c++ - 一起使用 MPI 和 OpenCV 时出现段错误

Question

我正在尝试用 C++ 学习 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，但出于学习的目的，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。

这就是我编译代码的方式:
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，屏幕上会出现一个图像几分之一秒，这就是我在终端上看到的

~/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;
}

谁能指出我哪里错了

谢谢

编辑:(在 user0815 的回答之后)

根据建议的更改，问题 Device or resource busy 已解决，但程序仍然出现段错误。

[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).
--------------------------------------------------------------------------

Answer 1

目前每个进程都尝试打开相机。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:

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:

我认为您的代码存在问题，您不能简单地使用 MPI_Send 传输对象。此外，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。

您可以通过发送大小为 img.rows * img.cols * sizeof(uint) 的 img.data 来实现这一点。然后您还可以使用 MPI_BYTE 作为数据类型，并且不需要自定义类型。

有关cv::Mat_ 内部结构的一些细节可以找到here .