Calling NASM function in C - 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. e.g.:
#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. e.g.:
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