我正在尝试使用OpenCV通过单独的线程连续传输来自2台摄像机的视频。以下代码显示Segmentation fault (core dumped)
这是什么原因以及如何解决此问题？
的main.cpp
#include <iostream>
#include <pthread.h>
#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/videoio.hpp>
using namespace std;
struct thread_data {
string path;
int thread_id;
};
void *capture(void *threadarg)
{
struct thread_data *data;
data = (struct thread_data *) threadarg;
cv::VideoCapture cap(data->path);
if( !cap.isOpened())
{
std::cout<<"Not good, open camera failed"<<std::endl;
}
std::cout<< "Opened IP camera successfully!"<<std::endl;
cv::Mat frame;
string ext = ".jpg";
string result;
while (true) {
cap >> frame;
cv::imshow("Frame",frame);
cv::waitKey(1);
}
pthread_exit(NULL);
}
int main(void) {
pthread_t threads[2];
struct thread_data td[2];
int rc=0;
for( int i = 0; i < 2; i++ ) {
cout <<"main() : creating thread, " << i << endl;
td[i].thread_id = i;
td[0].path = "rtsp://admin:opencv123@192.168.1.23:554/Streaming/Channels/101/";
td[1].path = "rtsp://admin:opencv123@192.168.1.24:554/Streaming/Channels/101/";
rc = pthread_create(&threads[i], NULL, capture, (void *)&td[i]);
if (rc) {
cout << "Error:unable to create thread," << rc << endl;
exit(-1);
}
}
pthread_exit(NULL);
return 0;
}
日志：
main() : creating thread, 0 main() : creating thread, 1
Segmentation fault (core dumped)
当我尝试多次运行时，我只能打开一个摄像头而且也不能连续播放。它会在几秒钟内启动和停止。
有时候我会收到错误消息
OpenCV Error: Insufficient memory (Failed to allocate 140703464366800 bytes) in OutOfMemoryError
我经历过各种Q＆amp; As＆＃39;在 StackOverflow 但没有帮助。
答案 0 :(得分：3)
这里的问题是代码面临竞争条件。我能够在我的系统上复制该问题，并确定了以下问题：
让我们详细研究这些问题。
OpenCV窗口由其标题唯一标识。在当前代码中，标题是硬编码字符串"Frame"。所以基本上，两个线程都以未知顺序创建/更新/销毁同一个窗口。这是一个竞争条件，可以通过向struct thread_data添加一个字符串字段来修复，该字段将作为唯一的窗口标识符。
在主线程中，子线程是异步创建的，因此for循环将在创建线程后立即退出，程序将在不等待生成线程完成执行的情况下提前退出。这个问题可以通过在程序退出之前添加函数调用来等待线程来解决。此过程称为加入，可以通过为每个衍生线程调用pthread_join来实现。
这个问题有点难以追查。由于某种原因，OpenCV使用的用于视频流捕获的后端库不以线程安全的方式运行。看起来，视频捕获打开过程容易出现竞争状态并需要同步锁定。通过在打开pthread_mutex_lock对象之前和之后调用函数pthread_mutex_unlock和VideoCapture，可以轻松实现锁定。
以下是修改后的代码，演示了所有上述问题的解决方案
#include <iostream>
#include <pthread.h>
#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/videoio.hpp>
using namespace std;
//Mutex for thread synchronization
static pthread_mutex_t foo_mutex = PTHREAD_MUTEX_INITIALIZER;
struct thread_data
{
string path;
int thread_id;
string window_title; //Unique window title for each thread
};
void *capture(void *threadarg)
{
struct thread_data *data;
data = (struct thread_data *) threadarg;
cv::VideoCapture cap;
//Safely open video stream
pthread_mutex_lock(&foo_mutex);
cap.open(data->path);
pthread_mutex_unlock(&foo_mutex);
if( !cap.isOpened())
{
std::cout<<"Not good, open camera failed"<<std::endl;
}
std::cout<< "Opened IP camera successfully!"<<std::endl;
cv::Mat frame;
string ext = ".jpg";
string result;
//Create window with unique title
cv::namedWindow(data->window_title);
while (true)
{
cap >> frame;
cv::imshow(data->window_title,frame);
cv::waitKey(10);
}
//Release VideoCapture object
cap.release();
//Destroy previously created window
cv::destroyWindow(data->window_title);
//Exit thread
pthread_exit(NULL);
}
int main(void)
{
const int thread_count = 2;
pthread_t threads[thread_count];
struct thread_data td[thread_count];
//Initialize thread data beforehand
td[0].path = "rtsp://admin:opencv123@192.168.1.23:554/Streaming/Channels/101/";
td[0].window_title = "First Window";
td[1].path = "rtsp://admin:opencv123@192.168.1.24:554/Streaming/Channels/101/";
td[1].window_title = "Second Window";
int rc=0;
for( int i = 0; i < thread_count; i++ )
{
cout <<"main() : creating thread, " << i << endl;
td[i].thread_id = i;
rc = pthread_create(&(threads[i]), NULL, capture, (void *)& (td[i]) );
if (rc)
{
cout << "Error:unable to create thread," << rc << endl;
exit(-1);
}
}
//Wait for the previously spawned threads to complete execution
for( int i = 0; i < thread_count; i++ )
pthread_join(threads[i], NULL);
pthread_exit(NULL);
return 0;
}
答案 1 :(得分：0)
void writeFrametoDisk(const cv::Mat *frame, std::string path, int frameNum, std::string windowName)
{
cv::imwrite(path.append(std::to_string(frameNum)).append(".png"), *frame);
return;
}
void openCameraStream(int deviceId, std::string dirName)
{
cv::VideoCapture cap;
cap.open(deviceId);
if(!cap.isOpened()){std::cerr << "Unable to open the camera " << std::endl;}
std::cout << cap.get(cv::CAP_PROP_FRAME_WIDTH) << " " << cap.get(cv::CAP_PROP_FRAME_HEIGHT) << std::endl;
std::string windowName = deviceId == 0 ? "Cam 0" : "Cam 1";
std::string outputDir = dirName;
bool frameStFg = false;
while(!frameStFg)
{
cv::Mat frame;
cap.read(frame);
if(frame.empty()){std::cerr << "frame buffer empty " << std::endl;}
else
{
frameStFg = true;
}
}
cv::TickMeter timer;
timer.start();
int frameCount = 0;
while(1)
{
cv::Mat frame;
cap.read(frame);
if(frame.empty()){std::cerr << "frame buffer empty " << std::endl;}
frameCount++;
std::thread th(writeFrametoDisk, &frame, outputDir, frameCount, windowName);
th.join();
//// a simple wayto exit the loop
if(frameCount > 500)
{
break;
}
}
timer.stop();
std::cout << "Device id " << deviceId << " Capture ran for " << timer.getTimeSec() << " seconds" << std::endl;
std::cout << "Device id " << deviceId << " Number of frames to be capture should be " << timer.getTimeSec() * 30 << std::endl;
std::cout << "Device id " << deviceId << " Number of frames captured " << frameCount << std::endl;
cap.release();
}
int main(int argc, char * argv[])
{
std::string outputDir1 = "";
std::string outputDir2 = "";
std::thread camera1Thread(openCameraStream, 0, outputDir1);
std::thread camera2Thread(openCameraStream, 1, outputDir2);
camera1Thread.join();
camera2Thread.join();
}
正如评论所提到的，没有图像流的任何imshow（）的流都可以很好地工作而不会出现任何问题。我的设置包括使用两个USB摄像机运行两个线程，并且这两个线程分别启动一个新线程，以保存从摄像机读取的图像。我没有观察到任何帧丢失或错误，也能够以大约30 fps的速度捕获和写入。
稍微调试一下原因，建议仅在主线程中使用任何imshow（）函数，即main（）函数。希望这对任何人都有帮助。