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# Thread Pools

## Name

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

## Description

Sometimes you wish to asyncronously 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 to 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 six 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 whether the threads are shared globally. 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
Returns : | the new GThreadPool

### g_thread_pool_push ()

    void        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.

`pool` : | a GThreadPool
`data` : | a new task for `pool`
`error` : | return location for error

### g_thread_pool_set_max_threads ()

    void        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.

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.

`pool` : | a GThreadPool
`max_threads` : | a new maximal number of threads for `pool`
`error` : | return location for error

### 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.

`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().

## See Also

GThread

  GLib thread system.

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