[FFmpeg-devel] [RFC] Global state into structure

[Nicolas George]

Diederick C. Niehorster (12021-06-12):
> As an API user, my main benefit would be to keep log for separate jobs
> going on in parallel separate. My application allows capturing data
> from one or more high-bandwidth machine vision cameras and storing
> them to file with ffmpeg. This often means multiple encodes are
> occurring in parallel, either because using multiple cameras or
> because a previous job didn't finish yet before the next one starts
> (these cameras generate so much data the encoder often can't quite
> keep up). I'd like each task to have its own log, not all of it mashed
> together into one.

Thanks for the feedback.

> What is the intention here, have one context per library (so my
> filters have a different log_ctx than my muxer)? I think it would make
> most sense to allow user let them all use the same context, or not,
> depending on their use case (e.g. may want one specific filter to use
> a different log context as you need its output, or want it to use
> different memory allocation function for some reason)? In any case, i

You raise an interesting question. For things like lists of registered
components, it makes sense to use a single instance for a large part of
the program, a whole decode-filter-encode system maybe. But indeed, for
logging, it makes sense to have a different context for each active
component.

I suspect here, log is more the exception than the norm. And if we
implement error messages to replace logging, the need for separate
logging callbacks would be significantly lowered.

> guess a bunch of different functions, like
> avformat_alloc_output_context2, avcodec_alloc_context3,
> avfilter_inout_alloc, avfilter_graph_alloc, would each need an extra
> input argument allowing user to set context to use.

Yes, this is what I am thinking about. On the other hand, for functions
that already have an AVSomethingContext as argument, no change in
prototype is necessary.

> Further, AVLibrary or what it'll be called user have a user data
> pointer. I would like use the log callback from C++ classes, and need
> the user data to point to a specific class instance. This also means
> that av_log should provide the relevant context when calling the
> callback.

This, for me, goes without saying: every time there's a user-provided
callback, there should be user-provided data too.

Regards,

-- 
  Nicolas George
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: not available
URL: <https://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20210617/f24de938/attachment.sig>