Making mods #1: Basics & setting up the SDK

First you should obtain the ModLoader SDK, You should go to the GitHub downloads page: https://github.com/minecraft-linux/server-modloader/releases, download mod_sdk.zip and extract it somewhere, making sure to write down the path, as you'll need it.

A very simple Example Mod

CoreMod provides us with a simple callback called when the server has been started: void modloader_on_server_start(ServerInstance* serverInstance).

The ModLoader also provides us with several logging functions, check out the log.h header for the list: https://github.com/minecraft-linux/server-modloader/blob/master/include/modloader/log.h. The easiest way to log something from a C mod is to use the modloader_log*() function family - the letter following the function name determines the log level.

The mod loader provides both a C and a C++ API. The C++ API is the recommended way to get started.

Writing mods in C

While you'll most likely want to write mods in C++, here's an example in C (save it as main.c):

#include <modloader/log.h>
  
void modloader_on_server_start(void* serverInstance) {
  modloader_logv("TestMod", "Server has been started!");
}

You can compile it using gcc main.c -I ${MODLOADER_SDK}/include/ -L ${MODLOADER_SDK}/lib -lserver_modloader -shared -o libTestMod.so. Make sure to define MODLOADER_SDK before running this command. You should then copy libTestMod.so to the server mods/ directory.

Writing mods in C++

The following example uses the C++ logging API instead of the C one (save it as main.cpp):

#include <modloader/log.h>
using namespace modloader;
  
extern "C" void modloader_on_server_start(void* serverInstance) {
  Log::verbose("TestMod", "Server has been started!");
}

You can compile it using g++ main.cpp -std=c++11 -I ${MODLOADER_SDK}/include/ -L ${MODLOADER_SDK}/lib/ -lserver_modloader -shared -fPIC -o libTestMod.so. Make sure to define MODLOADER_SDK before running this command. You should then copy libTestMod.so to the server mods/ directory.

Please note that it is heavily recommeded to use CMake instead of plainly compiling the mod using the command line g++ executable.

Using the CMake build system

In order to use the CMake build system please create a CMakeLists.txt file (in the same directory as the mod sources) with the following contents:

cmake_minimum_required(VERSION 3.0)
project(_YOUR_MOD_NAME_)

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_POSITION_INDEPENDENT_CODE ON)

find_package(ModLoader REQUIRED)

add_library(_YOUR_MOD_NAME_ SHARED main.cpp)
target_link_libraries(_YOUR_MOD_NAME_ PUBLIC ModLoader)

Replace all occurrences of _YOUR_MOD_NAME_ with the name of your mod, eg. ExampleMod

Then, to compile a mod using the CMake build system you can do the following:

export CMAKE_PREFIX_PATH=/Path/To/Mod/Sdk
mkdir -p build && cd build
cmake ..
make