<refentry id="glib-Thread-Pools">
<refmeta>
<refentrytitle>Thread Pools</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>GLIB Library</refmiscinfo>
</refmeta>
<refnamediv>
<refname>Thread Pools</refname><refpurpose>pools of threads to execute work concurrently.</refpurpose>
</refnamediv>
<refsynopsisdiv><title>Synopsis</title>
<synopsis>
#include <glib.h>
struct <link linkend="GThreadPool">GThreadPool</link>;
<link linkend="GThreadPool">GThreadPool</link>* <link linkend="g-thread-pool-new">g_thread_pool_new</link> (<link linkend="GFunc">GFunc</link> func,
<link linkend="gpointer">gpointer</link> user_data,
<link linkend="gint">gint</link> max_threads,
<link linkend="gboolean">gboolean</link> exclusive,
<link linkend="GError">GError</link> **error);
<link linkend="void">void</link> <link linkend="g-thread-pool-push">g_thread_pool_push</link> (<link linkend="GThreadPool">GThreadPool</link> *pool,
<link linkend="gpointer">gpointer</link> data,
<link linkend="GError">GError</link> **error);
<link linkend="void">void</link> <link linkend="g-thread-pool-set-max-threads">g_thread_pool_set_max_threads</link> (<link linkend="GThreadPool">GThreadPool</link> *pool,
<link linkend="gint">gint</link> max_threads,
<link linkend="GError">GError</link> **error);
<link linkend="gint">gint</link> <link linkend="g-thread-pool-get-max-threads">g_thread_pool_get_max_threads</link> (<link linkend="GThreadPool">GThreadPool</link> *pool);
<link linkend="guint">guint</link> <link linkend="g-thread-pool-get-num-threads">g_thread_pool_get_num_threads</link> (<link linkend="GThreadPool">GThreadPool</link> *pool);
<link linkend="guint">guint</link> <link linkend="g-thread-pool-unprocessed">g_thread_pool_unprocessed</link> (<link linkend="GThreadPool">GThreadPool</link> *pool);
<link linkend="void">void</link> <link linkend="g-thread-pool-free">g_thread_pool_free</link> (<link linkend="GThreadPool">GThreadPool</link> *pool,
<link linkend="gboolean">gboolean</link> immediate,
<link linkend="gboolean">gboolean</link> wait);
<link linkend="void">void</link> <link linkend="g-thread-pool-set-max-unused-threads">g_thread_pool_set_max_unused_threads</link>
(<link linkend="gint">gint</link> max_threads);
<link linkend="gint">gint</link> <link linkend="g-thread-pool-get-max-unused-threads">g_thread_pool_get_max_unused_threads</link>
(void);
<link linkend="guint">guint</link> <link linkend="g-thread-pool-get-num-unused-threads">g_thread_pool_get_num_unused_threads</link>
(void);
<link linkend="void">void</link> <link linkend="g-thread-pool-stop-unused-threads">g_thread_pool_stop_unused_threads</link>
(void);
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
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.
</para>
<para>
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.
</para>
<para>
To create a new thread pool, you use <link linkend="g-thread-pool-new"><function>g_thread_pool_new()</function></link>. It is
destroyed by <link linkend="g-thread-pool-free"><function>g_thread_pool_free()</function></link>.
</para>
<para>
If you want to execute a certain task within a thread pool, you call
<link linkend="g-thread-pool-push"><function>g_thread_pool_push()</function></link>.
</para>
<para>
To get the current number of running threads you call
<link linkend="g-thread-pool-get-num-threads"><function>g_thread_pool_get_num_threads()</function></link>. To get the number of still
unprocessed tasks you call <link linkend="g-thread-pool-unprocessed"><function>g_thread_pool_unprocessed()</function></link>. To control the
maximal number of threads for a thread pool, you use
<link linkend="g-thread-pool-get-max-threads"><function>g_thread_pool_get_max_threads()</function></link> and <link linkend="g-thread-pool-set-max-threads"><function>g_thread_pool_set_max_threads()</function></link>.
</para>
<para>
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
<link linkend="g-thread-pool-get-num-unused-threads"><function>g_thread_pool_get_num_unused_threads()</function></link>. The maximal number can be
controlled by <link linkend="g-thread-pool-get-max-unused-threads"><function>g_thread_pool_get_max_unused_threads()</function></link> and
<link linkend="g-thread-pool-set-max-unused-threads"><function>g_thread_pool_set_max_unused_threads()</function></link>. All currently unused threads
can be stopped by calling <link linkend="g-thread-pool-stop-unused-threads"><function>g_thread_pool_stop_unused_threads()</function></link>.
</para>
</refsect1>
<refsect1>
<title>Details</title>
<refsect2>
<title><anchor id="GThreadPool"/>struct GThreadPool</title>
<indexterm><primary>GThreadPool</primary></indexterm><programlisting>struct GThreadPool {
GFunc func;
gpointer user_data;
gboolean exclusive;
};
</programlisting>
<para>
The <link linkend="GThreadPool"><type>GThreadPool</type></link> 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.
</para><variablelist role="struct">
<varlistentry>
<term><link linkend="GFunc">GFunc</link> <structfield>func</structfield></term>
<listitem><simpara>the function to execute in the threads of this pool
</simpara></listitem>
</varlistentry>
<varlistentry>
<term><link linkend="gpointer">gpointer</link> <structfield>user_data</structfield></term>
<listitem><simpara>the user data for the threads of this pool
</simpara></listitem>
</varlistentry>
<varlistentry>
<term><link linkend="gboolean">gboolean</link> <structfield>exclusive</structfield></term>
<listitem><simpara>are all threads exclusive to this pool
</simpara></listitem>
</varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-new"/>g_thread_pool_new ()</title>
<indexterm><primary>g_thread_pool_new</primary></indexterm><programlisting><link linkend="GThreadPool">GThreadPool</link>* g_thread_pool_new (<link linkend="GFunc">GFunc</link> func,
<link linkend="gpointer">gpointer</link> user_data,
<link linkend="gint">gint</link> max_threads,
<link linkend="gboolean">gboolean</link> exclusive,
<link linkend="GError">GError</link> **error);</programlisting>
<para>
This function creates a new thread pool.
</para>
<para>
Whenever you call <link linkend="g-thread-pool-push"><function>g_thread_pool_push()</function></link>, either a new thread is
created or an unused one is reused. At most <parameter>max_threads</parameter> threads
are running concurrently for this thread pool. <parameter>max_threads</parameter> = -1
allows unlimited threads to be created for this thread pool. The
newly created or reused thread now executes the function <parameter>func</parameter> with
the two arguments. The first one is the parameter to
<link linkend="g-thread-pool-push"><function>g_thread_pool_push()</function></link> and the second one is <parameter>user_data</parameter>.
</para>
<para>
The parameter <parameter>exclusive</parameter> determines, whether the thread pool owns
all threads exclusive or whether the threads are shared
globally. If <parameter>exclusive</parameter> is <literal>TRUE</literal>, <parameter>max_threads</parameter> threads are started
immediately and they will run exclusively for this thread pool until
it is destroyed by <link linkend="g-thread-pool-free"><function>g_thread_pool_free()</function></link>. If <parameter>exclusive</parameter> is <literal>FALSE</literal>,
threads are created, when needed and shared between all
non-exclusive thread pools. This implies that <parameter>max_threads</parameter> may not
be -1 for exclusive thread pools.
</para>
<para>
<parameter>error</parameter> can be <literal>NULL</literal> to ignore errors, or non-<literal>NULL</literal> to report
errors. An error can only occur when <parameter>exclusive</parameter> is set to <literal>TRUE</literal> and
not all <parameter>max_threads</parameter> threads could be created.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><parameter>func</parameter> :</term>
<listitem><simpara> a function to execute in the threads of the new thread pool
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>user_data</parameter> :</term>
<listitem><simpara> user data that is handed over to <parameter>func</parameter> every time it
is called
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>max_threads</parameter> :</term>
<listitem><simpara> the maximal number of threads to execute concurrently in
the new thread pool, -1 means no limit
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>exclusive</parameter> :</term>
<listitem><simpara> should this thread pool be exclusive?
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>error</parameter> :</term>
<listitem><simpara> return location for error
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara> the new <link linkend="GThreadPool"><type>GThreadPool</type></link>
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-push"/>g_thread_pool_push ()</title>
<indexterm><primary>g_thread_pool_push</primary></indexterm><programlisting><link linkend="void">void</link> g_thread_pool_push (<link linkend="GThreadPool">GThreadPool</link> *pool,
<link linkend="gpointer">gpointer</link> data,
<link linkend="GError">GError</link> **error);</programlisting>
<para>
Inserts <parameter>data</parameter> into the list of tasks to be executed by <parameter>pool</parameter>. 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 <link linkend="g-thread-pool-new"><function>g_thread_pool_new()</function></link>. Otherwise <parameter>data</parameter> stays
in the queue until a thread in this pool finishes its previous task
and processes <parameter>data</parameter>.
</para>
<para>
<parameter>error</parameter> can be <literal>NULL</literal> to ignore errors, or non-<literal>NULL</literal> to report
errors. An error can only occur when a new thread couldn't be
created. In that case <parameter>data</parameter> is simply appended to the queue of work
to do.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><parameter>pool</parameter> :</term>
<listitem><simpara> a <link linkend="GThreadPool"><type>GThreadPool</type></link>
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>data</parameter> :</term>
<listitem><simpara> a new task for <parameter>pool</parameter>
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>error</parameter> :</term>
<listitem><simpara> return location for error
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-set-max-threads"/>g_thread_pool_set_max_threads ()</title>
<indexterm><primary>g_thread_pool_set_max_threads</primary></indexterm><programlisting><link linkend="void">void</link> g_thread_pool_set_max_threads (<link linkend="GThreadPool">GThreadPool</link> *pool,
<link linkend="gint">gint</link> max_threads,
<link linkend="GError">GError</link> **error);</programlisting>
<para>
Sets the maximal allowed number of threads for <parameter>pool</parameter>. A value of -1
means, that the maximal number of threads is unlimited.
</para>
<para>
Setting <parameter>max_threads</parameter> to 0 means stopping all work for <parameter>pool</parameter>. It is
effectively frozen until <parameter>max_threads</parameter> is set to a non-zero value
again.
</para>
<para>
A thread is never terminated while calling <parameter>func</parameter>, as supplied by
<link linkend="g-thread-pool-new"><function>g_thread_pool_new()</function></link>. Instead the maximal number of threads only
has effect for the allocation of new threads in <link linkend="g-thread-pool-push"><function>g_thread_pool_push()</function></link>.
A new thread is allocated, whenever the number of currently
running threads in <parameter>pool</parameter> is smaller than the maximal number.
</para>
<para>
<parameter>error</parameter> can be <literal>NULL</literal> to ignore errors, or non-<literal>NULL</literal> to report
errors. An error can only occur when a new thread couldn't be
created.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><parameter>pool</parameter> :</term>
<listitem><simpara> a <link linkend="GThreadPool"><type>GThreadPool</type></link>
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>max_threads</parameter> :</term>
<listitem><simpara> a new maximal number of threads for <parameter>pool</parameter>
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>error</parameter> :</term>
<listitem><simpara> return location for error
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-get-max-threads"/>g_thread_pool_get_max_threads ()</title>
<indexterm><primary>g_thread_pool_get_max_threads</primary></indexterm><programlisting><link linkend="gint">gint</link> g_thread_pool_get_max_threads (<link linkend="GThreadPool">GThreadPool</link> *pool);</programlisting>
<para>
Returns the maximal number of threads for <parameter>pool</parameter>.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><parameter>pool</parameter> :</term>
<listitem><simpara> a <link linkend="GThreadPool"><type>GThreadPool</type></link>
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara> the maximal number of threads
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-get-num-threads"/>g_thread_pool_get_num_threads ()</title>
<indexterm><primary>g_thread_pool_get_num_threads</primary></indexterm><programlisting><link linkend="guint">guint</link> g_thread_pool_get_num_threads (<link linkend="GThreadPool">GThreadPool</link> *pool);</programlisting>
<para>
Returns the number of threads currently running in <parameter>pool</parameter>.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><parameter>pool</parameter> :</term>
<listitem><simpara> a <link linkend="GThreadPool"><type>GThreadPool</type></link>
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara> the number of threads currently running
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-unprocessed"/>g_thread_pool_unprocessed ()</title>
<indexterm><primary>g_thread_pool_unprocessed</primary></indexterm><programlisting><link linkend="guint">guint</link> g_thread_pool_unprocessed (<link linkend="GThreadPool">GThreadPool</link> *pool);</programlisting>
<para>
Returns the number of tasks still unprocessed in <parameter>pool</parameter>.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><parameter>pool</parameter> :</term>
<listitem><simpara> a <link linkend="GThreadPool"><type>GThreadPool</type></link>
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara> the number of unprocessed tasks
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-free"/>g_thread_pool_free ()</title>
<indexterm><primary>g_thread_pool_free</primary></indexterm><programlisting><link linkend="void">void</link> g_thread_pool_free (<link linkend="GThreadPool">GThreadPool</link> *pool,
<link linkend="gboolean">gboolean</link> immediate,
<link linkend="gboolean">gboolean</link> wait);</programlisting>
<para>
Frees all resources allocated for <parameter>pool</parameter>.
</para>
<para>
If <parameter>immediate</parameter> is <literal>TRUE</literal>, no new task is processed for
<parameter>pool</parameter>. Otherwise <parameter>pool</parameter> 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 <parameter>pool</parameter> is freed.
</para>
<para>
If <parameter>wait</parameter> is <literal>TRUE</literal>, the functions does not return before all tasks
to be processed (dependent on <parameter>immediate</parameter>, whether all or only the
currently running) are ready. Otherwise the function returns immediately.
</para>
<para>
After calling this function <parameter>pool</parameter> must not be used anymore.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><parameter>pool</parameter> :</term>
<listitem><simpara> a <link linkend="GThreadPool"><type>GThreadPool</type></link>
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>immediate</parameter> :</term>
<listitem><simpara> should <parameter>pool</parameter> shut down immediately?
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>wait</parameter> :</term>
<listitem><simpara> should the function wait for all tasks to be finished?
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-set-max-unused-threads"/>g_thread_pool_set_max_unused_threads ()</title>
<indexterm><primary>g_thread_pool_set_max_unused_threads</primary></indexterm><programlisting><link linkend="void">void</link> g_thread_pool_set_max_unused_threads
(<link linkend="gint">gint</link> max_threads);</programlisting>
<para>
Sets the maximal number of unused threads to <parameter>max_threads</parameter>. If
<parameter>max_threads</parameter> is -1, no limit is imposed on the number of unused
threads.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><parameter>max_threads</parameter> :</term>
<listitem><simpara> maximal number of unused threads
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-get-max-unused-threads"/>g_thread_pool_get_max_unused_threads ()</title>
<indexterm><primary>g_thread_pool_get_max_unused_threads</primary></indexterm><programlisting><link linkend="gint">gint</link> g_thread_pool_get_max_unused_threads
(void);</programlisting>
<para>
Returns the maximal allowed number of unused threads.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara> the maximal number of unused threads
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-get-num-unused-threads"/>g_thread_pool_get_num_unused_threads ()</title>
<indexterm><primary>g_thread_pool_get_num_unused_threads</primary></indexterm><programlisting><link linkend="guint">guint</link> g_thread_pool_get_num_unused_threads
(void);</programlisting>
<para>
Returns the number of currently unused threads.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara> the number of currently unused threads
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-thread-pool-stop-unused-threads"/>g_thread_pool_stop_unused_threads ()</title>
<indexterm><primary>g_thread_pool_stop_unused_threads</primary></indexterm><programlisting><link linkend="void">void</link> g_thread_pool_stop_unused_threads
(void);</programlisting>
<para>
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 <link linkend="g-timeout-add"><function>g_timeout_add()</function></link>.</para>
<para>
</para></refsect2>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<variablelist>
<varlistentry>
<term><link linkend="GThread"><type>GThread</type></link></term>
<listitem><para>GLib thread system.</para></listitem>
</varlistentry>
</variablelist>
</para>
</refsect1>
</refentry>