MODDING.md

Modding

Modding Minecraft: Pi Edition is possible by patching the binary at runtime. To make this easier minecraft-pi-reborn includes a library called libreborn.so which provides several functions to help you patch the game.

Hex Addresses

Minecraft: Pi Edition has no symbols so you must patch the hex address of an instruction instead of using a function name. Hex addresses can be found using tools like Ghidra or RetDec. To find out what a function does, you can find its equivalent in MCPE 0.6.1 and use its name for reference because it includes export symbols.

Loading Directories

minecraft-pi-reborn loads mods from /usr/lib/minecraft-pi-reborn-client/mods and ~/.minecraft-pi/mods.

C++ Strings

Minecraft: Pi Edition was compiled with an old version of GCC, so when interacting with C++ strings, make sure you set -D_GLIBCXX_USE_CXX11_ABI=0.

__attribute__((constructor)) static void init() { }

You will see this a lot in the docs, it is the MCPI-Reborn equivalent of int main() {}. When a mod is loaded the code inside of it will be executed, for example:

#include <stdio.h>

__attribute__((constructor)) static void init() {
    printf("Helloworld\n");
}

This code will print "Helloworld" when loaded by MCPI-Reborn.

libreborn.so API

Header files can be download from Github.

void overwrite(void *start, void *target)

This method replaces a function with another function.

Parameters

• start: The function you are replacing.
• target: The function you are replacing it with.

Return Value

None

Warning

This should never be used on functions that are only 1 byte long because it overwrites 2 bytes.

Example

static int func_injection(int a, int b) {
    return a + 4;
}

__attribute__((constructor)) static void init() {
    overwrite((void *) 0xabcde, func_injection);
}

void overwrite_call(void *start, void *original)

This allows you to overwrite a specific call of a function rather than the function itself. This allows you to call the original function. However, this does not effect VTables.

Parameters

• start: The address of the function call to overwrite.
• target: The function call you are replacing it with.

Return Value

None

Warning

This method can only be safely used 512 times in total.

Example

typedef int (*func_t)(int a, int b);
static func_t func = (func_t) 0xabcde;
static void *func_original = NULL;

static int func_injection(int a, int b) {
    // Call Original Method
    (*func)(a, b);

    return a + 4;
}

__attribute__((constructor)) static void init() {
    overwrite_call((void *) 0xabcd, func_injection);
}

void overwrite_calls(void *start, void *original)

This allows you to overwrite all calls of a function rather than the function itself. This allows you to call the original function. However, this does not effect VTables.

Parameters

• start: The function call to overwrite;
• target: The function call you are replacing it with.

Return Value

None

Warning

This method can only be safely used 512 times in total.

Example

typedef int (*func_t)(int a, int b);
static func_t func = (func_t) 0xabcde;
static void *func_original = NULL;

static int func_injection(int a, int b) {
    (*func)(a, b);

    return a + 4;
}

__attribute__((constructor)) static void init() {
    overwrite_calls((void *) func, func_injection);
}

void patch(void *start, unsigned char patch[])

This allows you to replace a specific instruction.

Parameters

• start: The target instruction.
• patch: The new instruction (array length must be 4).

Return Value

None

Example

__attribute__((constructor)) static void init() {
    unsigned char patch_data[4] = {0x00, 0x00, 0x00, 0x00};
    patch((void *) 0xabcde, patch_data);
}

feature.so

int feature_has(const char *name, int server_default);

You may want a mod to only ativate if the user has selected a certain options. Example:

chat_enabled = feature_has("Implement Chat", 1);

Using custom/modified mods

To compile your mod use the following code for C++:

arm-linux-gnueabihf-g++ -shared -fPIC -DREBORN_HAS_COMPILED_CODE <MODFILE>.cpp -o lib<MODFILE>.so

And this for C:

arm-linux-gnueabihf-gcc -shared -fPIC -DREBORN_HAS_COMPILED_CODE <MODFILE>.c -o lib<MODFILE>.so

To load your mod you will need to move it to the mod folder throught the following snippet:

sudo mv lib<MODFILE>.so ~/.minecraft-pi/mods/lib<MODFILE>.so

For both, you will need to replace "<MODFILE>" with the name of the file you made. If you are making a server-side mod instead of a client-side mod you will need to change minecraft-pi-reborn-client to minecraft-pi-reborn-server.