]

I tried that too with no effect - getting "undefined reference to 'do_something'" >:(

[Solar]

Would you mind quoting the NASM command you use to assemble the object file, and the linker command you use to build the executable?

I've got this suspicion that either you don't have an object file coming out of NASM, or that you aren't giving that object file to the linker...

[crackers]

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nasm -f coff entry.asm -o entry.o
C:\djgpp\bin\gcc -fno-leading-underscore -c kernel.c -O2 -o kernel.o
C:\djgpp\bin\ld -Map kernel.map entry.o kernel.o -Tlink.ld -o kernel.bin

and linker script

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OUTPUT_FORMAT("binary")
ENTRY("k_main")
SECTIONS {
 .text 0x10000 : {
  code = . ; _code = . ;
  *(.text)
 }
 .data : {
  *(.data)
 }
 .bss : {
  bss = . ; _bss = . ;
  *(.bss)
  *(.COMMON)
 }
 end = . ; _end = . ;
} 

[Solar]

Well, I don't have DJGPP at hand, and the Cygwin 'ld' chokes on the classic "PE operation on non-PE file".

But [tt]objdump -t entry.o[/tt] and [tt]objdump -t kernel.o[/tt] will show you the symbol tables of the object files. It turns out the Cygwin gcc at least is happily ignoring -fno-leading-underscores...

[zloba]

It turns out the Cygwin gcc at least is happily ignoring -fno-leading-underscores

-underscores or -underscore?

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-fleading-underscore
   This option and its counterpart, -fno-leading-underscore, forcibly change the
   way C symbols are represented in the object file.  One use is to help link with
   legacy assembly code.

    Warning: the -fleading-underscore switch causes GCC to generate code that is not
   binary compatible with code generated without that switch.  Use it to conform to
   a non-default application binary interface.  Not all targets provide complete
   support for this switch.

[crackers]

After some time I've returned to this problem and discovered that I forgot to write

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section .text

in my asm code ;D