minecraft-pi-reborn/docs/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](https://ghidra-sre.org) or [RetDec](https://retdec.com). To find out what a function does, you can find its equivalent in Minecraft: Pocket Edition 0.6.1 and use its name for reference because Minecraft: Pocket Edition 0.6.1 includes symbols.

## Loading Directories
``minecraft-pi-reborn`` loads mods from two locations, ``/app/minecraft-pi/mods``, and ``~/.minecraft-pi/mods``. The first location only exists in the Docker container and is immutable, so you should place your mods in ``~/.minecraft-pi/mods`` which is mounted on the host.

## 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``.

## ``libreborn.so`` API
Header files and the shared library can be download from [Jenkins](https://jenkins.thebrokenrail.com/job/minecraft-pi-reborn/job/master/lastSuccessfulBuild/artifact/out/lib).

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