Using Lua with C++ — A short tutorial

Using Lua with C++ — A short tutorial

Written by Christian Stigen Larsen

This work is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License.

Using Lua is easy! In this short tutorial we'll show how to write a fully working host-program in C++ with Lua callbacks.

Since the static Lua libraries are written in C, you must import them as such:

extern "C" {#include "lua.h"}int main(){  lua_State *L = lua_open();  lua_close(L);  return 0;}

Update 2007-05-14: It has been reported that on some systems you need to include the headers lualib.h and lauxlib.h to compile the above example:

extern "C" {#include "lualib.h"#include "lauxlib.h"}

Compiling and linking with GNU g++:

g++ host.cpp -o host -Ilua-5.0.2/include/ -Llua-5.0.2/lib/ -llua

Including lualib.h and lauxlib.h makes it easy to write a fully working host:

#include <iostream>extern "C" {#include "lua.h"#include "lualib.h"#include "lauxlib.h"}void report_errors(lua_State *L, int status){  if ( status!=0 ) {    std::cerr << "-- " << lua_tostring(L, -1) << std::endl;    lua_pop(L, 1); // remove error message  }}int main(int argc, char** argv){  for ( int n=1; n<argc; ++n ) {    const char* file = argv[n];    lua_State *L = lua_open();    luaopen_io(L); // provides io.*    luaopen_base(L);    luaopen_table(L);    luaopen_string(L);    luaopen_math(L);    luaopen_loadlib(L);    std::cerr << "-- Loading file: " << file << std::endl;    int s = luaL_loadfile(L, file);    if ( s==0 ) {      // execute Lua program      s = lua_pcall(L, 0, LUA_MULTRET, 0);    }    report_errors(L, s);    lua_close(L);    std::cerr << std::endl;  }  return 0;}

Compilation and linking:

g++ host.cpp -o host -Ilua-5.0.2/include/ -Llua-5.0.2/lib/ -llua -llualib

Running Lua programs

Let's test this with some Lua programs. The files here are from the distribution, hello.lua is simply:

-- the first program in every languageio.write("Hello world, from ",_VERSION,"!\n")

Executing a couple of Lua programs with our host program produces:

[csl@eris:~/dev/lua/lua-5.0.2]$ ./host test/hello.lua test/printf.lua-- Loading file: test/hello.luaHello world, from Lua 5.0.2!-- Loading file: test/printf.luaHello csl from Lua 5.0.2 on Wed Mar  2 13:13:05 2005

Calling C functions from Lua

It gets very interesting when Lua programs call your own functions. In the following program, we define a function my_function() and register it with the Lua environment using lua_register(). Our function prints its arguments as strings and returns the integer value of 123.

#include <iostream>extern "C"{#include "lua.h"#include "lualib.h"#include "lauxlib.h"}int my_function(lua_State *L){  int argc = lua_gettop(L);  std::cerr << "-- my_function() called with " << argc    << " arguments:" << std::endl;  for ( int n=1; n<=argc; ++n ) {    std::cerr << "-- argument " << n << ": "      << lua_tostring(L, n) << std::endl;  }  lua_pushnumber(L, 123); // return value  return 1; // number of return values}void report_errors(lua_State *L, int status){  if ( status!=0 ) {    std::cerr << "-- " << lua_tostring(L, -1) << std::endl;    lua_pop(L, 1); // remove error message  }}int main(int argc, char** argv){  for ( int n=1; n<argc; ++n ) {    const char* file = argv[n];    lua_State *L = lua_open();    luaopen_io(L); // provides io.*    luaopen_base(L);    luaopen_table(L);    luaopen_string(L);    luaopen_math(L);    luaopen_loadlib(L);    // make my_function() available to Lua programs    lua_register(L, "my_function", my_function);    std::cerr << "-- Loading file: " << file << std::endl;    int s = luaL_loadfile(L, file);    if ( s==0 ) {      // execute Lua program      s = lua_pcall(L, 0, LUA_MULTRET, 0);    }    report_errors(L, s);    lua_close(L);    std::cerr << std::endl;  }  return 0;}