Appendix E. Video Grabber example using libv4l
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## Appendix E. Video Grabber example using libv4l

This program demonstrates how to grab V4L2 images in ppm format by using libv4l handlers. The advantage is that this grabber can potentially work with any V4L2 driver.

    /* V4L2 video picture grabber
       Copyright (C) 2009 Mauro Carvalho Chehab <mchehab@infradead.org>

       This program is free software; you can redistribute it and/or modify
       it under the terms of the GNU General Public License as published by
       the Free Software Foundation version 2 of the License.

       This program is distributed in the hope that it will be useful,
       but WITHOUT ANY WARRANTY; without even the implied warranty of
       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
       GNU General Public License for more details.
     */

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <fcntl.h>
    #include <errno.h>
    #include <sys/ioctl.h>
    #include <sys/types.h>
    #include <sys/time.h>
    #include <sys/mman.h>
    #include <linux/videodev2.h>
    #include "../libv4l/include/libv4l2.h"

    #define CLEAR(x) memset(&(x), 0, sizeof(x))

    struct buffer {
            void   *start;
            size_t length;
    };

    static void xioctl(int fh, int request, void *arg)
    {
            int r;

            do {
                    r = v4l2_ioctl(fh, request, arg);
            } while (r == -1 && ((errno == EINTR) || (errno == EAGAIN)));

            if (r == -1) {
                    fprintf(stderr, "error %d, %s\n", errno, strerror(errno));
                    exit(EXIT_FAILURE);
            }
    }

    int main(int argc, char **argv)
    {
            struct v4l2_format              fmt;
            struct v4l2_buffer              buf;
            struct v4l2_requestbuffers      req;
            enum v4l2_buf_type              type;
            fd_set                          fds;
            struct timeval                  tv;
            int                             r, fd = -1;
            unsigned int                    i, n_buffers;
            char                            *dev_name = "/dev/video0";
            char                            out_name[256];
            FILE                            *fout;
            struct buffer                   *buffers;

            fd = v4l2_open(dev_name, O_RDWR | O_NONBLOCK, 0);
            if (fd < 0) {
                    perror("Cannot open device");
                    exit(EXIT_FAILURE);
            }

            CLEAR(fmt);
            fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
            fmt.fmt.pix.width       = 640;
            fmt.fmt.pix.height      = 480;
            fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_RGB24;
            fmt.fmt.pix.field       = V4L2_FIELD_INTERLACED;
            xioctl(fd, VIDIOC_S_FMT, &fmt);
            if (fmt.fmt.pix.pixelformat != V4L2_PIX_FMT_RGB24) {
                    printf("Libv4l didn't accept RGB24 format. Can't proceed.\n");
                    exit(EXIT_FAILURE);
            }
            if ((fmt.fmt.pix.width != 640) || (fmt.fmt.pix.height != 480))
                    printf("Warning: driver is sending image at %dx%d\n",
                            fmt.fmt.pix.width, fmt.fmt.pix.height);

            CLEAR(req);
            req.count = 2;
            req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
            req.memory = V4L2_MEMORY_MMAP;
            xioctl(fd, VIDIOC_REQBUFS, &req);

            buffers = calloc(req.count, sizeof(*buffers));
            for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
                    CLEAR(buf);

                    buf.type        = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                    buf.memory      = V4L2_MEMORY_MMAP;
                    buf.index       = n_buffers;

                    xioctl(fd, VIDIOC_QUERYBUF, &buf);

                    buffers[n_buffers].length = buf.length;
                    buffers[n_buffers].start = v4l2_mmap(NULL, buf.length,
                                  PROT_READ | PROT_WRITE, MAP_SHARED,
                                  fd, buf.m.offset);

                    if (MAP_FAILED == buffers[n_buffers].start) {
                            perror("mmap");
                            exit(EXIT_FAILURE);
                    }
            }

            for (i = 0; i < n_buffers; ++i) {
                    CLEAR(buf);
                    buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                    buf.memory = V4L2_MEMORY_MMAP;
                    buf.index = i;
                    xioctl(fd, VIDIOC_QBUF, &buf);
            }
            type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

            xioctl(fd, VIDIOC_STREAMON, &type);
            for (i = 0; i < 20; i++) {
                    do {
                            FD_ZERO(&fds);
                            FD_SET(fd, &fds);

                            /* Timeout. */
                            tv.tv_sec = 2;
                            tv.tv_usec = 0;

                            r = select(fd + 1, &fds, NULL, NULL, &tv);
                    } while ((r == -1 && (errno = EINTR)));
                    if (r == -1) {
                            perror("select");
                            return errno;
                    }

                    CLEAR(buf);
                    buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                    buf.memory = V4L2_MEMORY_MMAP;
                    xioctl(fd, VIDIOC_DQBUF, &buf);

                    sprintf(out_name, "out%03d.ppm", i);
                    fout = fopen(out_name, "w");
                    if (!fout) {
                            perror("Cannot open image");
                            exit(EXIT_FAILURE);
                    }
                    fprintf(fout, "P6\n%d %d 255\n",
                            fmt.fmt.pix.width, fmt.fmt.pix.height);
                    fwrite(buffers[buf.index].start, buf.bytesused, 1, fout);
                    fclose(fout);

                    xioctl(fd, VIDIOC_QBUF, &buf);
            }

            type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
            xioctl(fd, VIDIOC_STREAMOFF, &type);
            for (i = 0; i < n_buffers; ++i)
                    v4l2_munmap(buffers[i].start, buffers[i].length);
            v4l2_close(fd);

            return 0;
    }

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