Building an Android executable gRPC server that uses protocol buffers (without APK)

I'm not quite sure why this happens. Something about once the linker loads helloworld_proto_lib, it overrides all loaded protobuf symbols, and for some reason another library that you had nothing to do with crashes your program. But that's not telling you anything new.

Here's one way to solve this problem:

1. Changes to grpc-helloworld.cc

Make the main extern "C", and change its name maybe. For example:

 extern "C" int my_main() {
  std::cout << "qwerty" << std::endl;
  return 0;
}

2. Add file grpc-avocado.cc

This will contain the actual main of the executable, which will dynamically load the libraries helloworld_proto_lib and grpc-helloworld. Here's how to do it:

#include <iostream>
#include <android/dlext.h>
#include <dlfcn.h>

int main() {
  android_dlextinfo extinfo;
  extinfo.flags = ANDROID_DLEXT_FORCE_LOAD;

  void* proto_lib = android_dlopen_ext("/path/to/libhelloworld_proto_lib.so", RTLD_LAZY, &extinfo);
  void* helloworld = dlopen("/path/to/libgrpc-helloworld.so", RTLD_LAZY);
  int (*my_main)() = (int (*)())dlsym(helloworld, "my_main");

  return my_main();
}

The function android_dlopen_ext from #include <android/dlext.h>, and its flag argument, are described here: https://developer.android.com/ndk/reference/group/libdl . In the above code we pass the flag ANDROID_DLEXT_FORCE_LOAD, which is documented as:

When set, do not use stat(2) to check if the library has already been loaded.

This flag allows forced loading of the library in the case when for some reason multiple ELF files share the same filename (because the already-loaded library has been removed and overwritten, for example).

Note that if the library has the same DT_SONAME as an old one and some other library has the soname in its DT_NEEDED list, the first one will be used to resolve any dependencies.

I think the text in bold is what explains why this solution works.

3. Change CMakeLists.txt

Since you'll be loading helloworld_proto_lib dynamically, you can now remove it from the executable definition, and there's no need for any proto headers:

add_executable(avocado
    src/main/cpp/grpc-avocado.cc)

target_link_libraries(avocado
  android
  ${log-lib})

Build, push, and run

You can now build, push the executable avocado and the two libraries libgrpc-helloworld.so, libhelloworld_proto_lib.so, and run. You don't need LD_LIBRARY_PATH. Good luck with the rest of your project!