index.rst

Ndk Shims

How to add new or missing native ndk function

Adding a new libc function to the launcher

Our libc-shim wrapper are split across multiple cpp files, all have a structure like https://github.com/minecraft-linux/libc-shim/blob/master/src/common.cpp#L334-L350.

void shim::add_common_shimmed_symbols(std::vector<shim::shimmed_symbol> &list) {
    list.insert(list.end(), {
        {"__errno", bionic::get_errno},
    });
}

• "__errno" is the name of the libc symbol
• bionic::get_errno is a function or global pointer with the same type and calling convention as the ndk provided function. Linux armhf has a different calling convention than android armeabi-v7a, you have to wrap all functions with floating point arguments or return values, use ARMHFREWRITE(bionic::get_errno) to automatically wrap it in c++. Windows uses different calling conventions than linux, macOS and Android.

Loading a stub library with mcpelauncher-linker

Stub libraries are libraries without the need of an android shared library file, which implements symbols of real shared libraries. You can call linker::dlopen and linker::dlsym on them to get the pointer of the function or global object.

std::unordered_map<std::string, void*> syms;
syms["__errno"] = (void*) bionic::get_errno;
linker::load_library("stub.so", syms);

• "__errno" is the name of the symbol
• bionic::get_errno is a function or global pointer with the same type and calling convention as the ndk provided function. Linux armhf has a different calling convention than android armeabi-v7a, you have to wrap all functions with floating point arguments or return values, use ARMHFREWRITE(bionic::get_errno) to automatically wrap it in c++. Windows uses different calling conventions than linux, macOS and Android.