Re: watching a file descriptor in gtk3

[Chris Vine <chris cvine freeserve co uk>]

On Sun, 22 Jan 2017 11:05:41 -1000 (HST)
rbd <rbd soest hawaii edu> wrote:
> Hi all,
>
> I need to monitor a Unix file descriptor for input within my gtk3
> program. I believe that I need to somehow be using
> g_source_add_unix_fd() and friends but am finding the documentation
> confusing. Are there any simple examples which show how to do this
> anywhere? (I looked but could not find, apologies for any oversight
> on my part.)
>
> I am porting an old Motif app which used
>
>          XtAppAddInput( ... fd ... callbackproc ...)
>
> to register an input handler callbackproc() to be called whenever
> there was input pending on fd. Whatever I use to replace this should
> NOT block for any user-perceivable amount of time when checking fd
> for pending input, which will typically be very small (under 80
> bytes) and occurring between a few times per second and a few times
> per hour.
>
> Absent a working understanding of how to use g_source_add_unix_fd(),
> etc., my inclination is to just use g_idle_add_full() and do my own
> non-blocking select() inside my callback. Seems like
> g_source_add_unix_fd() is perhaps better suited to this task, but I
> can't figure out what I need to do vs. what glib can do without me,
> i.e., do I really need to create and attach my own GSource and code
> my own entire set of GSourceFuncs? That would seem to be a hugely
> complicated effort compared to the simple XtAppAddInput() call, or
> even to coding my own select() stuff inside a g_idle_add_full()
> callback. Monitoring a few stray file descriptors within a gtk3
> program would seem to be a not-unusual need, so I am hoping someone
> will tell me that there is an existing GSource somewhere that I can
> just add my own fd into, but that is not at all obvious from the
> documentation.

I do not fully understand your question (especially I don't understand
your reference to using "g_idle_add_full() and do my own non-blocking
select() inside my callback", which would not work), but you can either
use glib's g_poll()/g_source_add_poll() implementation or (on unix) its
g_source_add_unix_fd() implementation to watch on a file descriptor.
The functionality of both is equivalent, and on unix both use poll()
underneath. This may (or may not) help:

https://sourceforge.net/p/cxx-gtk-utils/git/ci/master/tree/c++-gtk-utils/io_watch.cpp

If you want to use glib's g_source_add_poll() implementation rather
than its g_source_add_unix_fd() implementation, you can avoid some of
the boiler plate with g_io_add_watch() - you can use this to call up a
callback function whenever a file descriptor is ready (you don't need
to use the GIOChannel object for any other purpose, such as reading from
or writing to the descriptor):

https://developer.gnome.org/glib/stable/glib-IO-Channels.html#g-io-add-watch

Chris