Let's assume we want to create an unnecessary custommath
module for Lua.
local custommath = require "custommath"
print(custommath.add(10, 20))
#ifndef CUSTOM_MATH_H
#define CUSTOM_MATH_H
//c++ compatability
#ifdef __cplusplus
#define C_API_BEGIN extern "C" {
#define C_API_END }
#else
#define C_API_BEGIN
#define C_API_END
#endif // !__cplusplus
C_API_BEGIN
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <math.h>
//the functions we want to implement. names do not matter
int add(lua_State* lua);
int subtract(lua_State* lua);
int multiply(lua_State* lua);
int divide(lua_State* lua);
//the function which will be called when "require "custommath"" is called from lua
int __declspec(dllexport) luaopen_custommath(lua_State* lua);
C_API_END
#endif // !CUSTOM_MATH_H
The key part in this header file is this function:
int __declspec(dllexport) luaopen_custommath(lua_State* lua);
First of all you need the _declspec(dllexport)
because we want to create a shared library.
It is also required to have this specific signature int luaopen[MODULE](lua_State*)
so Lua can find the function. In our case [MODULE]
is custommath
.
Note: The shared library has to have the same name as the module. In this case custommath
.
I won't go into implementation details of the actual math functions. These are just basic Lua stack manipulations. If you are struggling with these please read the manual.
The int luaopen_custommath(lua_State*)
implementation:
int luaopen_custommath(lua_State* lua)
{
//create table with 4 entries
lua_createtable(lua, 0, 4);
//set key value pairs of the table
lua_pushstring(lua, "add");
lua_pushcfunction(lua, &add);
lua_settable(lua, -3);
lua_pushstring(lua, "subtract");
lua_pushcfunction(lua, &subtract);
lua_settable(lua, -3);
lua_pushstring(lua, "multiply");
lua_pushcfunction(lua, &multiply);
lua_settable(lua, -3);
lua_pushstring(lua, "divide");
lua_pushcfunction(lua, ÷);
lua_settable(lua, -3);
//there should now be a table on the stack with our 4 functions
return 1;
}
Running the following Lua script:
local custommath = require "custommath"
local a = 1.1
local b = 2.2
print(tostring(a) .. " + " .. tostring(b) .. " = " .. tostring(custommath.add(a, b)))
print(tostring(a) .. " - " .. tostring(b) .. " = " .. tostring(custommath.subtract(a, b)))
print(tostring(a) .. " * " .. tostring(b) .. " = " .. tostring(custommath.multiply(a, b)))
print(tostring(a) .. " / " .. tostring(b) .. " = " .. tostring(custommath.divide(a, b)))
will result in following output:
1.1 + 2.2 = 3.3
1.1 - 2.2 = -1.1
1.1 * 2.2 = 2.42
1.1 / 2.2 = 0.5
Note: The Lua module can only be imported if the Lua version of the interpreter is compatible with the Lua version of the used API libraries.
This example builds a lua.exe
and runs the test.lua script as post build process. The Lua versions are therefore exactly the same.
Note: You can create modules like this in C++ too. Just wrap a extern "C"
around your code and especially around the lua includes like I did in the header file.
This project uses CMake to generate platform and compiler-specific build files.
Build just tested on Windows. Will most likely not work on other platforms. You had to atleast change the __declspec(dllexport)
when you are using GCC to __attribute__((dllexport))
or __attribute__((visibility("default")))
when GCC version is greater equal than 4. But for simplicity I decided not to do it.
- Clone the repository with Lua submodule
git clone --recurse-submodules https://github.com/Seng3694/CLuaModuleExample
- Generate the build files
mkdir bin cd bin cmake -G"Your Generator" ../CLuaModuleExample
- Build the files
cmake --build . --config Release
If everything went correct you should see this after building:
1.1 + 2.2 = 3.3
1.1 - 2.2 = -1.1
1.1 * 2.2 = 2.42
1.1 / 2.2 = 0.5
If there are errors while building, remove the post_build
custom command in the CMakeLists.txt. You have to run the lua
executable on your own and pass the test.lua
script file as an argument.