Android NDK/JNI: Building a shared library that depends on other shared libraries

Question

I am writing an android app that wants to make JNI calls into a shared library built in using the NDK. The trick is this shared library calls functions provided by OTHER shared libraries. The other shared libraries are C libraries that have been compiled elsewhere.

Here's what I've tried:

My Environment: I'm working in Eclipse. I've added native support and have a jni library. In that library I have my code and a \lib directory where I have copied my other .so files.

Attempt #1 Android.mk: Just telling it where the libs are

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)

LOCAL_MODULE           := native_lib
LOCAL_SRC_FILES        := native_lib.cpp

LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib/support_lib1
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib/support_lib2

include $(BUILD_SHARED_LIBRARY)

This builds just fine, but when I try to run I get errors indicating that dlopen(libnative_lib) failed because it couldn't load libsupport_lib1.

Coming here I found this:

Can shared library call another shared library?

which said that I needed to call load library on all necessary libraries. Great!

Attempt #2 Opening each library first

static {
    System.loadLibrary("support_lib1");
    System.loadLibrary("support_lib2");
    System.loadLibrary("native_lib");
}

Again, this builds just fine, however when I run I get a new error:

couldn't load libsupport_lib1. findLibrary returned null.

Now we're getting somewhere. It must not be loading the libraries over to the target.

Attempt #3 Copying .so files into project/libs/armeabi

Didn't work. When Eclipse builds it deleted the files I dropped in there.

Attempt #4 Creating a new module for each library

So then I found this:

Android NDK: Link using a pre-compiled static library

It's about static libraries, but maybe I am having a similar problem. The gist is that I need to declare a module for each library. So my new Android.mk looks like this:

LOCAL_PATH := $(call my-dir)

#get support_lib1
include $(CLEAR_VARS)
LOCAL_MODULE           := support_lib1
LOCAL_SRC_FILES        := $(LOCAL_PATH)/lib/support_lib1.so
include $(BUILD_SHARED_LIBRARY)

#get support_lib2
include $(CLEAR_VARS)
LOCAL_MODULE           := support_lib2
LOCAL_SRC_FILES        := $(LOCAL_PATH)/lib/support_lib2.so
include $(BUILD_SHARED_LIBRARY)

#build native lib
include $(CLEAR_VARS)    
LOCAL_MODULE           := native_lib
LOCAL_SRC_FILES        := native_lib.cpp

LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib/support_lib1
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib/support_lib2

include $(BUILD_SHARED_LIBRARY)

This builds! Even better, armeabi has the sos now! Even BETTER I get the following messages when I try to run it (telling me that support_lib1 and 2 were opened by LoadLibrary:

Trying to load lib /data/app-lib/com.example.tst/libsupport_lib1.so added shared lib /data/app-lib/com.example.tst/libsupport_lib1.so no JNI_OnLoad found in /data/app-lib/com.example.tst/libsupport_lib1.so, skipping init

but then... dlopen failed: Could not locate symbol func_that_exists_in_libsupport_lib.so referenced by libnative_lib.so

Edit: Attempt 5: Use PREBUILT_SHARED_LIBRARY

So I found this: How can i Link prebuilt shared Library to Android NDK project?

which seems to be exactly what I'm asking. Their answer seems to be 'don't use 'build_shared_library' but instead 'use PREBUILT_SHARED_LIBRARY

Okay, let's try.

 LOCAL_PATH := $(call my-dir)

#get support_lib1
include $(CLEAR_VARS)
LOCAL_MODULE           := support_lib1
LOCAL_SRC_FILES        := $(LOCAL_PATH)/lib/support_lib1.so
include $(PREBUILT_SHARED_LIBRARY)

#get support_lib2
include $(CLEAR_VARS)
LOCAL_MODULE           := support_lib2
LOCAL_SRC_FILES        := $(LOCAL_PATH)/lib/support_lib2.so
include $(PREBUILT_SHARED_LIBRARY)

#build native lib
include $(CLEAR_VARS)    
LOCAL_MODULE           := native_lib
LOCAL_SRC_FILES        := native_lib.cpp

LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_SHARED_LIBRARIES := support_lib1 support_lib2

include $(BUILD_SHARED_LIBRARY)

Build... fails! The build complains about missing symbols now.

Edit: Attempt 6: Flatten everything

So I went back to the prebuilts documentation in the NDK. It says:

Each prebuilt library must be declared as a single independent module to the build system. Here is a trivial example where we assume that the file "libfoo.so" is located in the same directory than the Android.mk below:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE := foo-prebuilt
LOCAL_SRC_FILES := libfoo.so
include $(PREBUILT_SHARED_LIBRARY)

Notice that, to declare such a module, you really only need the following:

Give the module a name (here 'foo-prebuilt'). This does not need to correspond to the name of the prebuilt library itself.

Assign to LOCAL_SRC_FILES the path to the prebuilt library you are providing. As usual, the path is relative to your LOCAL_PATH.

Include PREBUILT_SHARED_LIBRARY, instead of BUILD_SHARED_LIBRARY, if you are providing a shared, library. For static ones, use PREBUILT_STATIC_LIBRARY. A prebuilt module does not build anything. However, a copy of your prebuilt shared library will be copied into $PROJECT/obj/local, and another will be copied and stripped into $PROJECT/libs/.

So let's try flattening everything out to match the trivial example. I copied my libraries out of their cozy /lib folder and put them in the jni root. I then did this:

 LOCAL_PATH := $(call my-dir)

#get support_lib1
include $(CLEAR_VARS)
LOCAL_MODULE           := support_lib1
LOCAL_SRC_FILES        := support_lib1.so
include $(PREBUILT_SHARED_LIBRARY)

#get support_lib2
include $(CLEAR_VARS)
LOCAL_MODULE           := support_lib2
LOCAL_SRC_FILES        := support_lib2.so
include $(PREBUILT_SHARED_LIBRARY)

#build native lib
include $(CLEAR_VARS)    
LOCAL_MODULE           := native_lib
LOCAL_SRC_FILES        := native_lib.cpp

LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_SHARED_LIBRARIES := support_lib1 support_lib2

include $(BUILD_SHARED_LIBRARY)

and... same error. Moreover I'm most definitely NOT seeing library files getting copied to $PROJECT/obj/local.

sooooo.... now what?

A clue! The libraries that eclipse put in my armeabi are NOT copies of the ones I tried to give it. libsupport_lib1.so in armeabi is NOT the same size as libsupport_lib1.so in jni/lib... so what am I doing wrong that's not copying it over to the proper location? — djc6535, Feb 21 '14 at 00:34
so unfortunate that the libraries can't be placed directly in libs/ without eclipse trashing them on build .. would be so much more simple .. I can't find a way around that anywhere. — TTimo, Jul 16 '15 at 23:13
Hi, I have same issue. Did you solve it ? If so, could you give me your solution ? — DreamInBox, Jul 24 '18 at 07:55

score 15 · Answer 1 · edited Oct 24 '15 at 08:13

Your problem is with the naming convention. NDK and Android insist on the shared library names to always begin with lib. Otherwise, the libraries will not be linked properly, and not copied to the libs/armeabi folder properly, and not installed on the device (copied to /data/data/package/lib directory properly.

If you rename support_lib1.so to libsupport_1.so and support_lib2.so to libsupport_2.so, and put these two files in jni/lib directory, then your Attempt #5 will work with minor change:

LOCAL_PATH := $(call my-dir)

#get support_lib1
include $(CLEAR_VARS)
LOCAL_MODULE           := support_lib1
LOCAL_SRC_FILES        := lib/libsupport_1.so
include $(PREBUILT_SHARED_LIBRARY)

#get support_lib2
include $(CLEAR_VARS)
LOCAL_MODULE           := support_lib2
LOCAL_SRC_FILES        := lib/libsupport_2.so
include $(PREBUILT_SHARED_LIBRARY)

#build native lib
include $(CLEAR_VARS)    
LOCAL_MODULE           := native_lib
LOCAL_SRC_FILES        := native_lib.cpp

LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_SHARED_LIBRARIES := support_lib1 support_lib2

include $(BUILD_SHARED_LIBRARY)

BTW, I don't think you need this -L$(SYSROOT)/../usr/lib.

PS Don't forget to update the Java side, too:

static {
    System.loadLibrary("support_lib1");
    System.loadLibrary("support_lib2");
    System.loadLibrary("native_lib");
}

Now gradle provides experimental plugin and we can configure all things inside build.gradle file. Could you provide build.gradle variant of above Android.mk file — support_ms, Mar 25 '16 at 11:34
What I haven't understood with this so far is how to set it up such that it uses the correct .so for the platform target. It only lets you select one .so and then distributes it to all armeabi, armeabi-v7a, arm64-v8a etc. If the source static library we are including is already segregated into these platforms, how do we pass them along as such? — Jay Snayder, Aug 24 '16 at 15:43
@JaySnayder : there is no automatic mechanism to split the shared libraries by **android-xx**, but this is rarely needed because generally speaking, NDK is backwards compatible. Which means, a library that works on Gingerbread, will still work on Nogut. There are some edge cases, and there is justification to provide different **.so** variants when you split your APK into separate versions with different target SDK. For example, you don't need armeabi for Lolliop and higher. — Alex Cohn, Aug 27 '16 at 19:42

score 5 · Answer 2 · answered Mar 28 '14 at 02:23

Not sure if this is exactly where you are at, but here's what I know about these sorts of things.

Make each prebuilt libary its own separate Makefile. Multiple targets in Android.mk tends to get wonky. Sad.
Include each make file using $(call import-add-path) and $(call import-module)
Export as much as you can from the prebuilt's make files, using the LOCAL_EXPORT_ family of variables.

Prebuilt Shared Library Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := my_module_name

MY_LIBRARY_NAME := shared_library_name

### export include path
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/include

### path to library
LOCAL_SRC_FILES := libs/$(TARGET_ARCH_ABI)/lib$(MY_LIBRARY_NAME).so

### export dependency on the library
LOCAL_EXPORT_LDLIBS := -L$(LOCAL_PATH)/libs/$(TARGET_ARCH_ABI)/
LOCAL_EXPORT_LDLIBS += -l$(MY_LIBRARY_NAME)

include $(PREBUILT_SHARED_LIBRARY)

This is assuming that the prebuilt libaries live in a dir structure like this

+ SharedProjectFolderName
+--- Android.mk
+--- include/
+-+- libs/$(TARGET_ARCH_ABI)/
  |- libshared_library_name.so

If you are not building for multiple ABI, I guess you can leave that bit out

The Project's Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := my_jni_module

## source files here, etc...

### define dependency on the other library
LOCAL_SHARED_LIBRARIES := my_module_name

include $(BUILD_SHARED_LIBRARY)

$(call import-add-path,$(LOCAL_PATH)/path/to/myLibraries/)
$(call import-module,SharedProjectFolderName)
$(call import-module,AnotherSharedProject)

I recommend you put all shared libraries in one folder. When you say $(call import-module,SharedProjectFolderName) it looks for a folder containing an Android.mk along the search path you told it (import-add-path)

By the way, you probably shouldn't specify LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib. It should be finding the proper libs from NDK by itself. Adding more linker paths will probably confuse it. The proper way is to export the linker paths as flags from the sub-modules.

ALSO, you can use ndk-build V=1 to get a ton of info on why it can't find paths, etc

I'm trying your solution and I get an error: "Cannot find module with tag 'AnalyzerPrebuild' in import path". Could you please mention where does SharedProjectFolderName is located relatively to the main Project? — Asaf Pinhassi, Dec 24 '14 at 10:13
You can add multiple paths with additional `$(call import-add-path,....)` directives. all of your modules must be able to be found that way — yano, Jan 05 '15 at 21:55

score 0 · Answer 3 · answered Mar 29 '14 at 17:58

The -L option gives the linker a directory path in which to look for libraries. The -l option gives the linker a library file name to link in. Library file names must begin with "lib". Your libraries should be named libsupport_lib1.so and libsupport_lib2.so. If you do that, then this is probably what you should do (replacing attempt #1):

LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog -lsupport_lib1 -lsupport_lib2
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib

The linker will prefix the library name you specify using -l with "lib" and suffix it with ".so". (Why do you have -L$(SYSROOT)/../usr/lib?)

I believe that attempts #1 and #2 failed because you did not link your libraries into your executable - they are not mentioned in a -l option. By the way, you can verify this yourself. Unzip the .apk file and look in the lib directory and subdirectories. Are your .so files in there?

Looking at the error:

but then... dlopen failed: Could not locate symbol func_that_exists_in_libsupport_lib.so referenced by libnative_lib.so

Can you supply the entire message? dlopen() loads and links libraries into the running process.

Android NDK/JNI: Building a shared library that depends on other shared libraries

3 Answers3

Prebuilt Shared Library Android.mk

The Project's Android.mk

Linked