如何在 C 代码中调用用 ARM 汇编编写的函数？

Question

To be specific:

1. The arm assembly function is written in a separated file such as .S file or .asm file.
2. I need to call this function in main.c
3. The ARM assembly is in ARMv8 architecture.

I've written some testing code but it fails to work.

#include <stdio.h>
extern int a_add(int a, int b);
int main(){
    int fi=5;
    int se=7;
    int result=a_add(fi, se);
    printf("result is %d", result);
    return 0;
}

And following is the assembly code.(a_add.S)

.section .text
.globl a_add
a_add:
    add x3,x1,x0
    mov x0,x3
    br x30

Does anyone know how I can fix these two files to let the a_add function work? I haven't tried .asm file yet.

Any help is appreciated and please forgive if I made mistakes on expression, but I hope my question is clear.

Answer 1

I have fixed the code by using extern "C" and have solved the problem. What needed is just claim the function as following in main.cpp or main.c

extern "C" {int a_add(int a, int b);}

Replace the old "extern" line by this line with "C". This is helpful if the compiler is for C++ code instead of C code, basically by letting the function compiled in the way of C code, not C++ code.

I haven't made such test but I suppose if the compiler is C compiler, there won't be such a problem.

Answer 2

main.c

#include<stdio.h>

extern int a_add(int, int);

int main() {
  int a = 5;
  int b = 3;
  printf("%d + %d = %d \n",  a, b, a_add(a, b));
}

add.s

        .text
        .global a_add
a_add:
        add     w0, w0, w1
        ret

Compiled with gcc (9.3.0) on an AArch64 machine (a docker container with ubuntu 20.04) using:

gcc main.c add.s -o run.exe && ./run.exe

A quick way to get the skeleton ready for a desired function in a target assembly would be:

1. Write a simple c function matching the signature of the desired function, eg, for the add function above:

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

2. Generate assembly for such C function to begin with.

gcc -S -O2 add.c

Note: Using higher optimisation level might produce less obvious code.

Tip : Compiler Explorer is a good tool to know while playing around with the assembly.