Chapter 29. Multi-threaded programs

Table of Contents

The constraints

Care is required when writing programs based on gtkmm using multiple threads of execution, arising from the fact that libsigc++, and in particular sigc::trackable, are not thread-safe. That's because none of the complex interactions that occur behind the scenes when using libsigc++ are protected by a mutex or other means of synchronization. [1]

The rules

This requires a number of rules to be observed when writing multi-threaded programs using gtkmm. These are set out below, but one point to note is that extra care is required when deriving classes from sigc::trackable, because the effects are unintuitive (see particularly points 4 and 5 below).

  1. Use Glib::Dispatcher to invoke gtkmm functions from worker threads (this is dealt with in more detail in the next section).

  2. A sigc::signal object should be regarded as owned by the thread which created it. Only that thread should connect a sigc::slot object to the signal object, and only that thread should emit() or call operator()() on the signal, or null any connected sigc::slot object. It follows (amongst other things) that any signal object provided by a gtkmm widget should only be operated on in the main GUI thread and any object deriving from sigc::trackable having its non-static methods referenced by slots connected to the signal object should only be destroyed in that thread.

  3. Any sigc::connection object should be regarded as owned by the thread in which the method returning the sigc::connection object was called. Only that thread should call sigc::connection methods on the object.

  4. A sigc::slot object created by a call to sigc::mem_fun() which references a method of a class deriving from sigc::trackable should never be copied to another thread, nor destroyed by a different thread than the one which created it.

  5. If a particular class object derives from sigc::trackable, only one thread should create sigc::slot objects representing any of the class's non-static methods by calling sigc::mem_fun(). The first thread to create such a slot should be regarded as owning the relevant object for the purpose of creating further slots referencing any of its non-static methods using that function, or nulling those slots by disconnecting them or destroying the trackable object.

  6. Although glib is itself thread-safe, any glibmm wrappers which use libsigc++ will not be. So for example, only the thread in which a main loop runs should call Glib::SignalIdle::connect(), Glib::SignalIO::connect(), Glib::SignalTimeout::connect(), Glib::SignalTimeout::connect_seconds for that main loop, or manipulate any sigc::connection object returned by them.

    The connect*_once() variants, Glib::SignalIdle::connect_once(), Glib::SignalTimeout::connect_once(), Glib::SignalTimeout::connect_seconds_once(), are thread-safe for any case where the slot is not created by a call to sigc::mem_fun() which represents a method of a class deriving from sigc::trackable.

[1] These interactions arise from the fact that, amongst other things, a class inheriting from sigc::trackable will, via that inheritance, have a std::list object keeping track of slots created by calls to sigc::mem_fun() representing any of its non-static methods (more particularly it keeps a list of callbacks which will null the connected slots on its destruction). Each sigc::slot object also keeps, via sigc::slot_rep, its own sigc::trackable object to track any sigc::connection objects which it needs to inform about its demise, and also has a function to deregister itself from any sigc::trackable on disconnection or destruction. sigc::signal objects also keep lists of slots, which will be updated by a call to their connect() method or calls to any sigc::connection object relating to such a connection.