[FFmpeg-soc] [PATCH] rtsp tunneling

[Martin Storsjö]

On Wed, 16 Jun 2010, Michael Niedermayer wrote:

> On Mon, Jun 14, 2010 at 08:12:30PM +0300, Martin Storsjö wrote:
> > 
> > Additionally, as far as I've understood AVOptions, they can only set 
> > something within one struct, since it simply uses offsets. For this case, 
> > we'd need AVOptions that would only apply if the URLContext happens to be 
> > a HTTP context, and then set the options within the priv_data, which can't 
> > be done as a simple offset (unless the priv_data is tacked on at the 
> > end of the context...)
> 
> I think we should check the past discussion about using AVOptions and private
> contexts. but from memory
> Adding an AVClass pointer to URLProtocol (and a priv_data_size of course too)
> allows us to allocate and init to defaults the private context in generic code

Not as far as I understand it. In libavcodec/opt.c, for example 
av_set_number2, lines 63-70, store values using this logic:

    dst= ((uint8_t*)obj) + o->offset;
    *(int*)dst= llrint(num/den)*intnum;

So the only things it can express are storing values with a plain offset 
from the start of the object. There's no code for dereferencing the 
priv_data pointer and using that as an object.

In order to set values within the private data, we'd need to make the 
private data an AVClass so that it can have different options depending on 
what the actual protocol is. Then in the generic AVClass for URLContext, 
we'd need to store the offset of the private data pointer, so that the 
generic AVOption code can check for options in the private data if the 
option wasn't found in the generic URLContext.

// Martin