Re: Newbie problems using g_spawn_async_with_pipes and watching for IO

[Chris Vine <chris cvine freeserve co uk>]

On Wed, 20 Feb 2013 18:42:28 -0000
"William Orr" <ay1244 gmail com> wrote:
> Chris Vine wrote:
>
>
> On Tue, 19 Feb 2013 20:28:16 -0500
> Will Orr <ay1244 gmail com> wrote:
>     > Here[1] is the problematic code.
>     >
>     > I'm trying to run a command and gather the output and return
>     code. I
>     > do this by watching for the process to return, as well as
>     waiting for
>     > the channels to HUP. However, my callbacks never get called with
>     cond
>     > set to G_IO_HUP, even after the process terminates. Nor do any
>     of the
>     > g_io_channel_read_* calls return errors. This seems inconsistent
>     with
>     > the documentation, as well as examples that I've found in the
>     hours of
>     > researching this problem.
>     >
>     > I'm clearly doing something wrong, I'm just not sure where yet.
>     Any
>     > advice or solutions would be great. Thanks so much! Let me know
>     if I
>     > need to provide anymore information. Please CC me, as I'm not a
>     member
>     > of this list.
>
> This will help you:
>
> http://www.greenend.org.uk/rjk/2001/06/poll.html
>
> Chris
>
> Is this really preferable to using g_io_add_watch? It doesn't seem
> like I can make use of that in my event loop…

I think you misunderstood me, and I can see that I was was overly
concise. g_poll() is a wrapper on unix-like OSes for poll().  The web
page to which I directed you explains why POLLHUP, and so G_IO_HUP, is
not a reliable indicator of end-of-file.

If you have received G_IO_HUP, you must have received a hang-up, which
means on a pipe file descriptor that you must have end-of-file.  However
the converse is not true.  You can have end-of-file without G_IO_HUP,
which is what you are experiencing.  You are receiving POLLIN/G_IO_IN
instead.

The only reliable way of detecting end-of-file is if read() returns 0.
This equates to one of the GIOChannel read functions returning a
GIOStatus value of G_IO_STATUS_EOF.  Your mistake was in not checking
return values.

I have not studied your code in detail but you do not seem to be
unref()ing consistently either, in particularly in a case of error.  The
best thing to do is to unref() the GIOChannel object as soon as you have
called g_io_add_watch() or g_source_attach().  Then, returning FALSE in
the callback will automatically release the object and its resources.

Chris