Preventing functions from being stripped from a static library when linked into a shared library?

Question

I have a static library, Foo, that is used by a shared library, Bar. Bar is the native shared library loaded by my Android app. Foo contains JNI functions that are only call

Answer 1

Let's start with the basic two-libs sample from NDK. Here is its original Android.mk file:

 1 # Copyright (C) 2009 The Android Open Source Project
 2 #
 3 # Licensed under the Apache License, Version 2.0 (the "License");
 4 # you may not use this file except in compliance with the License.
 5 # You may obtain a copy of the License at
 6 #
 7 #      http://www.apache.org/licenses/LICENSE-2.0
 8 #
 9 # Unless required by applicable law or agreed to in writing, software
10 # distributed under the License is distributed on an "AS IS" BASIS,
11 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 # See the License for the specific language governing permissions and
13 # limitations under the License.
14 #
15 
16 # the purpose of this sample is to demonstrate how one can
17 # generate two distinct shared libraries and have them both
18 # uploaded in
19 #
20 
21 LOCAL_PATH:= $(call my-dir)
22 
23 # first lib, which will be built statically
24 #
25 include $(CLEAR_VARS)
26
27 LOCAL_MODULE    := libtwolib-first
28 LOCAL_SRC_FILES := first.c
29 
30 include $(BUILD_STATIC_LIBRARY)
31 
32 # second lib, which will depend on and include the first one
33 #
34 include $(CLEAR_VARS)
35 
36 LOCAL_MODULE    := libtwolib-second
37 LOCAL_SRC_FILES := second.c
38 
39 LOCAL_STATIC_LIBRARIES := libtwolib-first
40 
41 include $(BUILD_SHARED_LIBRARY)

Note that the JNI function is located in second.c which is not part of the libtwolib-first static library.

First, let us reproduce your problem. The change is simple:

...
27 LOCAL_MODULE    := libtwolib-first
28 LOCAL_SRC_FILES := first.c second.c
...
36 LOCAL_MODULE    := libtwolib-second
37 LOCAL_SRC_FILES :=

If you run the modified project, you will get the following error:

E/AndroidRuntime( 4213): FATAL EXCEPTION: main
E/AndroidRuntime( 4213): java.lang.UnsatisfiedLinkError: add
E/AndroidRuntime( 4213):        at com.example.twolibs.TwoLibs.add(Native Method)
E/AndroidRuntime( 4213):        at com.example.twolibs.TwoLibs.onCreate(TwoLibs.java:39)
E/AndroidRuntime( 4213):        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
E/AndroidRuntime( 4213):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
E/AndroidRuntime( 4213):        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
E/AndroidRuntime( 4213):        at android.app.ActivityThread.access$2300(ActivityThread.java:125)
E/AndroidRuntime( 4213):        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
E/AndroidRuntime( 4213):        at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 4213):        at android.os.Looper.loop(Looper.java:123)
E/AndroidRuntime( 4213):        at android.app.ActivityThread.main(ActivityThread.java:4627)
E/AndroidRuntime( 4213):        at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 4213):        at java.lang.reflect.Method.invoke(Method.java:521)
E/AndroidRuntime( 4213):        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:871)
E/AndroidRuntime( 4213):        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629)
E/AndroidRuntime( 4213):        at dalvik.system.NativeStart.main(Native Method)

You explained this problem precisely: the linker "stripped out" the "unused" Java_com_example_twolibs_TwoLibs_add() entry.

Now let us fix this:

39 LOCAL_STATIC_LIBRARIES := libtwolib-first
39 LOCAL_WHOLE_STATIC_LIBRARIES := libtwolib-first

And again the sample works!