## Thread Pools

Thread Pools — pools of threads to execute work concurrently

## Synopsis

    #include <glib.h>

    struct              GThreadPool;
    GThreadPool *       g_thread_pool_new                   (GFunc func,
                                                             gpointer user_data,
                                                             gint max_threads,
                                                             gboolean exclusive,
                                                             GError **error);
    gboolean            g_thread_pool_push                  (GThreadPool *pool,
                                                             gpointer data,
                                                             GError **error);
    gboolean            g_thread_pool_set_max_threads       (GThreadPool *pool,
                                                             gint max_threads,
                                                             GError **error);
    gint                g_thread_pool_get_max_threads       (GThreadPool *pool);
    guint               g_thread_pool_get_num_threads       (GThreadPool *pool);
    guint               g_thread_pool_unprocessed           (GThreadPool *pool);
    void                g_thread_pool_free                  (GThreadPool *pool,
                                                             gboolean immediate,
                                                             gboolean wait_);
    void                g_thread_pool_set_max_unused_threads
                                                            (gint max_threads);
    gint                g_thread_pool_get_max_unused_threads
                                                            (void);
    guint               g_thread_pool_get_num_unused_threads
                                                            (void);
    void                g_thread_pool_stop_unused_threads   (void);
    void                g_thread_pool_set_sort_function     (GThreadPool *pool,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);
    void                g_thread_pool_set_max_idle_time     (guint interval);
    guint               g_thread_pool_get_max_idle_time     (void);

## Description

Sometimes you wish to asynchronously fork out the execution of work and continue working in your own thread. If that will happen often, the overhead of starting and destroying a thread each time might be too high. In such cases reusing already started threads seems like a good idea. And it indeed is, but implementing this can be tedious and error-prone.

Therefore GLib provides thread pools for your convenience. An added advantage is, that the threads can be shared between the different subsystems of your program, when they are using GLib.

To create a new thread pool, you use `g_thread_pool_new()`. It is destroyed by `g_thread_pool_free()`.

If you want to execute a certain task within a thread pool, you call `g_thread_pool_push()`.

To get the current number of running threads you call `g_thread_pool_get_num_threads()`. To get the number of still unprocessed tasks you call `g_thread_pool_unprocessed()`. To control the maximal number of threads for a thread pool, you use `g_thread_pool_get_max_threads()` and `g_thread_pool_set_max_threads()`.

Finally you can control the number of unused threads, that are kept alive by GLib for future use. The current number can be fetched with `g_thread_pool_get_num_unused_threads()`. The maximal number can be controlled by `g_thread_pool_get_max_unused_threads()` and `g_thread_pool_set_max_unused_threads()`. All currently unused threads can be stopped by calling `g_thread_pool_stop_unused_threads()`.

## Details

### struct GThreadPool

    struct GThreadPool {
      GFunc func;
      gpointer user_data;
      gboolean exclusive;
    };

The GThreadPool struct represents a thread pool. It has three public read-only members, but the underlying struct is bigger, so you must not copy this struct.

GFunc `func`; | the function to execute in the threads of this pool
gpointer `user_data`; | the user data for the threads of this pool
gboolean `exclusive`; | are all threads exclusive to this pool

### g_thread_pool_new ()

    GThreadPool *       g_thread_pool_new                   (GFunc func,
                                                             gpointer user_data,
                                                             gint max_threads,
                                                             gboolean exclusive,
                                                             GError **error);

This function creates a new thread pool.

Whenever you call `g_thread_pool_push()`, either a new thread is created or an unused one is reused. At most `max_threads` threads are running concurrently for this thread pool. `max_threads` = -1 allows unlimited threads to be created for this thread pool. The newly created or reused thread now executes the function `func` with the two arguments. The first one is the parameter to `g_thread_pool_push()` and the second one is `user_data`.

The parameter `exclusive` determines whether the thread pool owns all threads exclusive or shares them with other thread pools. If `exclusive` is `TRUE`, `max_threads` threads are started immediately and they will run exclusively for this thread pool until it is destroyed by `g_thread_pool_free()`. If `exclusive` is `FALSE`, threads are created when needed and shared between all non-exclusive thread pools. This implies that `max_threads` may not be -1 for exclusive thread pools.

`error` can be `NULL` to ignore errors, or non-`NULL` to report errors. An error can only occur when `exclusive` is set to `TRUE` and not all `max_threads` threads could be created.

`func` : | a function to execute in the threads of the new thread pool
`user_data` : | user data that is handed over to `func` every time it is called
`max_threads` : | the maximal number of threads to execute concurrently in the new thread pool, -1 means no limit
`exclusive` : | should this thread pool be exclusive?
`error` : | return location for error, or `NULL`
Returns : | the new GThreadPool

### g_thread_pool_push ()

    gboolean            g_thread_pool_push                  (GThreadPool *pool,
                                                             gpointer data,
                                                             GError **error);

Inserts `data` into the list of tasks to be executed by `pool`.

When the number of currently running threads is lower than the maximal allowed number of threads, a new thread is started (or reused) with the properties given to `g_thread_pool_new()`. Otherwise, `data` stays in the queue until a thread in this pool finishes its previous task and processes `data`.

`error` can be `NULL` to ignore errors, or non-`NULL` to report errors. An error can only occur when a new thread couldn't be created. In that case `data` is simply appended to the queue of work to do.

Before version 2.32, this function did not return a success status.

`pool` : | a GThreadPool
`data` : | a new task for `pool`
`error` : | return location for error, or `NULL`
Returns : | `TRUE` on success, `FALSE` if an error occurred

### g_thread_pool_set_max_threads ()

    gboolean            g_thread_pool_set_max_threads       (GThreadPool *pool,
                                                             gint max_threads,
                                                             GError **error);

Sets the maximal allowed number of threads for `pool`. A value of -1 means that the maximal number of threads is unlimited. If `pool` is an exclusive thread pool, setting the maximal number of threads to -1 is not allowed.

Setting `max_threads` to 0 means stopping all work for `pool`. It is effectively frozen until `max_threads` is set to a non-zero value again.

A thread is never terminated while calling `func`, as supplied by `g_thread_pool_new()`. Instead the maximal number of threads only has effect for the allocation of new threads in `g_thread_pool_push()`. A new thread is allocated, whenever the number of currently running threads in `pool` is smaller than the maximal number.

`error` can be `NULL` to ignore errors, or non-`NULL` to report errors. An error can only occur when a new thread couldn't be created.

Before version 2.32, this function did not return a success status.

`pool` : | a GThreadPool
`max_threads` : | a new maximal number of threads for `pool`, or -1 for unlimited
`error` : | return location for error, or `NULL`
Returns : | `TRUE` on success, `FALSE` if an error occurred

### g_thread_pool_get_max_threads ()

    gint                g_thread_pool_get_max_threads       (GThreadPool *pool);

Returns the maximal number of threads for `pool`.

`pool` : | a GThreadPool
Returns : | the maximal number of threads

### g_thread_pool_get_num_threads ()

    guint               g_thread_pool_get_num_threads       (GThreadPool *pool);

Returns the number of threads currently running in `pool`.

`pool` : | a GThreadPool
Returns : | the number of threads currently running

### g_thread_pool_unprocessed ()

    guint               g_thread_pool_unprocessed           (GThreadPool *pool);

Returns the number of tasks still unprocessed in `pool`.

`pool` : | a GThreadPool
Returns : | the number of unprocessed tasks

### g_thread_pool_free ()

    void                g_thread_pool_free                  (GThreadPool *pool,
                                                             gboolean immediate,
                                                             gboolean wait_);

Frees all resources allocated for `pool`.

If `immediate` is `TRUE`, no new task is processed for `pool`. Otherwise `pool` is not freed before the last task is processed. Note however, that no thread of this pool is interrupted while processing a task. Instead at least all still running threads can finish their tasks before the `pool` is freed.

If `wait_` is `TRUE`, the functions does not return before all tasks to be processed (dependent on `immediate`, whether all or only the currently running) are ready. Otherwise the function returns immediately.

After calling this function `pool` must not be used anymore.

`pool` : | a GThreadPool
`immediate` : | should `pool` shut down immediately?
`wait_` : | should the function wait for all tasks to be finished?

### g_thread_pool_set_max_unused_threads ()

    void                g_thread_pool_set_max_unused_threads
                                                            (gint max_threads);

Sets the maximal number of unused threads to `max_threads`. If `max_threads` is -1, no limit is imposed on the number of unused threads.

The default value is 2.

`max_threads` : | maximal number of unused threads

### g_thread_pool_get_max_unused_threads ()

    gint                g_thread_pool_get_max_unused_threads
                                                            (void);

Returns the maximal allowed number of unused threads.

Returns : | the maximal number of unused threads

### g_thread_pool_get_num_unused_threads ()

    guint               g_thread_pool_get_num_unused_threads
                                                            (void);

Returns the number of currently unused threads.

Returns : | the number of currently unused threads

### g_thread_pool_stop_unused_threads ()

    void                g_thread_pool_stop_unused_threads   (void);

Stops all currently unused threads. This does not change the maximal number of unused threads. This function can be used to regularly stop all unused threads e.g. from `g_timeout_add()`.

### g_thread_pool_set_sort_function ()

    void                g_thread_pool_set_sort_function     (GThreadPool *pool,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);

Sets the function used to sort the list of tasks. This allows the tasks to be processed by a priority determined by `func`, and not just in the order in which they were added to the pool.

Note, if the maximum number of threads is more than 1, the order that threads are executed cannot be guaranteed 100%. Threads are scheduled by the operating system and are executed at random. It cannot be assumed that threads are executed in the order they are created.

`pool` : | a GThreadPool
`func` : | the GCompareDataFunc used to sort the list of tasks. This function is passed two tasks. It should return 0 if the order in which they are handled does not matter, a negative value if the first task should be processed before the second or a positive value if the second task should be processed first.
`user_data` : | user data passed to `func`

Since 2.10

### g_thread_pool_set_max_idle_time ()

    void                g_thread_pool_set_max_idle_time     (guint interval);

This function will set the maximum `interval` that a thread waiting in the pool for new tasks can be idle for before being stopped. This function is similar to calling `g_thread_pool_stop_unused_threads()` on a regular timeout, except this is done on a per thread basis.

By setting `interval` to 0, idle threads will not be stopped.

The default value is 15000 (15 seconds).

`interval` : | the maximum `interval` (in milliseconds) a thread can be idle

Since 2.10

### g_thread_pool_get_max_idle_time ()

    guint               g_thread_pool_get_max_idle_time     (void);

This function will return the maximum `interval` that a thread will wait in the thread pool for new tasks before being stopped.

If this function returns 0, threads waiting in the thread pool for new work are not stopped.

Returns : | the maximum `interval` (milliseconds) to wait for new tasks in the thread pool before stopping the thread

Since 2.10

## See Also

GThread