Develop Reference

Android NDK: dlsym() returns "undefined symbol: JNI_OnLoad" error

I don't know if I'm wrong but I found this solution to bypass Android restriction on private shared library like OpenCL. It copies required libs from /system/vendor/lib64 to /data/data/package.name/ and then loads them by calling dlopen.
#include <math.h>
#include <stdio.h>
#include <string.h>
#include <jni.h>
#include <android/log.h>
#include <stdlib.h>
#include <dlfcn.h>
#include <errno.h>
#include <unistd.h>
#define LOG_TAG "JNI"
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR, LOG_TAG, __VA_ARGS__)
#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
void copy_file(const char* in_path, const char* out_path) {
FILE* fin = fopen(in_path, "rb");
check(fin, "fopen()");
FILE* fout = fopen(out_path, "wb");
check(fout, "fopen()");
unsigned char buffer[255];
size_t ret;
while ((ret = fread(buffer, sizeof(*buffer), ARRAY_SIZE(buffer), fin)) == ARRAY_SIZE(buffer)) {
fwrite(buffer, sizeof(*buffer), ret, fout);
}
if (ferror(fin) != 0) {
LOGE("Error on read");
exit(1);
}
fwrite(buffer, sizeof(*buffer), ret, fout);
fclose(fin);
fclose(fout);
}
void* sdlopen(const char* path){
void *handle;
handle = dlopen(path, RTLD_LAZY | RTLD_GLOBAL);
if (!handle) {
LOGE("#1 %s", dlerror());
exit(1);
}
return handle;
}
void load(){
void *handle, *handle1, *handle2, *handle3, *handle4, *handle5, *handle6;
cl_int (*_clGetPlatformIDs)(cl_uint, cl_platform_id*, cl_uint*);
char *error;
copy_file("/system/vendor/lib64/libOpenCL.so", "/data/data/com.app.opencl/libOpenCL.so");
copy_file("/system/lib64/libcutils.so", "/data/data/com.app.opencl/libcutils.so");
copy_file("/system/lib64/libbase.so", "/data/data/com.app.opencl/libbase.so");
copy_file("/system/lib64/libc++.so", "/data/data/com.app.opencl/libc++.so");
copy_file("/system/lib64/libvndksupport.so", "/data/data/com.app.opencl/libvndksupport.so");
copy_file("/system/lib64/libdl_android.so", "/data/data/com.app.opencl/libdl_android.so");
copy_file("/system/lib64/ld-android.so", "/data/data/com.app.opencl/ld-android.so");
handle1 = sdlopen("/data/data/com.app.opencl/libc++.so");
handle2 = sdlopen("/data/data/com.app.opencl/ld-android.so");
handle3 = sdlopen("/data/data/com.app.opencl/libdl_android.so");
handle4 = sdlopen("/data/data/com.app.opencl/libvndksupport.so");
handle5 = sdlopen("/data/data/com.app.opencl/libbase.so");
handle6 = sdlopen("/data/data/com.app.opencl/libcutils.so");
handle = sdlopen("/data/data/com.app.opencl/libOpenCL.so");
_clGetPlatformIDs = dlsym(handle, "clGetPlatformIDs");
if ((error = dlerror()) != NULL) {
LOGE("#2 %s", error);
exit(1);
}
dlclose(handle);
dlclose(handle1);
dlclose(handle2);
dlclose(handle3);
dlclose(handle4);
dlclose(handle5);
dlclose(handle6);
}
With the code above I can dlopen libOpenCL.so but
dlsym(handle, "clGetPlatformIDs");
returns undefined symbol: JNI_OnLoad. I think it shouldn't look for JNI_OnLoad function. What is wrong?

OpenCV and C++: "Can't resolve variable 'Mat`"

I'm trying to integrate OpenCV in Android Java native interface using C++ language. I have placed the OpenCV inside the jni folder, where I'm writing my C++ code. I have included the Opencv header files in my HelloNative.c file. But I'm still getting this error while trying to access Mat object.
"Can't resolve variable Mat".
I have tried using namespace cv, but it gives an error to predeclare using and namespace; which is not a solution. I'm posting my code below, please somebody have a look at it and let me know what I'm doing wrong.
#include <string.h>
#include <jni.h>
#include <stdio.h>
#include <stdlib.h>
#include <android/asset_manager.h>
#include <android/asset_manager_jni.h>
#include <android/log.h>
#include <malloc.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <OpenCV/include/opencv2/opencv.hpp>
#include <OpenCV/include/opencv/cv.h>
#include <OpenCV/modules/core/include/opencv2/core.hpp>
#include <OpenCV/modules/core/include/opencv2/core/core_c.h>
#include <OpenCV/modules/core/include/opencv2/core/mat.hpp>
#include <OpenCV/modules/imgproc/include/opencv2/imgproc/imgproc.hpp>
#include <OpenCV/modules/imgproc/include/opencv2/imgproc/imgproc_c.h>
#include <OpenCV/modules/legacy/include/opencv2/legacy/legacy.hpp>
#include <OpenCV/modules/videostab/include/opencv2/videostab/motion_stabilizing.hpp>
#include <OpenCV/modules/videostab/include/opencv2/videostab/global_motion.hpp>
JNIEXPORT jint JNICALL
Java_com_example_soimporttest_MainActivity_getStringFromJNI(JNIEnv *env, jobject thisObj,
jstring imagepath) {
const char *jnamestr = (*env)->GetStringUTFChars(env, imagepath, 0);
int width, height, gray;
FILE *fp = fopen(jnamestr, "rb");
if (fp) {
//Read .pgm file
int scan = fscanf(fp, "P5%d%d%d", &width, &height, &gray);
if (scan != 3 || &gray > 256 || &width > 0xfffff || &height > 0xfffff || &gray <= 1 ||
&width < 32 || &height < 32) {
fclose(fp);
return 9;
} else {
return 0;
}
} else {
return -1;
}
}
JNIEXPORT void JNICALL
Java_com_example_soimporttest_MainActivity_displayImage(JNIEnv *env, jobject thisObj, jclass type, jlong inpAddr, jlong outAddr) {
::Mat &src = *(Mat*)inpAddr;
Mat &dst = *(Mat*)outAddr;
applyFilter(src , dst);
}
There are two functions in this file, the first function getStringFromJNI is working perfectly fine.
Thank you.

After a lot of research, I finally managed to find out a good solution for my question. Please follow below mentioned tutorial:
https://github.com/leadrien/opencv_native_androidstudio
You'll be facing some issues while integrating it. You can post those solutions in the comments and I will guide you through each.
Thanks.

How to read a file in c using NDK in android

I am trying to read a file in c using NDK. When i debug the code, i see that i get a NULL pointer in f(file reading pointer). Also, my file which i need to read is in the same folder under cpp including all other header files. I have the read and write permissions for external storage in my app. Any help is appreciated.
C Code:
#ifndef client_h
#define client_h
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <pthread.h>
#include "utility.h"
#define SENDBUFSIZE 1399
#define RECVBUFSIZE 1399
void pingServer(char *hostname, int portno)
{
//Some other functionality which works correctly
// File Read operations
FILE *f = fopen("somefile.csv", "r"); //Get the null pointer here
if (f == NULL)
perror("Could not open input file");
char *currentRow[3];
char *nextRow[3];
int rows = 3;
for (int i=0; i<rows; i++)
{
currentRow[i] = (char*)malloc(20 * sizeof(char));
nextRow[i] = (char*)malloc(20 * sizeof(char));
}
double current, next;
double elapsed;
int isNextRow, isCurentRow;
readCsvLine(f,currentRow,rows); //Get the headers
int counter = 2;
isCurentRow = readCsvLine(f,currentRow,rows);
//sends the data
//Free the memory
for (int i=0; i<rows; i++)
{
free(currentRow[i]);
free(nextRow[i]);
}
free(buf);
return;
}
char* printName(){
char* ptr = "hello From C";
return ptr;
}
#endif
Java code for calling the c methods:
#include <jni.h>
#include <string>
#include "client.h"
extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_habbas_mobiledifferentiation_MainActivity_stringFromJNI(
JNIEnv *env,
jobject /* this */) {
char *hostname = "127.0.0.1";
int portno = 1254;
pingServer(hostname, portno);
std::string hello = printName();
return env->NewStringUTF(hello.c_str());
}

Reading frames from video file using openCV and Native code C++

I'm trying to write an app that gets all the frames of a video and manipulating them, I found oout that the best way to extract frames on Android is using OpenCv lib.
I saw in the sample code that uses VideoCapture object that receives the video path and can grab frames out of it, so I wrote the following code but the capture.open() doen't really open the video file, the capture.isOpen() is always false.
Source code:
#include <jni.h>
//opencv
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/video/background_segm.hpp>
#include <opencv2/core/mat.hpp>
//C
#include <stdio.h>
#include <string.h>
//C++
#include <iostream>
#include <sstream>
//Android
#include <android/log.h>
//#define LOGI(TAG,INFO) __android_log_print(ANDROID_LOG_INFO,INFO,__VA_ARGS__)
using namespace cv;
extern "C" {
JNIEXPORT void JNICALL Java_com_example_nativeopencvcheck_MainActivity_processVideo(JNIEnv* env,
jobject thsObj,jstring fileName) {
const char * fileNameNative;
jboolean isCopy;
fileNameNative = env->GetStringUTFChars(fileName, &isCopy);
//create the capture object
__android_log_print(ANDROID_LOG_ERROR, "From_Native",
"trying to open file: %s", fileNameNative);
VideoCapture capture(fileNameNative);
capture.open(fileNameNative);
if (!capture.isOpened()) { //!!!!!! ALWAYS CLOSED !!!!!
__android_log_write(ANDROID_LOG_ERROR, "From_Native",
"capture isn't open. closing..");
exit( EXIT_FAILURE);
}
Mat iplimage;
capture.retrieve(iplimage,0);
if (iplimage.size > 0) {
jclass cls = env->FindClass( "com/example/opencvframesext/MainActivity");
if (cls == 0) {
return;
}
jmethodID javamethod = env->GetMethodID(cls, "getCurrentFrameFromNative", "()V");
if (javamethod == 0) {
// LOGI("From_Native","GetMethodID error");
return;
}
jobject obj; // TODO
env->CallVoidMethod(obj, javamethod);
return;
}
/*bool gotFrame = capture.read(mat);
while (gotFrame) {
mats.addref(mat);
capture.read(mat);
}*/
//delete capture object
capture.release();
}
}

yea, that wont work, unfortunately.
there's no ffmpeg backend for VideoCapture on android.

The syscall __NR_perf_event_open seems doesn't work on Android

I want to use the syscall of __NR_perf_event_open for my Android app.
The code runs normally on linux but does not work on Android.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/ioctl.h>
#include <perf_event.h>
#include <asm/unistd.h>
long perf_event_open( struct perf_event_attr *hw_event, pid_t pid,
int cpu, int group_fd, unsigned long flags )
{
int ret;
ret = syscall( __NR_perf_event_open, hw_event, pid, cpu,
group_fd, flags );
return ret;
}
int main() {
//In the main function, I call perf_event_open:
struct perf_event_attr pe;
int fd;
fd = perf_event_open(&pe, 0, -1, -1, 0);
...
}
However, the fd always return the value -1. When I use "errno.h", it gave the error informaiton of:EBADF：bad file descriptor.

you have not configured "struct perf_event_attr pe;" yet

because pid == -1 and cpu == -1 is not valid. you can check it in http://web.eece.maine.edu/~vweaver/projects/perf_events/perf_event_open.html