Calling NASM function in C

Question

I'm trying to learn x86 assembler, and I want to call a NASM function in C . When i run my program I get this error:

Segmentation fault (Core dumped)

I've tried dozens of variations of my simple test function but it stops every time at the same position.

Here are my asm and c files:

div.asm:

global _test

_test:
    push    ebp
    mov     ebp, esp
    push    ebx
    mov     eax, [ebp+8]
    mov     ebx, [ebp+12]
    div     ebx
    pop     ebp
    ret

main.c:

#include <stdio.h>

extern unsigned int test (unsigned int, unsigned int);

int main(void)
{
    printf("%d\n", div(85,5));
    return 0;
}

I compile & link the files with:

nasm -f elf -o div.o div.asm
gcc -m32 -c -o main.o main.c
gcc -m32 -o run div.o main.o

I use a 64 Bit Linux in a Virtual Machine .

What is my mistake here, and how can I fix it?

Answer 1

You forget to pop ebx (or at least make the stack in order):

push    ebp
mov     ebp, esp
push    ebx         ; you push it here
mov     eax, [ebp+8]
mov     ebx, [ebp+12]
xor     edx,edx     ; ..and you must zero edx
div     ebx
pop     ebx         ; forgot to pop it here
pop     ebp
ret

Answer 2

It is unclear if you ever got your problem solved. In addition to the other issues, you would need to make your function call in your main.c match the call in div.asm . For example if you have created an assembly function _test , you need to declare it as extern and actually use the function in main . eg:

#include <stdio.h>

extern unsigned int _test (unsigned int, unsigned int);

int main(void)
{
    printf("%d\n", _test (85,5));    /* you are calling div here, not _test */
    return 0;
}

(your function name is not the name for your assembly object file div.o -- and as pointed out in the comments, div is an unsigned division declared in stdlib.h along with ldiv , and lldiv )

Your global declaration in your assembly function file must match the name you declared in main . eg:

    global _test

_test:
    push    ebp
    mov     ebp, esp
    mov     eax, [ebp+8]
    xor     edx, edx
    div     dword [ebp+12]
    mov     esp, ebp
    pop     ebp
    ret

Now, you can compile, link and run your test file:

$ nasm -f elf -o div.o div.asm
$ gcc -m32 -c -o main.o main.c
$ gcc -m32 -o run div.o main.o
$./run
17

or for the compilation/link, simply:

$ nasm -f elf -o div.o div.asm
$ gcc -m32 -o run main.c div.o