Exposing C Functions to Lua - https://chsasank.github.io/lua-c-wrapping.html

compile.sh

gcc main.c -shared -o mylib.so -fPIC  -llua

main.c

#ifdef __cplusplus
    #include "lua.hpp"
#else
    #include "lua.h"
    #include "lualib.h"
    #include "lauxlib.h"
#endif
#include <math.h>

//so that name mangling doesn't mess up function names
#ifdef __cplusplus
extern "C"{
#endif

static int c_swap (lua_State *L) {
    //check and fetch the arguments
    double arg1 = luaL_checknumber (L, 1);
    double arg2 = luaL_checknumber (L, 2);

    //push the results
    lua_pushnumber(L, arg2);
    lua_pushnumber(L, arg1);

    //return number of results
    return 2;
}

static int my_sin (lua_State *L) {
    double arg = luaL_checknumber (L, 1);
    lua_pushnumber(L, sin(arg));
    return 1;
}

//library to be registered
static const struct luaL_Reg mylib [] = {
      {"c_swap", c_swap},
      {"mysin", my_sin}, /* names can be different */
      {NULL, NULL}  /* sentinel */
    };

//name of this function is not flexible
int luaopen_mylib (lua_State *L){
    luaL_newlib(L, mylib);
    return 1;
}

#ifdef __cplusplus
}
#endif

test.lua

mylib = require 'mylib'
print(mylib.c_swap(2, 4))