Multiple Camera question

I'm new to v4l so forgive me if this question has been answered many times before.  I am using four USB webcams and would like to access them directly using v4l.  I am able to open and configure all four cameras but when I go to take snaphots with them I can only get one from each controller to work.  Does anyone know why this doesn't work?  Does anyone have any work arounds?

 

Thanks,

 

Mike

 

PS.  Code snippit trying to read from 4 cameras.  If I make the number of cameras 2 and just use /dev/video0 and /dev/video2 I get one image from each.  If I try to use /dev/video0 and /dev/video1 the program hangs waiting for sync from camera 1. 

 

    for (i = 0; i < NUM_CAMERAS; i++)
    {
        k = 0;
        log("taking shot for cam %i\n", i);
        if (ioctl(grab_fd[i], VIDIOCMCAPTURE, &grab_buf[i]) == -1)
        {
            perror("ioctl VIDIOCMCAPTURE");
            return;
        }
        log("waiting for sync for cam %i\n", i);
        while (ioctl(grab_fd[i], VIDIOCSYNC, &k) < 0) ;

        log("just took shot for cam %i\n", i);
     }