Does `shmsrc` / `shmsink` have a "stable" protocol?

[Greg Kennedy]

Okay, this is very helpful!  I had not seen shmpipe before but it's
exactly what I am looking for.

I do see that shmsrc tends to be followed by a caps filter and it
doesn't seem possible for shmsink to tell shmsrc what caps to use.
The proposed unixfdsrc / sink DOES have a command for sending caps -
would be useful.  One option might be to further wrap it in gdp
payload format and follow shmsrc with a gdpdepay to be able to send
additional commands across shm.

I think I have enough info to proceed now, thanks again for your reply!

On Mon, Sep 18, 2023 at 2:25 PM Nicolas Dufresne <nicolas at ndufresne.ca> wrote:
>
> Hi Greg,
>
> Le dimanche 17 septembre 2023 à 13:18 -0500, Greg Kennedy via gstreamer-devel a
> écrit :
> > I am trying to write an app that inserts frames into an external
> > gstreamer pipeline, but without marshalling through a socket or
> > container format.  I hit upon the idea of sharing the framebuffer via
> > shm IPC, avoiding a copy entirely.
> >
> > So I considered writing a plugin for this, but it occurred to me that
> > there is already shmsrc in plugins-bad, and it does exactly what I
> > would want: getting frames from shared mem and pushing them into a
> > pipeline, plus supporting other gstreamer features like changing caps
> > midstream.
> >
> > Normally of course shmsrc needs a matching shmsink; my question is
> > somewhat of a hack but I am curious if this could work:
> >
> > * Modify my app to "look like" an shmsink - exposing framebuffer and
> > control info just like shmsink would
>
> Inside the plugin gode, there is a BSD licence part for that purpose.
>
> https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-plugins-bad/sys/shm/shmpipe.h?ref_type=heads
> https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-plugins-bad/sys/shm/shmalloc.h?ref_type=heads
>
> And same for their associated .c files. Typically one would copy that into their
> project and use it.
>
> > * Run a pipeline with shmsrc reading from my app's masquerading interface
> >
> > I am curious how "stable" the interface is between shmsrc / shmsink.
> > If it changes frequently, this will be fragile - but if it's
> > well-defined, then I should be able to do it, and maybe it could be
> > documented (or spun off into a header) so others can do the same.
>
> -bad repo has unstable API, but we have never changed it in 10 years, mostly
> because of the fact that it may be used in some third party software. For a
> modern approach to this, Xavier posted this recently:
>
> https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5328
>
> This is similar, but the approach is different, there is FDs per buffer/memory,
> this allow passing through DMAbuf and/or other already FD located content.
>
> >
> > Thoughts?
>