c - How to wait for frame and alarm signal -

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i have problem webcam. can hardware 1 i'm convinced no. apps can see stream freezes. because of following output used app when problem occurs:

v4l: timeout (got sigalrm), hardware/driver problems?

i have checked out code , interesting part:

/* how many seconds wait before deciding it's driver problem. */ #define sync_timeout 3  int alarms;  void sigalarm(int signal) {     alarms++; }  .................................................................................  void wait_for_frame_v4l1( input_t *vidin, int frameid ) {     alarms = 0;     alarm(sync_timeout);     if (ioctl(vidin->fd, vidiocsync, vidin->buf + frameid) < 0 )         fprintf(stderr, "input: can't wait frame %d: %s\n", frameid, strerror(errno));     if (alarms)         fprintf(stderr, "v4l: timeout (got sigalrm), hardware/driver problems?");     alarm(0); }

from conclude sync_timeout problem. value 3 secondes seems quite enough.

my request me chage code don't block indefinitely waiting frames:

if no frame arrives within 100 ms, timeout , give gui chance update itself. not devices can free wheel, app should support such devices without blocking gui.

how can sub-second waiting?

v4l2 devices work this:

/* how many milliseconds wait before deciding it's driver problem. */ #define sync_timeout_msecs 100  int wait_for_frame_v4l2(input_t * vidin) {         struct timeval timeout;         fd_set rdset;         int n;          fd_zero(&rdset);         fd_set(vidin->fd, &rdset);          timeout.tv_sec = 0;         timeout.tv_usec = sync_timeout_msecs * 1000;          n = select(vidin->fd + 1, &rdset, 0, 0, &timeout);         if(n == -1) {             fprintf(stderr, "input: can't wait frame: %s\n", strerror(errno));         } else if(n == 0) {             sigalarm(0);             return 1;         }         return 0; }

but have v4l1 device.

what (usb) webcam , kernel version using?

  1. update driver/kernel
  2. if it's usb-cam, try connecting without usb-hub

the vidiocsync ioctl on vidin->fd suspends execution until vidin->buf has been filled. can wait filled buffer become available via select or poll


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