Linking C with NASM

Question

I have a NASM file and a C file. How do I call a function in the C file from the NASM file? How do I call a NASM function from the C file?

Many Thanks DD

like this: http://www.nasm.us/doc/nasmdoc9.html – Sam Liao Jul 28 '14 at 09:15 — Sam Liao, Jul 28 '14 at 09:15

score 33 · Accepted Answer · answered Jul 28 '14 at 09:47

33

Calling assembly function from C:

C file:

#include <stdio.h>

int add(int a, int b);

int main(int argc, char **argv)
{
  printf("%d\n", add(2, 6));
  return 0;
}

assembly file:

global add

section .data

section .text

add:
    mov   eax, [esp+4]   ; argument 1
    add   eax, [esp+8]   ; argument 2
    ret

compiling:

$ nasm -f elf add.asm 
$ gcc -Wall main.c add.o 
$ ./a.out 
8
$

Calling C function from assembly:

C file:

int add(int a, int b)
{
  return a + b;
}

assembly file:

extern add
extern printf
extern exit

global _start

section .data
  format db "%d", 10, 0
section .text

_start:
    push  6
    push  2
    call  add     ; add(2, 6)

    push  eax
    push  format
    call  printf  ; printf(format, eax)

    push  0
    call exit     ; exit(0)

compiling:

$ gcc -Wall -c add.c
$ nasm -f elf main.asm 
$ ld main.o add.o -lc -I /lib/ld-linux.so.2
$ ./a.out 
8
$

answered Jul 28 '14 at 09:47

5

This, of course, assumes a 32-bit platform; it won't work on 64-bit platforms – Drew McGowen Jul 28 '14 at 13:53
@DrewMcGowen What makes this program a 32-bit or 64 bit ? – Abhishek Sagar Aug 03 '19 at 09:00
@AbhishekSagar by default, gcc will compile to the same number of bits as what your OS is running – Drew McGowen Aug 03 '19 at 13:37
It won't work on 32bit because the way you pass parameters isn't the same. You don't push stuff on the stack. Instead you use the ABI and use registers as parameters. – E.T Apr 07 '20 at 22:53

Linking C with NASM

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