一起使用 MPI 和 OpenCV 时出现分段错误

  
本文介绍了一起使用 MPI 和 OpenCV 时出现分段错误的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我正在尝试学习 C++ 中的 MPI.我对 OpenCV 有一些了解，所以我尝试使用 MPI 和 OpenCV 编写程序.这听起来可能很愚蠢，但为了学习，我尝试在线程 0 上从网络摄像头捕获图像并将图像传递给线程 1 以转换为灰度并显示灰度图像.

I am trying to learn MPI in C++. I have some knowledge of OpenCV so I tried writing a program using both MPI and OpenCV. This may sound stupid but for the purpose of learning I tried capturing an image from webcam on thread 0 and passed the image to thread 1 for converting to grayscale and displaying the grayscale image.

这就是我编译代码的方式:
mpic++ opencv.cpp `pkg-config opencv --libs`

This is how I compile the code:
mpic++ opencv.cpp `pkg-config opencv --libs`

代码编译成功，但是当我运行可执行文件时，屏幕上会出现一小段时间的图像，这就是我在终端上看到的

The code compiles sucessfully but when I run the executable, an image shows up on the screen for a fraction of a second and this is what I see on the terminal

~/mpi$ mpirun -np 2 ./a.out
libv4l2: error setting pixformat: Device or resource busy
HIGHGUI ERROR: libv4l unable to ioctl S_FMT
libv4l2: error setting pixformat: Device or resource busy
libv4l1: error setting pixformat: Device or resource busy
HIGHGUI ERROR: libv4l unable to ioctl VIDIOCSPICT

[arch:09670] *** Process received signal *** 
[arch:09670] Signal: Segmentation fault (11)
[arch:09670] Signal code: Address not mapped (1) 
[arch:09670] Failing at address: 0x218ac50
[arch:09670] [ 0] /usr/lib/libpthread.so.0(+0x10740)[0x7f422fcac740]
[arch:09670] [ 1] /usr/lib/libopencv_core.so.2.4(_ZNK2cv11_InputArray6getMatEi+0x203)[0x7f4233c8c113]
[arch:09670] [ 2] /usr/lib/libopencv_imgproc.so.2.4(_ZN2cv8cvtColorERKNS_11_InputArrayERKNS_12_OutputArrayEii+0x50)[0x7f4232c25de0]
[arch:09670] [ 3] ./a.out[0x408f54]
[arch:09670] [ 4] /usr/lib/libc.so.6(__libc_start_main+0xf0)[0x7f422e9e9800]
[arch:09670] [ 5] ./a.out[0x408c19]
[arch:09670] *** End of error message *** 
--------------------------------------------------------------------------
mpirun noticed that process rank 1 with PID 9670 on node arch exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------

这是代码

#include <opencv2/opencv.hpp>
#include <mpi.h>

int main(int argc, char **argv) {
    cv::Mat_<uint> img(640,480);
    cv::Mat_<uint> gray(640,480);
    cv::VideoCapture cam(0);

    int rank, nproc, j=0;

    MPI_Status status;

    MPI_Init(&argc, &argv);

    // MPI datatype for 8UC3 image
    MPI_Datatype mat_8uc3;
    MPI_Type_contiguous(sizeof(img), MPI_BYTE, &mat_8uc3);
    MPI_Type_commit(&mat_8uc3);

    // MPI datatype for 8UC1 image
    MPI_Datatype mat_8uc1;
    MPI_Type_contiguous(sizeof(gray), MPI_BYTE, &mat_8uc1);
    MPI_Type_commit(&mat_8uc1);

    MPI_Comm_size(MPI_COMM_WORLD, &nproc); // number of processes
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);  // rank of the current process

    /*
     * Thread 0 captures the image from camera
     * and sends the image to process 1 for processing
     * thread 1 converts the image to grayscale and
     * displays the image
     */

    if (rank == 0) {
        // capture the image and send to thread 1
        while (1) { 
            cam >> img;
            cv::imshow("proc 0", img);
            MPI_Send(&img, 1, mat_8uc3, 1, j, MPI_COMM_WORLD);
            cv::waitKey(40);
            j++;
        }
    }
    else if (rank == 1) {
        // receive the image, convert to grayscale and display
        while (1) {
            MPI_Recv(&img, 1, mat_8uc3, 0, j, MPI_COMM_WORLD, &status);
            cv::cvtColor(img, gray, cv::COLOR_BGR2GRAY);
            cv::imshow("proc 1", gray);
            cv::waitKey(20);
            j++;
        }
    }

    MPI_Finalize();

    return 0;
}

谁能指出我哪里出错了

谢谢

(在 user0815 的回答之后)

(after user0815's answer)

在进行建议的更改时，设备或资源繁忙问题已解决，但程序仍会出现段错误.

On making the suggested changes the problem Device or resource busy is resolved but the program still gives a segfault.

[arch:01080] *** Process received signal *** 
[arch:01080] Signal: Segmentation fault (11)
[arch:01080] Signal code: Address not mapped (1) 
[arch:01080] Failing at address: 0x16bbf80
[arch:01080] [ 0] /usr/lib/libpthread.so.0(+0x10740)[0x7fea97322740]
[arch:01080] [ 1] /usr/lib/libopencv_core.so.2.4(_ZNK2cv11_InputArray6getMatEi+0x203)[0x7fea9b302113]
[arch:01080] [ 2] /usr/lib/libopencv_imgproc.so.2.4(_ZN2cv8cvtColorERKNS_11_InputArrayERKNS_12_OutputArrayEii+0x50)[0x7fea9a29bde0]
[arch:01080] [ 3] ./a.out[0x408fc3]
[arch:01080] [ 4] /usr/lib/libc.so.6(__libc_start_main+0xf0)[0x7fea9605f800]
[arch:01080] [ 5] ./a.out[0x408c79]
[arch:01080] *** End of error message *** 
--------------------------------------------------------------------------
mpirun noticed that process rank 1 with PID 1080 on node arch exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------

推荐答案

目前每个进程都在尝试打开摄像头.这很可能会导致问题.尝试将开口移动到根特定部分，如下所示:

Currently each process tries to open the camera. That is very likely to cause problems. Try to move the opening into the root specific section like so:

int main(int argc, char **argv) {
  cv::Mat_<uint> img(640,480);
  cv::Mat_<uint> gray(640,480);
  cv::VideoCapture cam;

  /* ... */
  if (rank == 0) {
    cam.open(0);
    /* ... */
  }
  /* ... */
}

更新:

我认为您的代码的问题在于，您不能简单地使用 MPI_Send 传输对象.sizeof 运算符通常对对象无效.如果要传输对象，则需要传输底层数据.

I think the problem with your code is, that you can't simply transfer objects with MPI_Send. Also is the sizeof operator not valid on objects in general. If you want to transfer an object, you need to transfer the underlying data.

您可以通过发送 img.data 大小为 img.rows * img.cols * sizeof(uint) 来实现这一点.然后你也可以使用 MPI_BYTE 作为数据类型，不需要自定义类型.

You could achieve this in your case by sending img.data with a size of img.rows * img.cols * sizeof(uint). Then you can also use MPI_BYTE as data type and no custom types are required.

cv::Mat_内部结构的一些细节可以参考这里.

Some details about the internal structure of cv::Mat_ can be found here.

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