Using ALSA mixer, GSource and g_thread_init() results in busy loop

[Thierry Reding <thierry reding avionic-design de>]

Hi,

I'm working on a Gtk+/GLib-based project that needs to monitor the ALSA mixer
for external changes in order to keep the internal state up-to-date. The
attached program is stripped down to the minimum required to demonstrate the
problem I'm seeing (I'm also attaching a small makefile). This happens for
glib 2.26.1 and with latest master.

This is what happens: the ALSA mixer is opened and initialized, and a GSource
created to monitors the file descriptor (to /dev/snd/controlC0 in my setup)
acquired from ALSA. That source is then added to the main loop and the main
loop is run.

If I run the attached program without parameters everything works as expected
and the underlying poll() call blocks while waiting for something to happen on
the ALSA mixer source. If I open alsamixer in a separate terminal and change
the controls, I can also see that the events are handled properly by the mixer
source functions.

However, if I run the program with an argument (it does not matter what, it's
just to differentiate both code paths) things go awry. The only difference is
that now g_thread_init(NULL) is called at startup, with the result of the
main loop running busy.

I've run the program through strace to see what's going on, and what I'm
seeing is that some file descriptor (one end of a pipe in fact) is
continuously being written with the character 'A', therefore unblocking the
poll() call via the other end of the pipe. I was able to track down the pipe
write to g_main_context_wakeup_unlocked(), but I am at a complete loss trying
to understand what would trigger these continuous calls.

Can anybody shed some light on this situation?

Cheers,
Thierry

CC = gcc
CFLAGS = -O2 -g -Wall -Werror `pkg-config --cflags gthread-2.0 alsa`
LIBS = `pkg-config --libs gthread-2.0 alsa`

all: alsa-monitor

clean:
        rm -f alsa-monitor

alsa-monitor: alsa-monitor.c Makefile
        $(CC) $(CFLAGS) -o $@ $< $(LIBS)