Compiling native_app_glue.c results in an invalid library file

Issue

I'm cross-compiling a C++ library to Android. I'm using CMake as my build generator and I'm using the toolchain file provided in the NDK (called android.toolchain.cmake). Note, I'm on Windows.

I'd like to compile the android_native_app_glue.c source - also provided by the NDK - to a static library so I can later link it to my final application. After having an error, that said I'm missing the symbol ANativeActivity_onCreate, I've started investigating. ANativeActivity_onCreate is a function defined in android_native_app_glue.c, so I've looked at the compiled library using nm -o libnative_glue.a and it was quite surprising to see this:

libnative_glue.a:android_native_app_glue.c.o:0000000000000000 T ANativeActivity_onCreate
libnative_glue.a:android_native_app_glue.c.o:0000000000000000 t android_app_create
libnative_glue.a:android_native_app_glue.c.o:0000000000000000 t android_app_destroy
libnative_glue.a:android_native_app_glue.c.o:0000000000000000 t android_app_entry
libnative_glue.a:android_native_app_glue.c.o:0000000000000000 t android_app_free
...

Basically my functions are "there", but are zero-sized. I'm assuming they are thrown away at the next link, since they are not valid - looking at the next artifact with nm confirmed that.

The CMake snippet to compile the library is fairly trivial, but I'll provide it for completeness's sake:

add_library(native_glue STATIC "${CMAKE_ANDROID_NDK}/sources/android/native_app_glue/android_native_app_glue.c")

I have two pre-defined cache-entries for CMake, ANDROID_ABI=arm64-v8a and ANDROID_PLATFORM=24.

What could cause this invalid library compilation?

Solution

Building a NativeActivity is usually done via libnative_app_glue.a, see https://developer.android.com/ndk/samples/sample_na.

Unfortunately, the instructions there have never been updated for CMake, so it could be easier to add the android_native_app_glue.c file to your shared lib.

Answered By - Alex Cohn