How to build an shared library and call it in other ndk program

Question

I want to build an shared library. To build it, I need to call another shared library. Here is what I did:

1.Create one Android project,named "BuildLib",and add a new folder "jni" under the project directory. Contents of jni folder:

jni-->Android.mk
-->Application.mk
-->add.cpp
-->add.h add.cpp just do two numbers addition:

add.h:

int add(int a,int b);

add.cpp:

#include "add.h"  
int add(int a,int b){
    return a+b;}

Android.mk:

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES  := add.cpp 
LOCAL_MODULE     := add
include $(BUILD_SHARED_LIBRARY)

After build the project,I got libadd.so under directory $(BUILDLIB)/libs/armeabi/.

Create another Android project, named "CallLib". Copy libadd.so and add.h to jni folder, create Android.mk, Application.mk, and call_add.cpp.

Android.mk:

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES := libadd.so
LOCAL_MODULE := add_prebuilt
include $(PREBUILD_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_C_INCLUDES := $(LOCAL_PATH）
LOCAL_SRC_FILES  := call_add.cpp 
LOCAL_MODULE     :=  native
LOCAL_SHARED_LIBRARIES := add_prebuilt
include $(BUILD_SHARED_LIBRARY)

call_add.cpp:

#include "add.h"
int call_add(){return add(1,2);}

After all above, I build the CallLib project, but got the error:

undefined reference to 'add(int, int)';

I think the libadd.so can not be found, but I don't know how to modify. Does anyone know how I can fix this? Any help will be appreciated.

Answer 1

In your second Android.mk, try replacing the first module with:

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES := libadd.so
LOCAL_MODULE := add_prebuilt
LOCAL_EXPORT_C_INCLUDES := add.h
include $(PREBUILD_SHARED_LIBRARY)

The LOCAL_EXPORT_C_INCLUDES flag should attach the header information to the add_prebuilt module, so it can be linked with your final library.