Re: gdk_input_add() and GdkInputCondition mapping

Emil Ong <onge mcs anl gov> writes:


I'm working on an application that creates a display based on input from
standard in.  I'm using gdk_input_add() to get the input from standard in,
but I'm having a problem when piping to my application: when the left hand
side of the pipe closes its standard out (usually on exit), my handler
continues to be called.

I'll give an example that illustrates this:

#include <stdio.h>
#include <unistd.h>
#include <sys/poll.h>
#include <gtk/gtk.h>

void handle_input(gpointer data, gint source, GdkInputCondition condition)
    char buf[1024];
    struct pollfd ufds[1];
    ufds[0].fd = source;
    ufds[0].events = POLLIN;
    poll(ufds, 1, 1000);
    printf("0x%x\n", ufds[0]. revents);

    buf[0] = '\0';
    read(source, buf, 1024);


int main(int argc, char *argv[])
    gtk_init(&argc, &argv);

    (void)gdk_input_add(STDIN_FILENO, GDK_INPUT_READ, handle_input, NULL);

    return 0;

Now, when I compile this into polltest and run it...

$ echo test | ./polltest

This effect appears to be freezing the GUI part of my app. I'm guessing
that this has something to do with the changes described in this message:

where the GDK_INPUT_* mappings were changed.

My question is, is there a portable and efficient way to work around
this?  Right now I'm calling poll() in my handler to see if stdin has a
hangup, but this seems redundant and something that glib and/or gdk should

This is how input watches in GDK have always worked; and how
select() works. EOF on a file descriptor is signaled by notification
that the file descriptor is ready for reading, and the following
read producing 0 bytes.

When that happens, you are responsible for closing your end
of the pipe and removing the input watch yourself.

(If you use g_io_channel_add_watch(), you will get poll() semantics
instead of select() semantics - that is, there is G_IO_HUP. However,
because poll may be emulated by select on some systems, you still
have to check for zero length reads as an indication of EOF.)


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