[Ffmpeg-cvslog] CVS: ffmpeg/libavformat rtp.c,1.24,1.25

[Rich Felker]

On Sat, Apr 01, 2006 at 12:34:51AM +0100, M?ns Rullg?rd wrote:
> Luca Barbato <lu_zero at gentoo.org> writes:
> 
> > Richard Felker CVS wrote:
> >> Update of /cvsroot/ffmpeg/ffmpeg/libavformat
> >> In directory mail:/var2/tmp/cvs-serv5251
> >> Modified Files:
> >> 	rtp.c Log Message:
> >> fix constraint violation: libavformat is not allowed to modify state of caller, including rng state
> >> Index: rtp.c
> >> ===================================================================
> >> RCS file: /cvsroot/ffmpeg/ffmpeg/libavformat/rtp.c,v
> >> retrieving revision 1.24
> >> retrieving revision 1.25
> >> diff -u -d -r1.24 -r1.25
> >> --- rtp.c	1 Mar 2006 22:45:27 -0000	1.24
> >> +++ rtp.c	30 Mar 2006 16:44:32 -0000	1.25
> >> @@ -532,9 +532,9 @@
> >>          payload_type = RTP_PT_PRIVATE; /* private payload type */
> >>      s->payload_type = payload_type;
> >>  -    s->base_timestamp = random();
> >> +    s->base_timestamp = 0; /* FIXME: was random(), what should this be? */
> >
> > random (check the rfc)
> >
> >>      s->timestamp = s->base_timestamp;
> >> -    s->ssrc = random();
> >> +    s->ssrc = 0; /* FIXME: was random(), what should this be? */
> >
> > any value would fit, random warmly suggested
> 
> What sort of sick spec mandates random values?  It's not like we're
> generating encryption keys here.

0 is a random number, and so is any arbitrarily long string of zeros.
It may be low probability, but if the protocol can't handle it then
the protocol is broken.

If someone wants to implement their own PRNG with internal (constant!)
seed or seed provided by the called, be my guest. However, keep in
mind the following are all illegal:

- corrupting the calling program's state (the caller may be using
  random and the internal state it keeps and expect deterministic
  results given the same seed!)
- use of non-reentrant library functions, since lavf muxers/demuxers
  may be used in multithreaded programs (random is non-reentrant).
- anything that leaks information, especially the time() function.

Rich