manpagez: man pages & more
html files: glib
Home | html | info | man

Thread Pools

Thread Pools — pools of threads to execute work concurrently

Types and Values

struct GThreadPool

Includes

#include <glib.h>

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().

Functions

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. Besides, exclusive thread pools are not affected by g_thread_pool_set_max_idle_time() since their threads are never considered idle and returned to the global pool.

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. See GThreadError for possible errors that may occur. Note, even in case of error a valid GThreadPool is returned.

Parameters

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.

Parameters

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.

Parameters

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 .

Parameters

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 .

Parameters

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 .

Parameters

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.

Parameters

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.

Parameters

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.

Parameters

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).

Parameters

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


g_thread_pool_move_to_front ()

gboolean
g_thread_pool_move_to_front (GThreadPool *pool,
                             gpointer data);

Moves the item to the front of the queue of unprocessed items, so that it will be processed next.

Parameters

pool

a GThreadPool

 

data

an unprocessed item in the pool

 

Returns

TRUE if the item was found and moved

Since: 2.46

Types and Values

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.

Members

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

 

See Also

GThread

© manpagez.com 2000-2017
Individual documents may contain additional copyright information.