Using Glib::Dispatcher
| Chapter 29. Multi-threaded programs |

## Using Glib::Dispatcher

The slots connected to `sigc::signal` objects execute in the thread which calls `emit()` or `operator()()` on the signal. `Glib::Dispatcher` does not behave this way: instead its connected slots execute in the thread in which the `Glib::Dispatcher` object was constructed (which must have a glib main loop). If a `Glib::Dispatcher` object is constructed in the main GUI thread (which will therefore be the receiver thread), any worker thread can emit on it and have the connected slots safely execute gtkmm functions.

Some thread safety rules on the use of `Glib::Dispatcher` still apply. As mentioned, a `Glib::Dispatcher` object must be constructed in the receiver thread (the thread in whose main loop it will execute its connected slots). By default this is the main program thread, although there is a `Glib::Dispatcher` constructor which can take the `Glib::MainContext` object of any thread which has a main loop. Only the receiver thread should call `connect()` on the `Glib::Dispatcher` object, or manipulate any related `sigc::connection` object, unless additional synchronization is employed. However, any worker thread can safely emit on the `Glib::Dispatcher` object without any locking once the receiver thread has connected the slots, provided that it is constructed before the worker thread is started (if it is constructed after the thread has started, additional synchronization will normally be required to ensure visibility).

Aside from the fact that connected slots always execute in the receiver thread, `Glib::Dispatcher` objects are similar to `sigc::signal<void>` objects. They therefore cannot pass unbound arguments nor return a value. The best way to pass unbound arguments is with a thread-safe (asynchronous) queue. At the time of writing glibmm does not have one, although most people writing multi-threaded code will have one available to them (they are relatively easy to write although there are subtleties in combining thread safety with strong exception safety).

A `Glib::Dispatcher` object can be emitted on by the receiver thread as well as by a worker thread, although this should be done within reasonable bounds. On unix-like systems `Glib::Dispatcher` objects share a single common pipe, which could in theory at least fill up on a very heavily loaded system running a program with a very large number of `Dispatcher` objects in use. Were the pipe to fill up before the receiver thread's main loop has had an opportunity to read from it to empty it, and the receiver thread attempt to emit and so write to it when it is in that condition, the receiver thread would block on the write, so deadlocking. Where the receiver thread is to emit, a normal `sigc::signal<void>` object could of course be used instead.

Chapter 29. Multi-threaded programs |  | Example