Lua Core

"Lua is an extension programming language designed to support general procedural programming with data description facilities. It also offers good support for object-oriented programming, functional programming, and data-driven programming. Lua is intended to be used as a powerful, light-weight scripting language for any program that needs one. " (from "Lua 5.1 Reference Manual" by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes)

This module provides a BlitzMax interface to the Lua main and auxiliary APIs. It is almost complete, the only functions missing are those with variable argument lists (which are not yet supported by BlitzMax).

The axe.lua package also contains the full Lua 5.1.4 distribution. The Lua source code is completely integrated into the module, additional DLLs (or shared libraries, resp.) are no longer required.

Lua Documentation

The Lua Reference Manual is part of this distribution and may be directly visited from here:

• Lua 5.1 Contents
• Lua 5.1 Reference Manual
• Lua 5.1 ReadMe

Additional information can be found on the Lua web site; the Lua wiki contains further material about this module and related packages.

Introduction

The following description is not meant as a tutorial, but may still help you to start Lua programming within BlitzMax. More API-related information can be found in the Lua Reference manual

Setting up a Lua VM

Running Lua scripts always require the instantiation of at least one Lua VM:

 local LuaState:byte ptr = luaL_newstate()

The result of this instantiation is a pointer to a structure which contains the full state of the new VM and has to be passed as the first argument to almost any other Lua API function.

It is now time to "open" the intrinsic Lua libraries (note: these libraries are part of axe.lua now, external DLLs or shared libraries are not necessary):

 luaL_openlibs(LuaState)

Always initialize the Lua VM by opening its libraries unless you really know what you are doing!

Usually, creating a single Lua state is sufficient, even further (Lua) threads share the basic Lua state (and its associated environment).

Shutting down a Lua VM

At the end, it's always a good idea to shut down the Lua VM

 lua_close(LuaState)

The Lua interpreter has now been terminated and its state variable is no longer valid.

Accessing Lua Globals

The code

  lua_pushstring(LuaState, "BMXString")
  lua_setglobal (LuaState, "luaglobal")

defines a global Lua variable (called luaglobal) which contains a string (namely "BMXString").

Registering BlitzMax Functions

In order to access BlitzMax features from within a Lua script, it is necessary to implement suitable BlitzMax functions and register them in the Lua VM.

Such BlitzMax functions typically look as follows:

 function BMXName:int (LuaState:Byte Ptr)
   ...      ' handling of parameters passed from Lua (if required)
     ...    ' actual function body
   ...      ' passing results back to Lua (if required)
   return 0 ' number of values returned to Lua
 end function

Such a function is then registered using

 lua_register(LuaState, "luaname", BMXName)

This command registers a BlitzMax function (called BMXName) with the (global) name luaname under Lua. Both names may be equal, but they do not have to.

Running Lua Scripts From Within BlitzMax

The code

 local Result:int = luaL_loadString(LuaState,LuaScript)
 if (Result <> 0) then
   ' ERROR!!!
   lua_close(LuaState) ' just to be complete
   end
 end if

loads and compiles a (BlitzMax) string containing a LuaScript. The result (of compilation) is left on the (Lua) stack.

 lua_getfield(LuaState, LUA_GLOBALSINDEX, "debug")' get global "debug"
 lua_getfield(LuaState, -1, "traceback")       ' get "debug.traceback"
 lua_remove (LuaState, -2)           ' remove "debug" table from stack

 Result = lua_pcall(LuaState,1,-1,-1)' use "debug.traceback" as err.hdlr
 if (Result <> 0) then
   ' ERROR
   lua_close(LuaState) ' just to be complete
   end
 end if

actually evaluates the previously loaded script. The initially mentioned Lua commands just prepare for proper error messages should the Lua script fail.