Simple C++ program to capture a webcam frame in Linux

webcam_capture.cpp

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <linux/ioctl.h>
#include <linux/types.h>
#include <linux/v4l2-common.h>
#include <linux/v4l2-controls.h>
#include <linux/videodev2.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <string.h>
#include <fstream>
#include <string>

using namespace std;

int main() {
    // 1.  Open the device
    int fd; // A file descriptor to the video device
    fd = open("/dev/video0",O_RDWR);
    if(fd < 0){
        perror("Failed to open device, OPEN");
        return 1;
    }


    // 2. Ask the device if it can capture frames
    v4l2_capability capability;
    if(ioctl(fd, VIDIOC_QUERYCAP, &capability) < 0){
        // something went wrong... exit
        perror("Failed to get device capabilities, VIDIOC_QUERYCAP");
        return 1;
    }


    // 3. Set Image format
    v4l2_format imageFormat;
    imageFormat.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    imageFormat.fmt.pix.width = 1024;
    imageFormat.fmt.pix.height = 1024;
    imageFormat.fmt.pix.pixelformat = V4L2_PIX_FMT_MJPEG;
    imageFormat.fmt.pix.field = V4L2_FIELD_NONE;
    // tell the device you are using this format
    if(ioctl(fd, VIDIOC_S_FMT, &imageFormat) < 0){
        perror("Device could not set format, VIDIOC_S_FMT");
        return 1;
    }


    // 4. Request Buffers from the device
    v4l2_requestbuffers requestBuffer = {0};
    requestBuffer.count = 1; // one request buffer
    requestBuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; // request a buffer wich we an use for capturing frames
    requestBuffer.memory = V4L2_MEMORY_MMAP;

    if(ioctl(fd, VIDIOC_REQBUFS, &requestBuffer) < 0){
        perror("Could not request buffer from device, VIDIOC_REQBUFS");
        return 1;
    }


    // 5. Quety the buffer to get raw data ie. ask for the you requested buffer
    // and allocate memory for it
    v4l2_buffer queryBuffer = {0};
    queryBuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    queryBuffer.memory = V4L2_MEMORY_MMAP;
    queryBuffer.index = 0;
    if(ioctl(fd, VIDIOC_QUERYBUF, &queryBuffer) < 0){
        perror("Device did not return the buffer information, VIDIOC_QUERYBUF");
        return 1;
    }
    // use a pointer to point to the newly created buffer
    // mmap() will map the memory address of the device to
    // an address in memory
    char* buffer = (char*)mmap(NULL, queryBuffer.length, PROT_READ | PROT_WRITE, MAP_SHARED,
                        fd, queryBuffer.m.offset);
    memset(buffer, 0, queryBuffer.length);


    // 6. Get a frame
    // Create a new buffer type so the device knows whichbuffer we are talking about
    v4l2_buffer bufferinfo;
    memset(&bufferinfo, 0, sizeof(bufferinfo));
    bufferinfo.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    bufferinfo.memory = V4L2_MEMORY_MMAP;
    bufferinfo.index = 0;

    // Activate streaming
    int type = bufferinfo.type;
    if(ioctl(fd, VIDIOC_STREAMON, &type) < 0){
        perror("Could not start streaming, VIDIOC_STREAMON");
        return 1;
    }

/***************************** Begin looping here *********************/
    // Queue the buffer
    if(ioctl(fd, VIDIOC_QBUF, &bufferinfo) < 0){
        perror("Could not queue buffer, VIDIOC_QBUF");
        return 1;
    }

    // Dequeue the buffer
    if(ioctl(fd, VIDIOC_DQBUF, &bufferinfo) < 0){
        perror("Could not dequeue the buffer, VIDIOC_DQBUF");
        return 1;
    }
    // Frames get written after dequeuing the buffer

    cout << "Buffer has: " << (double)bufferinfo.bytesused / 1024
            << " KBytes of data" << endl;

    // Write the data out to file
    ofstream outFile;
    outFile.open("webcam_output.jpeg", ios::binary| ios::app);

    int bufPos = 0, outFileMemBlockSize = 0;  // the position in the buffer and the amoun to copy from
                                        // the buffer
    int remainingBufferSize = bufferinfo.bytesused; // the remaining buffer size, is decremented by
                                                    // memBlockSize amount on each loop so we do not overwrite the buffer
    char* outFileMemBlock = NULL;  // a pointer to a new memory block
    int itr = 0; // counts thenumber of iterations
    while(remainingBufferSize > 0) {
        bufPos += outFileMemBlockSize;  // increment the buffer pointer on each loop
                                        // initialise bufPos before outFileMemBlockSize so we can start
                                        // at the begining of the buffer

        outFileMemBlockSize = 1024;    // set the output block size to a preferable size. 1024 :)
        outFileMemBlock = new char[sizeof(char) * outFileMemBlockSize];

        // copy 1024 bytes of data starting from buffer+bufPos
        memcpy(outFileMemBlock, buffer+bufPos, outFileMemBlockSize);
        outFile.write(outFileMemBlock,outFileMemBlockSize);

        // calculate the amount of memory left to read
        // if the memory block size is greater than the remaining
        // amount of data we have to copy
        if(outFileMemBlockSize > remainingBufferSize)
            outFileMemBlockSize = remainingBufferSize;

        // subtract the amount of data we have to copy
        // from the remaining buffer size
        remainingBufferSize -= outFileMemBlockSize;

        // display the remaining buffer size
        cout << itr++ << " Remaining bytes: "<< remainingBufferSize << endl;
        
        delete outFileMemBlock;
    }

    // Close the file
    outFile.close();


/******************************** end looping here **********************/

    // end streaming
    if(ioctl(fd, VIDIOC_STREAMOFF, &type) < 0){
        perror("Could not end streaming, VIDIOC_STREAMOFF");
        return 1;
    }

    close(fd);
    return 0;
}

Hey @mike168m. Just wanted to say that I've been struggling to work out how the V4L2 kernel works, the documentations is simultaneously way too specific and surprisingly vague.

This has cleared up the questions I have surrounding a few aspects and I just wanted to say thanks for uploading your work for us all to learn from!

Exactly a simple camera capture kind of code with minor glitches that I was looking for!! Before I compile , link and execute the code I checked
/dev/audio* and /dev/video on bash and they don't seem to exist!!! My integrated camera didn't work on windows for some reason and so I added an external logitech camera which was sitting in my drawer and seemed to work in windows. But UBUNTU linux /dev/video* doesn't seem to pick it up!! Is there any libs I have to install to experience the thrill of a camera rendition!! Any help definitely appreciated!!

LoveCoder625

@DbDibyendu did you find how to capture multiple frames ?