Android NDK: how to build and then use a prebuilt library - android-ndk

Question

I have read many posts on how to share prebuilt libraries using the Android.mk system. The solutions boil down to two steps:
A directory with the already-built library uses *include $(PREBUILT_SHARED_LIBRARY)*
The project consuming the library uses *LOCAL_SHARED_LIBRARIES*
(you can substitute "STATIC" for "SHARED" to build and use a .a instead of a .so)
What I am trying to do is add a step 0: build the library from sources. If I change a source file that contributes to the library I want the Android.mk system to execute steps 0, 1, and 2 in order.
I have two projects in Eclipse/ADT:
MyApp - uses MyLibrary
MyLibrary - contains the source files for the library
The question I'm asking here focuses on MyLibrary project. Here is the Android.mk for MyLibrary:
LOCAL_PATH := $(call my-dir)
# step 0: build my library
include $(CLEAR_VARS)
LOCAL_MODULE := mylibrary
LOCAL_LDLIBS := -llog
LOCAL_SRC_FILES := libsrc1.c libsrc2.c
include $(BUILD_SHARED_LIBRARY)
# step 1: export my library (PREBUILT_SHARED_LIBRARY):
include $(CLEAR_VARS)
LOCAL_MODULE := mylibrary-prebuilt
LOCAL_SRC_FILES := ../libs/$(TARGET_ARCH_ABI)/libmylibrary.so
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)
include $(PREBUILT_SHARED_LIBRARY)
If I do a project->clean... and then a project->build project in this project I get the error:
ERROR:jni/Android.mk:mylibrary-prebuilt: LOCAL_SRC_FILES points to a missing file.
The clean removes the .so and I'm guessing PREBUILT_SHARED_LIBRARY detects the missing .so before BUILD_SHARED_LIBRARY rebuilds the library and copies it (even though the steps are in the correct order).
If I comment out step 1 and build, libmylibrary.so is correctly built and copied to libs/armeabi/libmylibrary.so. If I then uncomment step 2 and do a project->build project, that is without doing a clean first, I get these warnings and error:
warning: overriding commands for target `obj/local/armeabi/libmylibrary.so'
warning: ignoring old commands for target `obj/local/armeabi/libmylibrary.so'
warning: overriding commands for target `libs/armeabi/libmylibrary.so'
warning: ignoring old commands for target `libs/armeabi/libmylibrary.so'
make: *** No rule to make target `jni/../libs/armeabi/libmylibrary.so', needed by `obj/local/armeabi/libmylibrary.so'. Stop.
I think I understand these errors, but I do not see a way to accomplish what I want.
What am I missing?

Answer 1

You can simply avoid step 1 entirely and just rebuild the library from sources. I'm not sure I understand the benefits of having the three steps your describe. Feel free to clarify.
In all cases, your analysis of ndk-build's behaviour is spot on, and all of this is completely intentional.
The NDK itself provides several prebuilt libraries and allows you to rebuild them from sources if you want. For example, if you look at sources/cxx-stl/stlport/Android.mk, you will see that a variable (STLPORT_FORCE_REBUILD) is used to control whether to use the prebuilt binaries, or rebuild them from sources.
Maybe using a similar method for your project would work.