Compile JNI library that includes OpenCV headers

Question

Hy!

I have an Android project with a cpp file that needs to be compiled. This file is including some headers of the OpenCV library. The OpenCV source files are under jni/include/opencv2 folder as you can see in Android.mk file. When I try to compile with the ndk-build the file the compiler gives the following error:

"undefined reference to"

It seems that is a "error: undefined reference to" problem to all this files.

The Android.mk file have the following content:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

OPENCV_PACKAGE_DIR := $(LOCAL_PATH)/include

LOCAL_MODULE := iqindex

LOCAL_C_INCLUDES := \
    $(LOCAL_PATH) \
    $(OPENCV_PACKAGE_DIR)

LOCAL_SRC_FILES := \
    main.cpp

LOCAL_SHARED_LIBRARIES := iqindex

LOCAL_LDLIBS := -llog -lz

include $(BUILD_SHARED_LIBRARY)

I do not have enough experience with jni/ndk stuff and my questions are:

1. To compile a .cpp file that needs another header files is enough to compile it with ndk-build command and it will be all done ?

2. What is wrong in my attempt to compile it ? And what is wrong in Android.mk ?

EDIT: The console error is the following:

C:\MyName\My Project App>ndk-build

C:/MyName/android-ndk-r8e/build/core/add-application.mk:128: Android NDK: WARNING: APP_PLATFORM android-14 is larger than android:minSdkVersion 8 in ./AndroidManifest.xml
SharedLibrary  : libiqindex.so

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o:(.ARM.extab.text.unlikely._ZL6jthrowP7_JNIEnv+0x2c): error: undefined reference to 'typeinfo for cv::Exception'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function cv::Mat::Mat(cv::Mat const&):jni/include/opencv2/core/mat.hpp:125: error: undefined reference to 'cv::Mat::copySize(cv::Mat const&)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function cv::Mat::release():jni/include/opencv2/core/mat.hpp:364: error: undefined reference to 'cv::Mat::deallocate()'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function cv::Mat::~Mat():jni/include/opencv2/core/mat.hpp:278: error: undefined reference to 'cv::fastFree(void*)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_Mat_submat_1n:jni/include/opencv2/core/mat.hpp:376: error: undefined reference to 'cv::Mat::Mat(cv::Mat const&, cv::Rect_<int> const&)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_Mat_resize_1n:jni/main.cpp:111: error: undefined reference to 'cv::_InputArray::_InputArray(cv::Mat const&)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_Mat_resize_1n:jni/main.cpp:111: error: undefined reference to 'cv::_OutputArray::_OutputArray(cv::Mat&)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_Mat_resize_1n:jni/main.cpp:111: error: undefined reference to 'cv::resize(cv::_InputArray const&, cv::_OutputArray const&, cv::Size_<int>, double, double, int)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_Utils_rotateData:jni/main.cpp:124: error: undefined reference to'cv::_InputArray::_InputArray(cv::Mat const&)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_Utils_rotateData:jni/main.cpp:124: error: undefined reference to'cv::_OutputArray::_OutputArray(cv::Mat&)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_Utils_rotateData:jni/main.cpp:124: error: undefined reference to'cv::flip(cv::_InputArray const&, cv::_OutputArray const&, int)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_Utils_rotateData:jni/main.cpp:125: error: undefined reference to'cv::Mat::t() const'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_IQLocal_nativeCreate:jni/main.cpp:139: error: undefined reference to 'get_instance()'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_IQLocal_load:jni/main.cpp:156: error: undefined reference to 'IQIndex::load(std::string, std::string)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_IQLocal_match:jni/main.cpp:171: error: undefined reference to 'IQIndex::match(cv::Mat&)'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_IQLocal_train:jni/main.cpp:182: error: undefined reference to 'IQIndex::train()'

C:/MyName/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi-v7a/objs/iqindex/main.o: in function Java_project_name_zxing_IQLocal_compute:jni/main.cpp:198: error: undefined reference to 'IQIndex::compute(cv::Mat const&, char const*, char const*)'

collect2: ld returned 1 exit status

make: *** [obj/local/armeabi-v7a/libiqindex.so] Error 1

SECOND EDIT: Now the Android.mk looks like this:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

# OpenCV
OPENCV_CAMERA_MODULES:=on
OPENCV_INSTALL_MODULES:=on

include C:/Emil/downloads/OpenCV-2.4.6-android-sdk-r2/OpenCV-2.4.6-android-sdk/sdk    /native/jni/OpenCV.mk

LOCAL_MODULE := iqindex

LOCAL_SRC_FILES := \
    main.cpp

LOCAL_C_INCLUDES := \
    $(LOCAL_PATH)/iqindex.h

LOCAL_LDLIBS := -llog -lz

include $(BUILD_SHARED_LIBRARY)

Answer 1

This is late, but in response to :

Now I have another problem: I have in the jni folder a header file named iqindex.h . The cpp file includes it, but now I have "undefined reference to" iqindex.h when I try to build. I tried the LOCAL_C_INCLUDES command in Android.mk which pointed to the header file, but then it doesn't see openCV anymore at compile time. How do I include this header to make the files to compile ?

The problem probably is that you used LOCAL_C_INCLUDES :=/foo/bar

Try LOCAL_C_INCLUDES +=/foo/bar. You should add the include directory, not overwrite the existing includes.

Answer 2

I suppose that you forgot to add OpenCV libriries to your project. So linker can't find OpenCV related functions.

From OpenCV documentation:

If your application project doesn’t have a JNI part, just copy the corresponding OpenCV native libs from /sdk/native/libs/ to your project directory to folder libs/.

In case of the application project with a JNI part, instead of manual libraries copying you need to modify your Android.mk file: add the following two code lines after the "include $(CLEAR_VARS)" and before "include path_to_OpenCV-2.4.6-android-sdk/sdk/native/jni/OpenCV.mk OPENCV_CAMERA_MODULES:=on OPENCV_INSTALL_MODULES:=on

So you should have following lines in your Android.mk:

include $(CLEAR_VARS)
# OpenCV
OPENCV_CAMERA_MODULES:=on
OPENCV_INSTALL_MODULES:=on
include ../../sdk/native/jni/OpenCV.mk