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Let's assume we want to create an unnecessarycustommath module for Lua.

localcustommath=require"custommath"print(custommath.add(10,20))

#ifndefCUSTOM_MATH_H#defineCUSTOM_MATH_H//c++ compatability#ifdef__cplusplus#defineC_API_BEGIN extern "C" {#defineC_API_END }#else#defineC_API_BEGIN#defineC_API_END#endif// !__cplusplusC_API_BEGIN#include<lua.h>#include<lualib.h>#include<lauxlib.h>#include<math.h>//the functions we want to implement. names do not matterintadd(lua_State*lua);intsubtract(lua_State*lua);intmultiply(lua_State*lua);intdivide(lua_State*lua);//the function which will be called when "require "custommath"" is called from luaint __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 signatureint luaopen[MODULE](lua_State*) so Lua can find the function. In our case[MODULE] iscustommath.

Note: The shared library has to have the same name as the module. In this casecustommath.

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.

Theint luaopen_custommath(lua_State*) implementation:

intluaopen_custommath(lua_State*lua){//create table with 4 entrieslua_createtable(lua,0,4);//set key value pairs of the tablelua_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,&divide);lua_settable(lua,-3);//there should now be a table on the stack with our 4 functionsreturn1;}

Running the following Lua script:

localcustommath=require"custommath"locala=1.1localb=2.2print(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 = 3.31.1 - 2.2 = -1.11.1 * 2.2 = 2.421.1 / 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 alua.exe and runs thetest.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 aextern "C" around your code and especially around the lua includes like I did in the header file.

• Lua

This project usesCMake 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.

1. Clone the repository with Lua submodule

   git clone --recurse-submodules https://github.com/Seng3694/CLuaModuleExample

2. Generate the build files

   mkdir bincd bincmake -G"Your Generator" ../CLuaModuleExample

3. Build the files

   cmake --build . --config Release

If everything went correct you should see this after building:

1.1 + 2.2 = 3.31.1 - 2.2 = -1.11.1 * 2.2 = 2.421.1 / 2.2 = 0.5

If there are errors while building, remove thepost_build custom command in theCMakeLists.txt. You have to run thelua executable on your own and pass thetest.lua script file as an argument.