A ABI is a contract between the program and the Operating system / Hardware.

Lets write a library in Zig and have it “exported” in such a way that it conforms to C ABI (the API required by C to call functions, pass arguments and return values)

export fn multiplyAndAdd(x: c_int, y: c_int) c_int {
    var k: c_int = x * x;
    return k + y;
}

export -> This fn is to be compiled in a C ABI compatible way. c_int -> an integer whose bit layout can be understood by C.

Now lets build it into a shared dynamic library zig build-lib -fPIC -dynamic clib.zig

This will output a libclib.dylib on the macOS (will be libclib.so on Linux)

Lets see if we can call this fn from C code. After all, this is why we exported the fn in the first place.

#include "clib.h"
#include<stdio.h>


int main() {
  int k = 12;
  int j = multiplyAndAdd(k , 16);
  printf("%d\n", j);
}

int multiplyAndAdd(int, int);

gcc -I. -L. main.c -o main -lclib

Executing this, we get

./main
160

Cool. If this is C ABI, can we use it from other languages as well ? Turns out, we can. Lets call this fn from Python

import ctypes
import pathlib

if __name__ == "__main__":
    # Load the shared library into ctypes
    libname = pathlib.Path().absolute() / "libclib.dylib"
    c_lib = ctypes.CDLL(libname)
    c_lib.multiplyAndAdd.restype = ctypes.c_int
    answer = c_lib.multiplyAndAdd(ctypes.c_int(12), ctypes.c_int(16))
    print(answer)

> python pythonclib.py
160

C is THE universal ABI