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---
title: "Calling assembly functions from C"
url: calling-assembly-functions-from-c.html
date: 2024-06-17T16:12:13+02:00
type: note
draft: false
---
This is using the portable GNU assembler and TinyCC compiler but GCC or Clang
can be used as well.
First lets define a simple function in assembly.
```asm
# sum.s
.intel_syntax noprefix
.global sum
.text
sum:
add rdi, rsi
mov rax, rdi
ret
```
Lets compile this with GNU assembler `as sum.s -o sum.o`.
Now we need a C program that calls this function.
```c
// main.c
#include <stdio.h>
// We need to define the signature of the function.
int sum(int a, int b);
int main() {
for(int i=0; i<10; ++i) {
printf("SUM of 5+%d is %d\n", i, sum(5, i));
}
return 0;
}
```
Now lets compile and link into final program with `tcc main.c sum.o -o main`.
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