CUSTOM C FUNCTIONS

Custom C Function Definition

In this example, I have a C file named sum.c with a function named sum and an assembly file:

#include <stdio.h>

extern int sum(int a, int b);

int sum(int a, int b)
{
    printf("Here!\n");
    return (a + b);
}

extern sum
extern exit

section .text
    global main

main:
    push 1
    push 2
    call sum
    push eax
    call exit

• extern is used to tell the assembler that the function is defined elsewhere;
• I use the call instruction to call my custom function sum from the sum.c file;
• The eax register contains the return value of the function;

---

Compilation Process

Now I can compile and link my assembly code with my file sum.c.

First, generate the object file using nasm. Second, compile the sum.c file and link it with the object file.

nasm -f elf32 -o first.o first.s
gcc -m32 -o first first.o sum.c -no-pie