lesen Funktion für Webcam Gerät in v4l2 schlägt mit ungültigem Argument

Question

Ich habe ein einfaches Programm in C99 für v4l2 schreiben und nach vielen Linux-Bücher sollte es funktionieren, aber ich bekomme Invalid argument Fehler für lesen API. was habe ich vergessen?

int main(int argc, char** argv) 
{ 
    int fd = open("/dev/video0", O_RDWR); 
    if (fd == -1) { 
     perror("Error opening"); 
     exit(EXIT_FAILURE); 
    } 


    struct v4l2_format format; //Query Format structure 
    memset(&format, 0, sizeof(format)); 
    format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; 
    int status = ioctl(fd, VIDIOC_G_FMT, &format); 
    if (status == -1) { 
     perror("Error querying format"); 
     exit(EXIT_FAILURE); 
    } 

    size_t width = format.fmt.pix.width; //Image width 
    size_t height = format.fmt.pix.height; //Image height 
    size_t imageSize = format.fmt.pix.sizeimage; // Total image size in bytes 
    size_t pixelFmt = format.fmt.pix.pixelformat; // Pixel format 
    printf("width:%u, height:%u, size:%u", width, height, imageSize); 
    switch(pixelFmt) { 
    case V4L2_PIX_FMT_YUYV: 
     printf(" & format: YuYv\n"); 
     break; 
    case V4L2_PIX_FMT_RGB24: 
     printf(" & format: RGB24\n"); 
     break; 
    default: 
     printf(" & format: %u\n", pixelFmt); 
    } 

    char* buf = malloc(imageSize); // Image buffer 
    if(buf == NULL) { 
     perror("Error allocating buffer"); 
     exit(EXIT_FAILURE); 
    } 

    fd_set fds; //Select descriptors 
    FD_ZERO(&fds); 
    FD_SET(fd, &fds); 

    struct timeval tv; //Timeout specification structure 
    tv.tv_sec = 20; 
    tv.tv_usec = 0; 

    while(true) { 

     status = select(fd+1, &fds, NULL, NULL, &tv); 

     if (status == -1) { 
      perror("Error selecting"); 
      exit(EXIT_FAILURE); 
     } else if(status == 0) { 
      perror("Select timeout"); 
      exit(EXIT_FAILURE); 
     } 

     status = read(fd, buf, imageSize); 
     if (status == -1) { 
      perror("Error reading"); 
      exit(EXIT_FAILURE); 
     } 

    } 

    free(buf); 
    close(fd); 

    return EXIT_SUCCESS; 
} 

Ich bin sicher, dass die webcame arbeitet und v4l2 Bibliothek ist in meinem Linux installiert finden Sie die komplette Ausgabe:

width:640, height:480, size:614400 & format: YuYv 
Error reading: Invalid argument 

Answer 1

Sie sollten das folgende Codefragment verwenden, um Videodaten

zu erfassen

struct v4l2_requestbuffers reqbuf; 
reqbuf.count = BUFFER_COUNT; 
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; 
reqbuf.memory = V4L2_MEMORY_MMAP; 

ret = ioctl(fd , VIDIOC_REQBUFS, &reqbuf); 
if(ret < 0) { 
    perror("VIDIOC_REQBUFS: "); 
    return ret; 
} 

struct v4l2_buffer buf; 
for (i = 0; i < reqbuf.count; i++) { 
    buf.index = i; 
    buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; 
    buf.memory = V4L2_MEMORY_MMAP; 
    ret = ioctl(fd , VIDIOC_QUERYBUF, &buf); 
    if(ret < 0) { 
     perror("VIDIOC_QUERYBUF: "); 
     return ret; 
    } 
    framebuf[i].length = buf.length; 
    framebuf[i].start = (char *) mmap(0, buf.length, PROT_READ|PROT_WRITE, MAP_SHARED, fd, buf.m.offset); 
} 

enum v4l2_buf_type type = V4L2_BUF_TYPE_VIDEO_CAPTURE; 
ret = ioctl(fd, VIDIOC_STREAMON, &type); 
if (ret < 0) { 
    perror("VIDIOC_STREAMON: "); 
    return ret; 
} 

ret = ioctl(fd, VIDIOC_DQBUF, &buf); 
if (ret < 0) { 
    perror("VIDIOC_DQBUF "); 
    return ret; 
} 

// access the captured data via framebuf[buf.index].start