Threads

Threads act almost like processes, but unlike processes all threads of one process share the same memory. This is good, as it provides easy communication between the involved threads via this shared memory, and it is bad, because strange things (so called “Heisenbugs”) might happen if the program is not carefully designed. In particular, due to the concurrent nature of threads, no assumptions on the order of execution of code running in different threads can be made, unless order is explicitly forced by the programmer through synchronization primitives.

The aim of the thread-related functions in GLib is to provide a portable means for writing multi-threaded software. There are primitives for mutexes to protect the access to portions of memory (GMutex, GRecMutex and GRWLock). There is a facility to use individual bits for locks (g_bit_lock()). There are primitives for condition variables to allow synchronization of threads (GCond). There are primitives for thread-private data - data that every thread has a private instance of (GPrivate). There are facilities for one-time initialization (GOnce, g_once_init_enter()). Finally, there are primitives to create and manage threads (GThread).

The GLib threading system used to be initialized with g_thread_init(). This is no longer necessary. Since version 2.32, the GLib threading system is automatically initialized at the start of your program, and all thread-creation functions and synchronization primitives are available right away.

Note that it is not safe to assume that your program has no threads even if you don’t call g_thread_new() yourself. GLib and GIO can and will create threads for their own purposes in some cases, such as when using g_unix_signal_source_new() or when using GDBus.

Originally, UNIX did not have threads, and therefore some traditional UNIX APIs are problematic in threaded programs. Some notable examples are

  • C library functions that return data in statically allocated buffers, such as strtok() or strerror(). For many of these, there are thread-safe variants with a _r suffix, or you can look at corresponding GLib APIs (likeg_strsplit()org_strerror()“).
  • The functions setenv() and unsetenv() manipulate the process environment in a not thread-safe way, and may interfere with getenv() calls in other threads. Note that getenv() calls may be hidden behind other APIs. For example, GNU gettext() calls getenv() under the covers. In general, it is best to treat the environment as readonly. If you absolutely have to modify the environment, do it early in main(), when no other threads are around yet.
  • The setlocale() function changes the locale for the entire process, affecting all threads. Temporary changes to the locale are often made to change the behavior of string scanning or formatting functions like scanf() or printf(). GLib offers a number of string APIs (like g_ascii_formatd() or g_ascii_strtod()) that can often be used as an alternative. Or you can use the uselocale() function to change the locale only for the current thread.
  • The fork() function only takes the calling thread into the child’s copy of the process image. If other threads were executing in critical sections they could have left mutexes locked which could easily cause deadlocks in the new child. For this reason, you should call exit() or exec() as soon as possible in the child and only make signal-safe library calls before that.
  • The daemon() function uses fork() in a way contrary to what is described above. It should not be used with GLib programs.

GLib itself is internally completely thread-safe (all global data is automatically locked), but individual data structure instances are not automatically locked for performance reasons. For example, you must coordinate accesses to the same GHashTable from multiple threads. The two notable exceptions from this rule are GMainLoop and GAsyncQueue, which are thread-safe and need no further application-level locking to be accessed from multiple threads. Most refcounting functions such as g_object_ref() are also thread-safe.

A common use for GThreads is to move a long-running blocking operation out of the main thread and into a worker thread. For GLib functions, such as single GIO operations, this is not necessary, and complicates the code. Instead, the …_async() version of the function should be used from the main thread, eliminating the need for locking and synchronisation between multiple threads. If an operation does need to be moved to a worker thread, consider using g_task_run_in_thread(), or a GThreadPool. GThreadPool is often a better choice than GThread, as it handles thread reuse and task queueing; GTask uses this internally.

However, if multiple blocking operations need to be performed in sequence, and it is not possible to use GTask for them, moving them to a worker thread can clarify the code.