On Wednesday 18 July 2007 21:52:39 Laurent Pinchart wrote:
> > + /* copy the frame */
> > + mem = video->queue.mem + buf->buf.m.offset;
> > + if (copy_to_user(data, mem, len)) {
> > + uvc_trace(UVC_TRACE_READ, "copy_to_user error\n");
> > + mutex_unlock(&video->queue.mutex);
> > + uvc_queue_buffer(&video->queue, &v4l2_b);
> > + return -EFAULT;
> > + }
>
> This won't work. read() can be called with data buffers smaller than a
> video frame.
> You should not assume that a read() call will read a whole
> buffer at once.
This is not correct. One read() should provide one frame exactly, In facts,
the V4L2 API states:
"read() attempts to read up to count bytes from file descriptor fd into the
buffer starting at buf. [...] Regardless of the count value each read() call
will provide at most one frame (two fields) worth of data.
[...]
read() never returns a "snapshot" of a buffer being filled. Using a single
buffer the driver will stop capturing when the application starts reading the
buffer until the read is finished. [...]"
Best regards
Luca Risolia
_______________________________________________
Linux-uvc-devel mailing list
Linux-uvc-devel@lists.berlios.de
https://lists.berlios.de/mailman/listinfo/linux-uvc-devel
