//test.lua
width = 10
height = 20

//test.c
#include <stdio.h>
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>

int main() {
  lua_State *L = luaL_newstate();
  luaL_openlibs(L);

  if(luaL_loadfile(L, "test.lua") || lua_pcall(L, 0,0,0)){
    printf("error %s\n", lua_tostring(L,-1));
    return -1;
  }
  lua_getglobal(L,"width");
  lua_getglobal(L,"length");
  printf("width = %d\n", lua_tointeger(L,-2));
  printf("length = %d\n", lua_tointeger(L,-1));
  lua_close(L);
  return 0;
}

//test.lua
function add(x, y)
  return x+y
end
//test.c
#include <stdio.h>
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <math.h>

int main() {
  lua_State *L = luaL_newstate();
  luaL_openlibs(L);

  if(luaL_loadfile(L, "test.lua") || lua_pcall(L, 0,0,0)){
    printf("error %s\n", lua_tostring(L,-1));
    return -1;
  }
  lua_getglobal(L,"add");
  lua_pushnumber(L, 10);
  lua_pushnumber(L, 20);
  if(lua_pcall(L, 2, 1, 0) != 0){
    printf("error %s\n", lua_tostring(L,-1));
    return -1;
  }
  double z = lua_tonumber(L, -1);
  printf("z = %f \n", z);
  lua_pop(L, 1);
  lua_close(L);
  return 0;
}

//mylib.c
#include <stdio.h>
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <math.h>

static int myadd(lua_State *L){
                int a = luaL_checknumber(L, 1);
                int b = luaL_checknumber(L, 2);
                lua_pushnumber(L, a+b);
                return 1;
}

static const struct luaL_Reg mylib [] = {
        {"add", myadd}, 
        {NULL, NULL}
};

int luaopen_mylib(lua_State *L){
  luaL_newlib(L, mylib);
  return 1;
}

//call.lua
#!/usr/local/bin/lua

lib=require "mylib"
print(lib.add(1, 2))