Android NDK: Create shared library from a static library

Let’s assume you want to create one shared library from two static libraries. Here is how to do it:

In your Android.mk

include $(CLEAR_VARS)
LOCAL_MODULE := lib1
LOCAL_SRC_FILES := lib1.cpp
include $(BUILD_STATIC_LIBRARY)

include $(CLEAR_VARS)
LOCAL_MODULE := lib2
LOCAL_SRC_FILES := lib2.cpp
include $(BUILD_STATIC_LIBRARY)

# this shared library will have all symbols from two above libraries
include $(CLEAR_VARS)
LOCAL_MODULE := lib_shared
LOCAL_SRC_FILES := empty.cpp
LOCAL_WHOLE_STATIC_LIBRARIES := lib1 lib2
include $(BUILD_SHARED_LIBRARY)

Important option to note is LOCAL_WHOLE_STATIC_LIBRARIES. If you use regular LOCAL_STATIC_LIBRARIES, because you don’t use any symbols from lib1 or lib2 in lib_shared, they will be stripped at link time. To prevent this from happening LOCAL_WHOLE_STATIC_LIBRARIES adds following options to link line to make sure symbols are not stripped:

-Wl,--whole-archive -llib1 -llib2 -Wl,--no-whole-archive

Here is documentation for --whole-archive:

--whole-archive

For each archive mentioned on the command line after the --whole-archive option, include every object file in the archive in the link, rather than searching the archive for the required object files. This is normally used to turn an archive file into a shared library, forcing every object to be included in the resulting shared library. This option may be used more than once.