main.c

#include <stdio.h>
#include <stdlib.h>

#include "lauxlib.h"
#include "lua.h"
#include "lualib.h"

// custom allocator for lua runtime, use luaL_newstate() instead if you don't care
void *lua_alloc(void *custom_arg, void *old_pointer, size_t old_size, size_t new_size) {
 (void)custom_arg; (void)old_size;

 if (new_size == 0) {
 free(old_pointer);
 return NULL;
 }
 return realloc(old_pointer, new_size);
}

// will be called from lua
static int lua_get_third_number(lua_State *L) {
 int value = 20;
 lua_pushinteger(L, value);
 return 1;
}

int main(void) {
 lua_State *lua = lua_newstate(lua_alloc, /* custom_arg: */ NULL);
 luaL_openlibs(lua);

 lua_register(lua, "get_third_number", lua_get_third_number);

 const char *lua_code =
 "function add_numbers(a, b)" "\n"
 " c = get_third_number()" "\n"
 " print('Adding:', a, b, c)" "\n"
 " return a + b + c" "\n"
 "end" "\n";

 if (luaL_dostring(lua, lua_code) != LUA_OK) {
 printf("Error running code: %s\n", lua_tostring(lua, -1));
 return 1;
 }

 // push function on the stack
 lua_getglobal(lua, "add_numbers");

 // (optional) verify function
 if (!lua_isfunction(lua, -1)) {
 printf("last stack value (should be 'add') is not a function\n");
 return 1;
 }
 
 // push function arguments
 lua_pushinteger(lua, 25); // a
 lua_pushinteger(lua, 24); // b

 if (lua_pcall(lua, 2, 1, /* error_function: */ 0) != LUA_OK) {
 printf("Error calling function: %s\n", lua_tostring(lua, -1));
 return 1;
 }

 // (optional) verify the result value
 if (!lua_isinteger(lua, -1)) {
 printf("Function 'add' did not return a number\n");
 return 1;
 }

 int result = lua_tonumber(lua, -1);
 printf("Result of add is %d.\n", result);

 lua_pop(lua, 1);
 lua_close(lua);
 return 0;
}