[Ffmpeg-devel] Eliminating some memcpy() & memory allocationsfrom the v4l2 input

[Luca Abeni]

Hi Michael,

On Wed, 2006-02-01 at 17:12 +0100, Michael Niedermayer wrote:
[...]
> > > VIDIOC_QBUF will be called only when libavformat does not
> > > need the packet anymore, and I am planning to use the destruct()
> > > method
> > > of AVPacket for this. Does this usage of AVPacket (and of its
> > > destruct()
> > > method) make sense? 
> 
> yes
Thanks for confirming

> > Or am I misunderstanding its purpose?
> > 
> > You could add some args to av_new_packet(), 
> 
> no not without forking :)
Of course I do not want to fork :)
My goal is to implement this feature without modifying the libavformat
infrastructure.

> > or an extended version
> > like:
> > av_new_packet_dest(void *data, void (*destruct)(void *, void *), void *
> > cookie);
> > 
> > and keep the cookie in AVPacket, then call destruct() with both data
> > and cookie.
> > So you can pass the fd to ioctl() in the cookie.
> 
> well, yes thats possible but why would we want that extra layer? is there
> some advatge i dont see?
I guess Fran?ois does not like these parts of my patch:
+       pkt->priv = (void *)((res & 0xFF) | ((s->fd & 0xFFFFFF) << 8));

+    buf.index = (int)pkt->priv & 0xFF;
+    fd = (int)pkt->priv >> 8;

This was just a quick hack to get things working in a short time.
- I think on some architectures there might be problems casting int to
void*, but the C standard probably defines an int type that can be
casted to pointers (going to search for it...).
- The "res & 0xFF" and "buf.index = (int)pkt->priv & 0xFF" part is safe
if I change desired_video_buffers to 255
- There still is a problem if s->fd > (2 << 24) - 1, but I hope I can
reasonably assume this will not happen? I'll add a check anyway.

Or do you think it is better to define a
struct buffer_descriptor{
  int fd;
  int index;
};
and to use pkt->priv as a pointer to it?

			Thanks,
				Luca
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