<page xmlns="http://projectmallard.org/1.0/"
      xmlns:its="http://www.w3.org/2005/11/its"
      xmlns:xi="http://www.w3.org/2003/XInclude"
      type="topic"
      id="main-contexts">

  <info>
    <link type="guide" xref="index#specific-how-tos"/>

    <credit type="author copyright">
      <name>Philip Withnall</name>
      <email its:translate="no">philip.withnall@collabora.co.uk</email>
      <years>2014–2015</years>
    </credit>

    <include href="cc-by-sa-3-0.xml" xmlns="http://www.w3.org/2001/XInclude"/>

    <desc>
      GLib main contexts, invoking functions in other threads, and the event
      loop
    </desc>
  </info>

  <title>GLib Main Contexts</title>

  <synopsis>
    <title>Summary</title>

    <list>
      <item><p>
        Use
        <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-main-context-invoke-full"><code>g_main_context_invoke_full()</code></link>
        to invoke functions in other threads, assuming every thread has a thread
        default main context which runs throughout the lifetime of that thread
        (<link xref="#g-main-context-invoke-full"/>)
      </p></item>
      <item><p>
        Use
        <link href="https://developer.gnome.org/gio/stable/GTask.html"><code>GTask</code></link>
        to run a function in the background without caring about the specific
        thread used (<link xref="#gtask"/>)
      </p></item>
      <item><p>
        Liberally use assertions to check which context executes each function,
        and add these assertions when first writing the code
        (<link xref="#checking-threading"/>)
      </p></item>
      <item><p>
        Explicitly document contexts a function is expected to be called in, a
        callback will be invoked in, or a signal will be emitted in
        (<link xref="#using-gmaincontext-in-a-library"/>)
      </p></item>
      <item><p>
        Beware of <code>g_idle_add()</code> and similar functions which
        implicitly use the global-default main context
        (<link xref="#implicit-use-of-the-global-default-main-context"/>)
      </p></item>
    </list>
  </synopsis>

  <section id="what-is-gmaincontext">
    <title>What is <code>GMainContext</code>?</title>

    <p>
      <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#GMainContext"><code>GMainContext</code></link>
      is a generalized implementation of an
      <link href="http://en.wikipedia.org/wiki/Event_loop">event loop</link>,
      useful for implementing polled file I/O or event-based widget systems
      (such as GTK+). It is at the core of almost every GLib application. To
      understand <code>GMainContext</code> requires understanding
      <link href="man:poll(2)">poll()</link> and polled I/O.
    </p>

    <p>
      A <code>GMainContext</code> has a set of
      <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#GSource"><code>GSource</code></link>s
      which are ‘attached’ to it, each of which can be thought of as an expected
      event with an associated callback function which will be invoked when that
      event is received; or equivalently as a set of file descriptors (FDs) to
      check. An event could be a timeout or data being received on a socket, for
      example. One iteration of the event loop will:
    </p>
    <list type="enumerated">
      <item><p>
        Prepare sources, determining if any of them are ready to dispatch
        immediately.
      </p></item>
      <item><p>
        Poll the sources, blocking the current thread until an event is received
        for one of the sources.
      </p></item>
      <item><p>
        Check which of the sources received an event (several could have).
      </p></item>
      <item><p>
        Dispatch callbacks from those sources.
      </p></item>
    </list>

    <p>
      This is
      <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#mainloop-states">explained
      very well</link> in the
      <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#GSourceFuncs">GLib
      documentation</link>.
    </p>

    <p>
      At its core, <code>GMainContext</code> is just a <code>poll()</code> loop,
      with the preparation, check and dispatch stages of the loop corresponding
      to the normal preamble and postamble in a typical <code>poll()</code> loop
      implementation, such as listing 1 from
      <link href="http://www.linux-mag.com/id/357/">this article</link>.
      Typically, some complexity is needed in non-trivial
      <code>poll()</code>-using applications to track the lists of FDs which are
      being polled. Additionally, <code>GMainContext</code> adds a lot of useful
      functionality which vanilla <code>poll()</code> doesn’t support. Most
      importantly, it adds thread safety.
    </p>

    <p>
      <code>GMainContext</code> is completely thread safe, meaning that a
      <code>GSource</code> can be created in one thread and attached to a
      <code>GMainContext</code> running in another thread. (See
      also: <link xref="threading"/>.) A typical use for this might be to allow
      worker threads to control which sockets are being listened to by a
      <code>GMainContext</code> in a central I/O thread. Each
      <code>GMainContext</code> is ‘acquired’ by a thread for each iteration
      it’s put through. Other threads cannot iterate a <code>GMainContext</code>
      without acquiring it, which guarantees that a <code>GSource</code> and its
      FDs will only be polled by one thread at once (since each
      <code>GSource</code> is attached to at most one
      <code>GMainContext</code>). A <code>GMainContext</code> can be swapped
      between threads across iterations, but this is expensive.
    </p>

    <p>
      <code>GMainContext</code> is used instead of <code>poll()</code> mostly
      for convenience, as it transparently handles dynamically managing
      the array of FDs to pass to <code>poll()</code>, especially when operating
      over multiple threads. This is done by encapsulating FDs in
      <code>GSource</code>s, which decide whether those FDs should be passed to
      the <code>poll()</code> call on each ‘prepare’ stage of the main context
      iteration.
    </p>
  </section>

  <section id="what-is-gmainloop">
    <title>What is <code>GMainLoop</code>?</title>

    <p>
      <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#GMainLoop"><code>GMainLoop</code></link>
      is essentially the following few lines of code, once reference counting
      and locking have been removed (from
      <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-main-loop-run"><code>g_main_loop_run()</code></link>):
    </p>
    <code mime="text/x-csrc">loop->is_running = TRUE;
while (loop->is_running)
  {
    g_main_context_iteration (context, TRUE);
  }</code>

    <p>
      Plus a fourth line in
      <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-main-loop-quit"><code>g_main_loop_quit()</code></link>
      which sets <code>loop-&gt;is_running = FALSE</code> and which will cause
      the loop to terminate once the current main context iteration ends.
    </p>

    <p>
      Hence, <code>GMainLoop</code> is a convenient, thread-safe way of running
      a <code>GMainContext</code> to process events until a desired exit
      condition is met, at which point <code>g_main_loop_quit()</code> should be
      called. Typically, in a UI program, this will be the user clicking ‘exit’.
      In a socket handling program, this might be the final socket closing.
    </p>

    <p>
      It is important not to confuse main contexts with main loops. Main
      contexts do the bulk of the work: preparing source lists, waiting for
      events, and dispatching callbacks. A main loop simply iterates a context.
    </p>
  </section>

  <section id="default-contexts">
    <title>Default Contexts</title>

    <p>
      One of the important features of <code>GMainContext</code> is its support
      for ‘default’ contexts. There are two levels of default context: the
      thread-default, and the global-default. The global-default (accessed using
      <code>g_main_context_default()</code>) is run by GTK+ when
      <code>gtk_main()</code> is called. It’s also used for timeouts
      (<code>g_timeout_add()</code>) and idle callbacks
      (<code>g_idle_add()</code>) — these won’t be dispatched unless the default
      context is running! (See:
      <link xref="#implicit-use-of-the-global-default-main-context"/>.)
    </p>

    <p>
      Thread-default contexts are a later addition to GLib (since version 2.22),
      and are generally used for I/O operations which need to run and dispatch
      callbacks in a thread. By calling
      <code>g_main_context_push_thread_default()</code> before starting an I/O
      operation, the thread-default context is set and the I/O operation can add
      its sources to that context. The context can then be run in a new main
      loop in an I/O thread, causing the callbacks to be dispatched on that
      thread’s stack rather than on the stack of the thread running the
      global-default main context. This allows I/O operations to be run entirely
      in a separate thread without explicitly passing a specific
      <code>GMainContext</code> pointer around everywhere.
    </p>

    <p>
      Conversely, by starting a long-running operation with a specific
      thread-default context set, the calling code can guarantee that the
      operation’s callbacks will be emitted in that context, even if the
      operation itself runs in a worker thread. This is the principle behind
      <link href="https://developer.gnome.org/gio/stable/GTask.html"><code>GTask</code></link>:
      when a new <code>GTask</code> is created, it stores a reference to the
      current thread-default context, and dispatches its completion callback in
      that context, even if the task itself is run using
      <link href="https://developer.gnome.org/gio/stable/GTask.html#g-task-run-in-thread"><code>g_task_run_in_thread()</code></link>.
    </p>

    <example>
      <p>
        For example, the code below will run a <code>GTask</code> which performs
        two writes in parallel from a thread. The callbacks for the writes will
        be dispatched in the worker thread, whereas the callback from the task
        as a whole will be dispatched in the <code>interesting_context</code>.
      </p>

      <code mime="text/x-csrc" style="valid">
typedef struct {
  GMainLoop *main_loop;
  guint n_remaining;
} WriteData;

/* This is always called in the same thread as thread_cb() because
 * it’s always dispatched in the @worker_context. */
static void
write_cb (GObject      *source_object,
          GAsyncResult *result,
          gpointer      user_data)
{
  WriteData *data = user_data;
  GOutputStream *stream = G_OUTPUT_STREAM (source_object);
  GError *error = NULL;
  gssize len;

  /* Finish the write. */
  len = g_output_stream_write_finish (stream, result, &amp;error);
  if (error != NULL)
    {
      g_error ("Error: %s", error->message);
      g_error_free (error);
    }

  /* Check whether all parallel operations have finished. */
  write_data->n_remaining--;

  if (write_data->n_remaining == 0)
    {
      g_main_loop_quit (write_data->main_loop);
    }
}

/* This is called in a new thread. */
static void
thread_cb (GTask        *task,
           gpointer      source_object,
           gpointer      task_data,
           GCancellable *cancellable)
{
  /* These streams come from somewhere else in the program: */
  GOutputStream *output_stream1, *output_stream;
  GMainContext *worker_context;
  GBytes *data;
  const guint8 *buf;
  gsize len;

  /* Set up a worker context for the writes’ callbacks. */
  worker_context = g_main_context_new ();
  g_main_context_push_thread_default (worker_context);

  /* Set up the writes. */
  write_data.n_remaining = 2;
  write_data.main_loop = g_main_loop_new (worker_context, FALSE);

  data = g_task_get_task_data (task);
  buf = g_bytes_get_data (data, &amp;len);

  g_output_stream_write_async (output_stream1, buf, len,
                               G_PRIORITY_DEFAULT, NULL, write_cb,
                               &amp;write_data);
  g_output_stream_write_async (output_stream2, buf, len,
                               G_PRIORITY_DEFAULT, NULL, write_cb,
                               &amp;write_data);

  /* Run the main loop until both writes have finished. */
  g_main_loop_run (write_data.main_loop);
  g_task_return_boolean (task, TRUE);  /* ignore errors */

  g_main_loop_unref (write_data.main_loop);

  g_main_context_pop_thread_default (worker_context);
  g_main_context_unref (worker_context);
}

/* This can be called from any thread. Its @callback will always be
 * dispatched in the thread which currently owns
 * @interesting_context. */
void
parallel_writes_async (GBytes              *data,
                       GMainContext        *interesting_context,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  GTask *task;

  g_main_context_push_thread_default (interesting_context);

  task = g_task_new (NULL, cancellable, callback, user_data);
  g_task_set_task_data (task, data,
                        (GDestroyNotify) g_bytes_unref);
  g_task_run_in_thread (task, thread_cb);
  g_object_unref (task);

  g_main_context_pop_thread_default (interesting_context);
}</code>
    </example>

    <section id="implicit-use-of-the-global-default-main-context">
      <title>Implicit Use of the Global-Default Main Context</title>

      <p>
        Several functions implicitly add sources to the global-default main
        context. They should <em>not</em> be used in threaded code. Instead, use
        <code>g_source_attach()</code> with the <code>GSource</code> created by
        the replacement function from the table below.
      </p>

      <p>
        Implicit use of the global-default main context means the callback
        functions are invoked in the main thread, typically resulting in work
        being brought back from a worker thread into the main thread.
      </p>

      <table shade="rows">
        <colgroup><col /></colgroup>
        <colgroup><col /><col /></colgroup>
        <thead>
          <tr>
            <td><p>Do not use</p></td>
            <td><p>Use instead</p></td>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td><p><code>g_timeout_add()</code></p></td>
            <td><p><code>g_timeout_source_new()</code></p></td>
          </tr>
          <tr>
            <td><p><code>g_idle_add()</code></p></td>
            <td><p><code>g_idle_source_new()</code></p></td>
          </tr>
          <tr>
            <td><p><code>g_child_watch_add()</code></p></td>
            <td><p><code>g_child_watch_source_new()</code></p></td>
          </tr>
        </tbody>
      </table>

      <example>
        <p>
          So to delay some computation in a worker thread, use the following
          code:
        </p>
        <code mime="text/x-csrc">
static guint
schedule_computation (guint delay_seconds)
{
  GSource *source = NULL;
  GMainContext *context;
  guint id;

  /* Get the calling context. */
  context = g_main_context_get_thread_default ();

  source = g_timeout_source_new_seconds (delay_seconds);
  g_source_set_callback (source, do_computation, NULL, NULL);
  id = g_source_attach (source, context);
  g_source_unref (source);

  /* The ID can be used with the same @context to
   * cancel the scheduled computation if needed. */
  return id;
}

static void
do_computation (gpointer user_data)
{
  /* … */
}</code>
      </example>
    </section>
  </section>

  <section id="using-gmaincontext-in-a-library">
    <title>Using <code>GMainContext</code> in a Library</title>

    <p>
      At a high level, library code must not make changes to main contexts which
      could affect the execution of an application using the library, for
      example by changing when the application’s <code>GSource</code>s are
      dispatched. There are various best practices which can be followed to aid
      this.
    </p>

    <p>
      Never iterate a context created outside the library, including the
      global-default or thread-default contexts. Otherwise,
      <code>GSource</code>s created in the application may be dispatched
      when the application is not expecting it, causing
      <link href="http://en.wikipedia.org/wiki/Reentrancy_%28computing%29">re-entrancy
      problems</link> for the application code.
    </p>

    <p>
      Always remove <code>GSource</code>s from a main context before dropping
      the library’s last reference to the context, especially if it may have
      been exposed to the application (for example, as a thread-default).
      Otherwise the application may keep a reference to the main context and
      continue iterating it after the library has returned, potentially causing
      unexpected source dispatches in the library. This is equivalent to not
      assuming that dropping the library’s last reference to a main context will
      finalize that context.
    </p>

    <p>
      If the library is designed to be used from multiple threads, or in a
      context-aware fashion, always document which context each callback will be
      dispatched in. For example, “callbacks will always be dispatched in the
      context which is the thread-default at the time of the object’s
      construction”. Developers using the library’s API need to know this
      information.
    </p>

    <p>
      Use <code>g_main_context_invoke()</code> to ensure callbacks are
      dispatched in the right context. It’s much easier than manually using
      <code>g_idle_source_new()</code> to transfer work between contexts.
      (See: <link xref="#ensuring-functions-are-called-in-the-right-context"/>.)
    </p>

    <p>
      Libraries should never use <code>g_main_context_default()</code> (or,
      equivalently, pass <code>NULL</code> to a <code>GMainContext</code>-typed
      parameter). Always store and explicitly use a specific
      <code>GMainContext</code>, even if it often points to some default
      context. This makes the code easier to split out into threads in future,
      if needed, without causing hard-to-debug problems caused by callbacks
      being invoked in the wrong context.
    </p>

    <p>
      Always write things asynchronously internally (using
      <link xref="#gtask"><code>GTask</code></link> where appropriate), and keep
      synchronous wrappers at the very top level of an API, where they can be
      implemented by calling <code>g_main_context_iteration()</code> on a
      specific <code>GMainContext</code>. Again, this makes future refactoring
      easier. This is demonstrated in the above example: the thread uses
      <code>g_output_stream_write_async()</code> rather than
      <code>g_output_stream_write()</code>.
    </p>

    <p>
      Always match pushes and pops of the thread-default main context:
      <code>g_main_context_push_thread_default()</code> and
      <code>g_main_context_pop_thread_default()</code>.
    </p>
  </section>

  <section id="ensuring-functions-are-called-in-the-right-context">
    <title>Ensuring Functions are Called in the Right Context</title>

    <p>
      The ‘right context’ is the thread-default main context of the <em>thread
      the function should be executing in</em>. This assumes the typical case
      that every thread has a <em>single</em> main context running in a main
      loop. A main context effectively provides a work or
      <link href="http://en.wikipedia.org/wiki/Message_queue">message
      queue</link> for the thread — something which the thread can
      periodically check to determine if there is work pending from
      another thread. Putting a message on this queue – invoking a function in
      another main context – will result in it eventually being dispatched in
      that thread.
    </p>

    <example>
      <p>
        For example, if an application does a long and CPU-intensive computation
        it should schedule this in a background thread so that UI updates in the
        main thread are not blocked. The results of the computation, however,
        might need to be displayed in the UI, so some UI update function must be
        called in the main thread once the computation’s complete.
      </p>

      <p>
        Furthermore, if the computation function can be limited to a single
        thread, it becomes easy to eliminate the need for locking a lot of the
        data it accesses. This assumes that other threads are implemented
        similarly and hence most data is only accessed by a single thread, with
        threads communicating by
        <link href="http://en.wikipedia.org/wiki/Message_passing">message
        passing</link>. This allows each thread to update its data at its
        leisure, which significantly simplifies locking.
     </p>
   </example>

    <p>
      For some functions, there might be no reason to care which context they’re
      executed in, perhaps because they’re asynchronous and hence do not block
      the context. However, it is still advisable to be explicit about which
      context is used, since those functions may emit signals or invoke
      callbacks, and for reasons of thread safety it’s necessary to know which
      threads those signal handlers or callbacks are going to be invoked in.
    </p>

    <example>
      <p>
        For example, the progress callback in
        <link href="https://developer.gnome.org/gio/stable/GFile.html#g-file-copy-async"><code>g_file_copy_async()</code></link>
        is documented as being called in the thread-default main context
        at the time of the initial call.
      </p>
    </example>

    <section id="invocation-core-principle">
      <title>Principles of Invocation</title>

      <p>
        The core principle of invoking a function in a specific context is
        simple, and is walked through below to explain the concepts. In practice
        the <link xref="#g-main-context-invoke-full">convenience method,
        <code>g_main_context_invoke_full()</code></link> should be used instead.
      </p>

      <p>
        A
        <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#GSource"><code>GSource</code></link>
        has to be added to the target <code>GMainContext</code>, which will invoke
        the function when it’s dispatched. This <code>GSource</code> should almost
        always be an idle source created with
        <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-idle-source-new"><code>g_idle_source_new()</code></link>,
        but this doesn’t have to be the case. It could be a timeout source
        so that the function is executed after a delay, for example.
      </p>

      <p>
        The <code>GSource</code> will be
        <link xref="#what-is-gmaincontext">dispatched as soon as it’s ready</link>,
        calling the function on the thread’s stack. In the case of an idle source,
        this will be as soon as all sources at a higher priority have been
        dispatched — this can be tweaked using the idle source’s priority
        parameter with
        <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-source-set-priority"><code>g_source_set_priority()</code></link>.
        The source will typically then be destroyed so the function is only
        executed once (though again, this doesn’t have to be the case).
      </p>

      <p>
        Data can be passed between threads as the <code>user_data</code> passed to
        the <code>GSource</code>’s callback. This is set on the source using
        <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-source-set-callback"><code>g_source_set_callback()</code></link>,
        along with the callback function to invoke. Only a single pointer
        is provided, so if multiple data fields need passing, they must be wrapped
        in an allocated structure.
      </p>

      <example>
        <p>
          The example below demonstrates the underlying principles, but there are
          convenience methods explained below which simplify things.
        </p>

        <code mime="text/x-csrc">
/* Main function for the background thread, thread1. */
static gpointer
thread1_main (gpointer user_data)
{
  GMainContext *thread1_main_context = user_data;
  GMainLoop *main_loop;

  /* Set up the thread’s context and run it forever. */
  g_main_context_push_thread_default (thread1_main_context);

  main_loop = g_main_loop_new (thread1_main_context, FALSE);
  g_main_loop_run (main_loop);
  g_main_loop_unref (main_loop);

  g_main_context_pop_thread_default (thread1_main_context);
  g_main_context_unref (thread1_main_context);

  return NULL;
}

/* A data closure structure to carry multiple variables between
 * threads. */
typedef struct {
  gchar   *some_string;  /* owned */
  guint    some_int;
  GObject *some_object;  /* owned */
} MyFuncData;

static void
my_func_data_free (MyFuncData *data)
{
  g_free (data->some_string);
  g_clear_object (&amp;data->some_object);
  g_free (data);
}

static void
my_func (const gchar *some_string,
         guint        some_int,
         GObject     *some_object)
{
  /* Do something long and CPU intensive! */
}

/* Convert an idle callback into a call to my_func(). */
static gboolean
my_func_idle (gpointer user_data)
{
  MyFuncData *data = user_data;

  my_func (data->some_string, data->some_int, data->some_object);

  return G_SOURCE_REMOVE;
}

/* Function to be called in the main thread to schedule a call to
 * my_func() in thread1, passing the given parameters along. */
static void
invoke_my_func (GMainContext *thread1_main_context,
                const gchar  *some_string,
                guint         some_int,
                GObject      *some_object)
{
  GSource *idle_source;
  MyFuncData *data;

  /* Create a data closure to pass all the desired variables
   * between threads. */
  data = g_new0 (MyFuncData, 1);
  data->some_string = g_strdup (some_string);
  data->some_int = some_int;
  data->some_object = g_object_ref (some_object);

  /* Create a new idle source, set my_func() as the callback with
   * some data to be passed between threads, bump up the priority
   * and schedule it by attaching it to thread1’s context. */
  idle_source = g_idle_source_new ();
  g_source_set_callback (idle_source, my_func_idle, data,
                         (GDestroyNotify) my_func_data_free);
  g_source_set_priority (idle_source, G_PRIORITY_DEFAULT);
  g_source_attach (idle_source, thread1_main_context);
  g_source_unref (idle_source);
}

/* Main function for the main thread. */
static void
main (void)
{
  GThread *thread1;
  GMainContext *thread1_main_context;

  /* Spawn a background thread and pass it a reference to its
   * GMainContext. Retain a reference for use in this thread
   * too. */
  thread1_main_context = g_main_context_new ();
  g_thread_new ("thread1", thread1_main,
                g_main_context_ref (thread1_main_context));

  /* Maybe set up your UI here, for example. */

  /* Invoke my_func() in the other thread. */
  invoke_my_func (thread1_main_context,
                  "some data which needs passing between threads",
                  123456, some_object);

  /* Continue doing other work. */
}</code>

        <p>
          This invocation is <em style="strong">uni-directional</em>: it calls
          <code>my_func()</code> in <code>thread1</code>, but there’s no way to
          return a value to the main thread. To do that, the same principle needs
          to be used again, invoking a callback function in the main thread. It’s
          a straightforward extension which isn’t covered here.
        </p>

        <p>
          To maintain thread safety, data which is potentially accessed by
          multiple threads must make those accesses mutually exclusive using a
          <link href="http://en.wikipedia.org/wiki/Mutual_exclusion">mutex</link>.
          Data potentially accessed by multiple threads:
          <code>thread1_main_context</code>, passed in the fork call to
          <code>thread1_main</code>; and <code>some_object</code>, a reference to
          which is passed in the data closure. Critically, GLib guarantees that
          <code>GMainContext</code> is thread safe, so sharing
          <code>thread1_main_context</code> between threads is safe. The example
          assumes that other code accessing <code>some_object</code> is thread
          safe.
        </p>

        <p>
          Note that <code>some_string</code> and <code>some_int</code> cannot be
          accessed from both threads, because <em>copies</em> of them are passed
          to <code>thread1</code>, rather than the originals. This is a standard
          technique for making cross-thread calls thread safe without requiring
          locking. It also avoids the problem of synchronizing freeing
          <code>some_string</code>.
        </p>

        <p>
          Similarly, a reference to <code>some_object</code> is transferred to
          <code>thread1</code>, which works around the issue of synchronizing
          destruction of the object (see <link xref="memory-management"/>).
        </p>

        <p>
          <code>g_idle_source_new()</code> is used rather than the simpler
          <code>g_idle_add()</code> so the <code>GMainContext</code> to attach to
          can be specified.
        </p>
      </example>
    </section>

    <section id="g-main-context-invoke-full">
      <title>
        Convenience Method: <code>g_main_context_invoke_full()</code>
      </title>

      <p>
        This is simplified greatly by the convenience method,
        <link href="https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#g-main-context-invoke-full"><code>g_main_context_invoke_full()</code></link>.
        It invokes a callback so that the specified <code>GMainContext</code> is
        owned during the invocation. Owning a main context is almost always
        equivalent to running it, and hence the function is invoked in the
        thread for which the specified context is the thread-default.
      </p>

      <p>
        <code>g_main_context_invoke()</code> can be used instead if the user
        data does not need to be freed by a <code>GDestroyNotify</code> callback
        after the invocation returns.
      </p>

      <example>
        <p>
          Modifying the earlier example, the <code>invoke_my_func()</code>
          function can be replaced by the following:
        </p>

        <code mime="text/x-csrc">
static void
invoke_my_func (GMainContext *thread1_main_context,
                const gchar  *some_string,
                guint         some_int,
                GObject      *some_object)
{
  MyFuncData *data;

  /* Create a data closure to pass all the desired variables
   * between threads. */
  data = g_new0 (MyFuncData, 1);
  data->some_string = g_strdup (some_string);
  data->some_int = some_int;
  data->some_object = g_object_ref (some_object);

  /* Invoke the function. */
  g_main_context_invoke_full (thread1_main_context,
                              G_PRIORITY_DEFAULT, my_func_idle,
                              data,
                              (GDestroyNotify) my_func_data_free);
}</code>

        <p>
          Consider what happens if <code>invoke_my_func()</code> were called
          from <code>thread1</code>, rather than from the main thread. With the
          original implementation, the idle source would be added to
          <code>thread1</code>’s context and dispatched on the context’s next
          iteration (assuming no pending dispatches with higher priorities).
          With the improved implementation,
          <code>g_main_context_invoke_full()</code> will notice that the
          specified context is already owned by the thread (or ownership can be
          acquired by it), and will call <code>my_func_idle()</code> directly,
          rather than attaching a source to the context and delaying the
          invocation to the next context iteration.
        </p>

        <p>
          This subtle behavior difference doesn’t matter in most cases, but is
          worth bearing in mind since it can affect blocking behavior
          (<code>invoke_my_func()</code> would go from taking negligible time,
          to taking the same amount of time as <code>my_func()</code> before
          returning).
        </p>
      </example>
    </section>
  </section>

  <section id="checking-threading">
    <title>Checking Threading</title>

    <p>
      It is useful to document which thread each function should be called in,
      in the form of an assertion:
    </p>
    <code mime="text/x-csrc">
g_assert (g_main_context_is_owner (expected_main_context));</code>

    <p>
      If that’s put at the top of each function, any assertion failure will
      highlight a case where a function has been called from the wrong
      thread. It is much easier to write these assertions when initially
      developing code, rather than debuging race conditions which can easily
      result from a function being called in the wrong thread.
    </p>

    <p>
      This technique can also be applied to signal emissions and callbacks,
      improving type safety as well as asserting the right context is used. Note
      that signal emission via
      <link href="https://developer.gnome.org/gobject/stable/gobject-Signals.html#g-signal-emit"><code>g_signal_emit()</code></link>
      is synchronous, and doesn’t involve a main context at all.
    </p>

    <example>
      <p>
        For example, instead of using the following when emitting a signal:
      </p>
      <code mime="text/x-csrc" style="invalid">
guint param1;  /* arbitrary example parameters */
gchar *param2;
guint retval = 0;

g_signal_emit_by_name (my_object, "some-signal",
                       param1, param2, &amp;retval);</code>

      <p>
        The following can be used:
      </p>
      <code mime="text/x-csrc" style="valid">
static guint
emit_some_signal (GObject     *my_object,
                  guint        param1,
                  const gchar *param2)
{
  guint retval = 0;

  g_assert (g_main_context_is_owner (expected_main_context));

  g_signal_emit_by_name (my_object, "some-signal",
                         param1, param2, &amp;retval);

  return retval;
}</code>
    </example>
  </section>

  <section id="gtask">
    <title><code>GTask</code></title>

    <p>
      <link href="https://developer.gnome.org/gio/stable/GTask.html"><code>GTask</code></link>
      provides a slightly different approach to invoking functions in other
      threads, which is more suited to the case where a function should be
      executed in <em>some</em> background thread, but not a specific one.
    </p>

    <p>
      <code>GTask</code> takes a data closure and a function to execute, and
      provides ways to return the result from this function. It handles
      everything necessary to run that function in an arbitrary thread belonging
      to some thread pool internal to GLib.
    </p>

    <example>
      <p>
        By combining
        <link xref="#g-main-context-invoke-full"><code>g_main_context_invoke_full()</code></link>
        and <code>GTask</code>, it is possible to run a task in a specific context
        and effortlessly return its result to the current context:
      </p>
      <code mime="text/x-csrc">
/* This will be invoked in thread1. */
static gboolean
my_func_idle (gpointer user_data)
{
  GTask *task = G_TASK (user_data);
  MyFuncData *data;
  gboolean retval;

  /* Call my_func() and propagate its returned boolean to
   * the main thread. */
  data = g_task_get_task_data (task);
  retval = my_func (data->some_string, data->some_int,
                    data->some_object);
  g_task_return_boolean (task, retval);

  return G_SOURCE_REMOVE;
}

/* Whichever thread this is invoked in, the @callback will be
 * invoked in, once my_func() has finished and returned a result. */
static void
invoke_my_func_with_result (GMainContext        *thread1_main_context,
                            const gchar         *some_string,
                            guint                some_int,
                            GObject             *some_object,
                            GAsyncReadyCallback  callback,
                            gpointer             user_data)
{
  MyFuncData *data;

  /* Create a data closure to pass all the desired variables
   * between threads. */
  data = g_new0 (MyFuncData, 1);
  data->some_string = g_strdup (some_string);
  data->some_int = some_int;
  data->some_object = g_object_ref (some_object);

  /* Create a GTask to handle returning the result to the current
   * thread-default main context. */
  task = g_task_new (NULL, NULL, callback, user_data);
  g_task_set_task_data (task, data,
                        (GDestroyNotify) my_func_data_free);

  /* Invoke the function. */
  g_main_context_invoke_full (thread1_main_context,
                              G_PRIORITY_DEFAULT, my_func_idle,
                              task,
                              (GDestroyNotify) g_object_unref);
}</code>
    </example>
  </section>
</page>
