are glib signals asynchronous? - asynchronous

Question

When using glib to dispatch signals through emit, are all the "listeners"/handlers called back-to-back or is control relinquished to the event loop after each listener/handler?

Answer 1

The callbacks are all called back-to-back without relinquishing control to the main loop.
Actually, as far as I know, g_signal_emit() does not even return control until all handlers are called, so there is no opportunity for the main-loop to kick-in.
So to answer the question in the title of this post: no, glib signals are not asynchronous.

Answer 2

GLib signals can be handled synchronously or asynchronously. GObject signals are always synchronous, i.e. when you emit a signal it does not return until the signal is handled.
To have a signal asynchronously handled with GLib, (I am using vala for brevity - use the vala compiler to convert the code into plain C) you must define a signal Source, or use a predefined one, as for example IdleSource or TimeoutSource (when I/O is out of question). For example assume that you have a function
void my_func() {
stdout.puts("Hello world! (async)\n");
}
and you want to call it asynchronously (from the same thread!) from
void caller() {
// Here you want to insert the asynchronous call
// that will be invoked AFTER caller has returned.
// Body of caller follows:
stdout.puts("Hello world!\n");
}
Here is how you do it:
void caller() {
// Code for the asynchronous call:
var ev = new IdleSource();
ev.set_callback(() => {
my_func();
return Source.REMOVE; // Source.REMOVE = false
});
ev.attach(MainContext.default());
// Body of caller follows:
stdout.puts("Hello world!\n");
}
You will get the following output:
Hello world!
Hello world! (async)
The my_func() function will be executed when MainLoop is idle (i.e. it has no other signals to process). To trigger it after a specific time interval has elapsed use the TimeoutSource signal source. A MainLoop must be running, otherwise this will not work.
Documentation:
https://valadoc.org/glib-2.0/index.htm
https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html