When using libunicap over libv4l to read a laptop's built-in USB webcam, the following simple test case:

#include <stdlib.h>
#include <unicap.h>

int main(int argc, char *argv[])
{
  unicap_handle_t unihand;
  unicap_device_t unidev;

  unicap_enumerate_devices(NULL, &unidev, 0);
  unicap_open(&unihand, &unidev);
  unicap_start_capture(unihand);

  sleep(1);

  unicap_stop_capture(unihand);
  unicap_close(unihand);

  return 0;
}

(Compile with "g++ v4lt.cpp `pkg-config --cflags --libs libunicap` -o v4lt".)

...produces multiple error messages from within unicap_stop_capture(). These are generated by libv4l. From stracing the process it looks like v4l first issues ioctl(VIDIOC_STREAMOFF), then attempts to call ioctl(VIDIOC_DQBUF) on each of its previously queued buffers. These ioctls fail with EINVAL.

(I'm guessing that it's something like STREAMOFF automatically dequeues the buffers, so they are no longer valid for the manual de-queue operations.)

This causes the line "libv4l2: error dequeuing buf: Invalid argument" to be output 16 times, which is a bit messy.

Furthermore, attempting to use UNICAP_BUFFER_TYPE_USER doesn't work: unicap_wait_buffer() blocks forever. Using the default system buffers and registering a frame callback (which is called from within libv4l's capture thread) does work, but this is not documented anywhere as a restriction of using the libv4l driver so I assume it's *supposed* to work. (I've no idea whether that is a problem within libv4l or on the libunicap side of things.)