Asynchronous operations

The most common usage of GTask is as a GAsyncResult, to manage data during an asynchronous operation. You call g_task_new() in the "start" method, followed by g_task_set_task_data() and the like if you need to keep some additional data associated with the task, and then pass the task object around through your asynchronous operation. Eventually, you will call a method such as g_task_return_pointer() or g_task_return_error(), which will save the value you give it and then invoke the task's callback function (waiting until the next iteration of the main loop first, if necessary). The caller will pass the GTask back to the operation's finish function (as a GAsyncResult), and you can use g_task_propagate_pointer() or the like to extract the return value.

Here is an example for using GTask as a GAsyncResult:

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typedef struct {
  CakeFrostingType frosting;
  char *message;
} DecorationData;

static void
decoration_data_free (DecorationData *decoration)
{
  g_free (decoration->message);
  g_slice_free (DecorationData, decoration);
}

static void
baked_cb (Cake     *cake,
          gpointer  user_data)
{
  GTask *task = user_data;
  DecorationData *decoration = g_task_get_task_data (task);
  GError *error = NULL;

  if (cake == NULL)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_NO_FLOUR,
                               "Go to the supermarket");
      g_object_unref (task);
      return;
    }

  if (!cake_decorate (cake, decoration->frosting, decoration->message, &error))
    {
      g_object_unref (cake);
      // g_task_return_error() takes ownership of error
      g_task_return_error (task, error);
      g_object_unref (task);
      return;
    }

  g_task_return_pointer (task, cake, g_object_unref);
  g_object_unref (task);
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  GTask *task;
  DecorationData *decoration;
  Cake  *cake;

  task = g_task_new (self, cancellable, callback, user_data);
  if (radius < 3)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_TOO_SMALL,
                               "%ucm radius cakes are silly",
                               radius);
      g_object_unref (task);
      return;
    }

  cake = _baker_get_cached_cake (self, radius, flavor, frosting, message);
  if (cake != NULL)
    {
      // _baker_get_cached_cake() returns a reffed cake
      g_task_return_pointer (task, cake, g_object_unref);
      g_object_unref (task);
      return;
    }

  decoration = g_slice_new (DecorationData);
  decoration->frosting = frosting;
  decoration->message = g_strdup (message);
  g_task_set_task_data (task, decoration, (GDestroyNotify) decoration_data_free);

  _baker_begin_cake (self, radius, flavor, cancellable, baked_cb, task);
}

Cake *
baker_bake_cake_finish (Baker         *self,
                        GAsyncResult  *result,
                        GError       **error)
{
  g_return_val_if_fail (g_task_is_valid (result, self), NULL);

  return g_task_propagate_pointer (G_TASK (result), error);
}

Chained asynchronous operations

GTask also tries to simplify asynchronous operations that internally chain together several smaller asynchronous operations. g_task_get_cancellable(), g_task_get_context(), and g_task_get_priority() allow you to get back the task's GCancellable, GMainContext, and I/O priority when starting a new subtask, so you don't have to keep track of them yourself. g_task_attach_source() simplifies the case of waiting for a source to fire (automatically using the correct GMainContext and priority).

Here is an example for chained asynchronous operations:

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typedef struct {
  Cake *cake;
  CakeFrostingType frosting;
  char *message;
} BakingData;

static void
decoration_data_free (BakingData *bd)
{
  if (bd->cake)
    g_object_unref (bd->cake);
  g_free (bd->message);
  g_slice_free (BakingData, bd);
}

static void
decorated_cb (Cake         *cake,
              GAsyncResult *result,
              gpointer      user_data)
{
  GTask *task = user_data;
  GError *error = NULL;

  if (!cake_decorate_finish (cake, result, &error))
    {
      g_object_unref (cake);
      g_task_return_error (task, error);
      g_object_unref (task);
      return;
    }

  // baking_data_free() will drop its ref on the cake, so we have to
  // take another here to give to the caller.
  g_task_return_pointer (result, g_object_ref (cake), g_object_unref);
  g_object_unref (task);
}

static void
decorator_ready (gpointer user_data)
{
  GTask *task = user_data;
  BakingData *bd = g_task_get_task_data (task);

  cake_decorate_async (bd->cake, bd->frosting, bd->message,
                       g_task_get_cancellable (task),
                       decorated_cb, task);
}

static void
baked_cb (Cake     *cake,
          gpointer  user_data)
{
  GTask *task = user_data;
  BakingData *bd = g_task_get_task_data (task);
  GError *error = NULL;

  if (cake == NULL)
    {
      g_task_return_new_error (task, BAKER_ERROR, BAKER_ERROR_NO_FLOUR,
                               "Go to the supermarket");
      g_object_unref (task);
      return;
    }

  bd->cake = cake;

  // Bail out now if the user has already cancelled
  if (g_task_return_error_if_cancelled (task))
    {
      g_object_unref (task);
      return;
    }

  if (cake_decorator_available (cake))
    decorator_ready (task);
  else
    {
      GSource *source;

      source = cake_decorator_wait_source_new (cake);
      // Attach @source to @task's GMainContext and have it call
      // decorator_ready() when it is ready.
      g_task_attach_source (task, source,
                            G_CALLBACK (decorator_ready));
      g_source_unref (source);
    }
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       gint                 priority,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  GTask *task;
  BakingData *bd;

  task = g_task_new (self, cancellable, callback, user_data);
  g_task_set_priority (task, priority);

  bd = g_slice_new0 (BakingData);
  bd->frosting = frosting;
  bd->message = g_strdup (message);
  g_task_set_task_data (task, bd, (GDestroyNotify) baking_data_free);

  _baker_begin_cake (self, radius, flavor, cancellable, baked_cb, task);
}

Cake *
baker_bake_cake_finish (Baker         *self,
                        GAsyncResult  *result,
                        GError       **error)
{
  g_return_val_if_fail (g_task_is_valid (result, self), NULL);

  return g_task_propagate_pointer (G_TASK (result), error);
}

Asynchronous operations from synchronous ones

You can use g_task_run_in_thread() to turn a synchronous operation into an asynchronous one, by running it in a thread which will then dispatch the result back to the caller's GMainContext when it completes.

Running a task in a thread:

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typedef struct {
  guint radius;
  CakeFlavor flavor;
  CakeFrostingType frosting;
  char *message;
} CakeData;

static void
cake_data_free (CakeData *cake_data)
{
  g_free (cake_data->message);
  g_slice_free (CakeData, cake_data);
}

static void
bake_cake_thread (GTask         *task,
                  gpointer       source_object,
                  gpointer       task_data,
                  GCancellable  *cancellable)
{
  Baker *self = source_object;
  CakeData *cake_data = task_data;
  Cake *cake;
  GError *error = NULL;

  cake = bake_cake (baker, cake_data->radius, cake_data->flavor,
                    cake_data->frosting, cake_data->message,
                    cancellable, &error);
  if (cake)
    g_task_return_pointer (task, cake, g_object_unref);
  else
    g_task_return_error (task, error);
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  CakeData *cake_data;
  GTask *task;

  cake_data = g_slice_new (CakeData);
  cake_data->radius = radius;
  cake_data->flavor = flavor;
  cake_data->frosting = frosting;
  cake_data->message = g_strdup (message);
  task = g_task_new (self, cancellable, callback, user_data);
  g_task_set_task_data (task, cake_data, (GDestroyNotify) cake_data_free);
  g_task_run_in_thread (task, bake_cake_thread);
  g_object_unref (task);
}

Cake *
baker_bake_cake_finish (Baker         *self,
                        GAsyncResult  *result,
                        GError       **error)
{
  g_return_val_if_fail (g_task_is_valid (result, self), NULL);

  return g_task_propagate_pointer (G_TASK (result), error);
}

Adding cancellability to uncancellable tasks

Finally, g_task_run_in_thread() and g_task_run_in_thread_sync() can be used to turn an uncancellable operation into a cancellable one. If you call g_task_set_return_on_cancel(), passing TRUE, then if the task's GCancellable is cancelled, it will return control back to the caller immediately, while allowing the task thread to continue running in the background (and simply discarding its result when it finally does finish). Provided that the task thread is careful about how it uses locks and other externally-visible resources, this allows you to make "GLib-friendly" asynchronous and cancellable synchronous variants of blocking APIs.

Cancelling a task:

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static void
bake_cake_thread (GTask         *task,
                  gpointer       source_object,
                  gpointer       task_data,
                  GCancellable  *cancellable)
{
  Baker *self = source_object;
  CakeData *cake_data = task_data;
  Cake *cake;
  GError *error = NULL;

  cake = bake_cake (baker, cake_data->radius, cake_data->flavor,
                    cake_data->frosting, cake_data->message,
                    &error);
  if (error)
    {
      g_task_return_error (task, error);
      return;
    }

  // If the task has already been cancelled, then we don't want to add
  // the cake to the cake cache. Likewise, we don't  want to have the
  // task get cancelled in the middle of updating the cache.
  // g_task_set_return_on_cancel() will return %TRUE here if it managed
  // to disable return-on-cancel, or %FALSE if the task was cancelled
  // before it could.
  if (g_task_set_return_on_cancel (task, FALSE))
    {
      // If the caller cancels at this point, their
      // GAsyncReadyCallback won't be invoked until we return,
      // so we don't have to worry that this code will run at
      // the same time as that code does. But if there were
      // other functions that might look at the cake cache,
      // then we'd probably need a GMutex here as well.
      baker_add_cake_to_cache (baker, cake);
      g_task_return_pointer (task, cake, g_object_unref);
    }
}

void
baker_bake_cake_async (Baker               *self,
                       guint                radius,
                       CakeFlavor           flavor,
                       CakeFrostingType     frosting,
                       const char          *message,
                       GCancellable        *cancellable,
                       GAsyncReadyCallback  callback,
                       gpointer             user_data)
{
  CakeData *cake_data;
  GTask *task;

  cake_data = g_slice_new (CakeData);

  ...

  task = g_task_new (self, cancellable, callback, user_data);
  g_task_set_task_data (task, cake_data, (GDestroyNotify) cake_data_free);
  g_task_set_return_on_cancel (task, TRUE);
  g_task_run_in_thread (task, bake_cake_thread);
}

Cake *
baker_bake_cake_sync (Baker               *self,
                      guint                radius,
                      CakeFlavor           flavor,
                      CakeFrostingType     frosting,
                      const char          *message,
                      GCancellable        *cancellable,
                      GError             **error)
{
  CakeData *cake_data;
  GTask *task;
  Cake *cake;

  cake_data = g_slice_new (CakeData);

  ...

  task = g_task_new (self, cancellable, NULL, NULL);
  g_task_set_task_data (task, cake_data, (GDestroyNotify) cake_data_free);
  g_task_set_return_on_cancel (task, TRUE);
  g_task_run_in_thread_sync (task, bake_cake_thread);

  cake = g_task_propagate_pointer (task, error);
  g_object_unref (task);
  return cake;
}

Porting from GSimpleAsyncResult

GTask's API attempts to be simpler than GSimpleAsyncResult's in several ways:

g_task_new ()

GTask *
g_task_new (gpointer source_object,
            GCancellable *cancellable,
            GAsyncReadyCallback callback,
            gpointer callback_data);

Creates a GTask acting on source_object , which will eventually be used to invoke callback in the current thread-default main context.

Call this in the "start" method of your asynchronous method, and pass the GTask around throughout the asynchronous operation. You can use g_task_set_task_data() to attach task-specific data to the object, which you can retrieve later via g_task_get_task_data().

By default, if cancellable is cancelled, then the return value of the task will always be G_IO_ERROR_CANCELLED, even if the task had already completed before the cancellation. This allows for simplified handling in cases where cancellation may imply that other objects that the task depends on have been destroyed. If you do not want this behavior, you can use g_task_set_check_cancellable() to change it.

Since 2.36

g_task_set_task_data ()

void
g_task_set_task_data (GTask *task,
                      gpointer task_data,
                      GDestroyNotify task_data_destroy);

Sets task 's task data (freeing the existing task data, if any).

Since 2.36

g_task_set_priority ()

void
g_task_set_priority (GTask *task,
                     gint priority);

Sets task 's priority. If you do not call this, it will default to G_PRIORITY_DEFAULT.

This will affect the priority of GSources created with g_task_attach_source() and the scheduling of tasks run in threads, and can also be explicitly retrieved later via g_task_get_priority().

Since 2.36

g_task_set_check_cancellable ()

void
g_task_set_check_cancellable (GTask *task,
                              gboolean check_cancellable);

Sets or clears task 's check-cancellable flag. If this is TRUE (the default), then g_task_propagate_pointer(), etc, and g_task_had_error() will check the task's GCancellable first, and if it has been cancelled, then they will consider the task to have returned an "Operation was cancelled" error (G_IO_ERROR_CANCELLED), regardless of any other error or return value the task may have had.

If check_cancellable is FALSE, then the GTask will not check the cancellable itself, and it is up to task 's owner to do this (eg, via g_task_return_error_if_cancelled()).

If you are using g_task_set_return_on_cancel() as well, then you must leave check-cancellable set TRUE.

Since 2.36

g_task_set_return_on_cancel ()

gboolean
g_task_set_return_on_cancel (GTask *task,
                             gboolean return_on_cancel);

Sets or clears task 's return-on-cancel flag. This is only meaningful for tasks run via g_task_run_in_thread() or g_task_run_in_thread_sync().

If return_on_cancel is TRUE, then cancelling task 's GCancellable will immediately cause it to return, as though the task's GTaskThreadFunc had called g_task_return_error_if_cancelled() and then returned.

This allows you to create a cancellable wrapper around an uninterruptable function. The GTaskThreadFunc just needs to be careful that it does not modify any externally-visible state after it has been cancelled. To do that, the thread should call g_task_set_return_on_cancel() again to (atomically) set return-on-cancel FALSE before making externally-visible changes; if the task gets cancelled before the return-on-cancel flag could be changed, g_task_set_return_on_cancel() will indicate this by returning FALSE.

You can disable and re-enable this flag multiple times if you wish. If the task's GCancellable is cancelled while return-on-cancel is FALSE, then calling g_task_set_return_on_cancel() to set it TRUE again will cause the task to be cancelled at that point.

If the task's GCancellable is already cancelled before you call g_task_run_in_thread()/g_task_run_in_thread_sync(), then the GTaskThreadFunc will still be run (for consistency), but the task will also be completed right away.

Since 2.36

g_task_set_source_tag ()

void
g_task_set_source_tag (GTask *task,
                       gpointer source_tag);

Sets task 's source tag. You can use this to tag a task return value with a particular pointer (usually a pointer to the function doing the tagging) and then later check it using g_task_get_source_tag() (or g_async_result_is_tagged()) in the task's "finish" function, to figure out if the response came from a particular place.

Since 2.36

g_task_report_error ()

void
g_task_report_error (gpointer source_object,
                     GAsyncReadyCallback callback,
                     gpointer callback_data,
                     gpointer source_tag,
                     GError *error);

Creates a GTask and then immediately calls g_task_return_error() on it. Use this in the wrapper function of an asynchronous method when you want to avoid even calling the virtual method. You can then use g_async_result_is_tagged() in the finish method wrapper to check if the result there is tagged as having been created by the wrapper method, and deal with it appropriately if so.

See also g_task_report_new_error().

Since 2.36

g_task_report_new_error ()

void
g_task_report_new_error (gpointer source_object,
                         GAsyncReadyCallback callback,
                         gpointer callback_data,
                         gpointer source_tag,
                         GQuark domain,
                         gint code,
                         const char *format,
                         ...);

Creates a GTask and then immediately calls g_task_return_new_error() on it. Use this in the wrapper function of an asynchronous method when you want to avoid even calling the virtual method. You can then use g_async_result_is_tagged() in the finish method wrapper to check if the result there is tagged as having been created by the wrapper method, and deal with it appropriately if so.

See also g_task_report_error().

Since 2.36

g_task_get_task_data ()

gpointer
g_task_get_task_data (GTask *task);

Gets task 's task_data.

Since 2.36

g_task_get_priority ()

gint
g_task_get_priority (GTask *task);

Gets task 's priority

Since 2.36

g_task_get_cancellable ()

GCancellable *
g_task_get_cancellable (GTask *task);

Gets task 's GCancellable

Since 2.36

g_task_get_check_cancellable ()

gboolean
g_task_get_check_cancellable (GTask *task);

Gets task 's check-cancellable flag. See g_task_set_check_cancellable() for more details.

Since 2.36

g_task_get_return_on_cancel ()

gboolean
g_task_get_return_on_cancel (GTask *task);

Gets task 's return-on-cancel flag. See g_task_set_return_on_cancel() for more details.

Since 2.36

g_task_get_context ()

GMainContext *
g_task_get_context (GTask *task);

Gets the GMainContext that task will return its result in (that is, the context that was the thread-default main context at the point when task was created).

This will always return a non-NULL value, even if the task's context is the default GMainContext.

Since 2.36

g_task_get_source_object ()

gpointer
g_task_get_source_object (GTask *task);

Gets the source object from task . Like g_async_result_get_source_object(), but does not ref the object.

Since 2.36

g_task_get_source_tag ()

gpointer
g_task_get_source_tag (GTask *task);

Gets task 's source tag. See g_task_set_source_tag().

Since 2.36

g_task_return_boolean ()

void
g_task_return_boolean (GTask *task,
                       gboolean result);

Sets task 's result to result and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

Since 2.36

g_task_return_int ()

void
g_task_return_int (GTask *task,
                   gssize result);

Sets task 's result to result and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

Since 2.36

g_task_return_pointer ()

void
g_task_return_pointer (GTask *task,
                       gpointer result,
                       GDestroyNotify result_destroy);

Sets task 's result to result and completes the task. If result is not NULL, then result_destroy will be used to free result if the caller does not take ownership of it with g_task_propagate_pointer().

"Completes the task" means that for an ordinary asynchronous task it will either invoke the task's callback, or else queue that callback to be invoked in the proper GMainContext, or in the next iteration of the current GMainContext. For a task run via g_task_run_in_thread() or g_task_run_in_thread_sync(), calling this method will save result to be returned to the caller later, but the task will not actually be completed until the GTaskThreadFunc exits.

Note that since the task may be completed before returning from g_task_return_pointer(), you cannot assume that result is still valid after calling this, unless you are still holding another reference on it.

Since 2.36

g_task_return_error ()

void
g_task_return_error (GTask *task,
                     GError *error);

Sets task 's result to error (which task assumes ownership of) and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

Note that since the task takes ownership of error , and since the task may be completed before returning from g_task_return_error(), you cannot assume that error is still valid after calling this. Call g_error_copy() on the error if you need to keep a local copy as well.

See also g_task_return_new_error().

Since 2.36

g_task_return_new_error ()

void
g_task_return_new_error (GTask *task,
                         GQuark domain,
                         gint code,
                         const char *format,
                         ...);

Sets task 's result to a new GError created from domain , code , format , and the remaining arguments, and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

See also g_task_return_error().

Since 2.36

g_task_return_error_if_cancelled ()

gboolean
g_task_return_error_if_cancelled (GTask *task);

Checks if task 's GCancellable has been cancelled, and if so, sets task 's error accordingly and completes the task (see g_task_return_pointer() for more discussion of exactly what this means).

Since 2.36

g_task_propagate_boolean ()

gboolean
g_task_propagate_boolean (GTask *task,
                          GError **error);

Gets the result of task as a gboolean.

If the task resulted in an error, or was cancelled, then this will instead return FALSE and set error .

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

Since 2.36

g_task_propagate_int ()

gssize
g_task_propagate_int (GTask *task,
                      GError **error);

Gets the result of task as an integer (gssize).

If the task resulted in an error, or was cancelled, then this will instead return -1 and set error .

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

Since 2.36

g_task_propagate_pointer ()

gpointer
g_task_propagate_pointer (GTask *task,
                          GError **error);

Gets the result of task as a pointer, and transfers ownership of that value to the caller.

If the task resulted in an error, or was cancelled, then this will instead return NULL and set error .

Since this method transfers ownership of the return value (or error) to the caller, you may only call it once.

Since 2.36

g_task_had_error ()

gboolean
g_task_had_error (GTask *task);

Tests if task resulted in an error.

Since 2.36

g_task_get_completed ()

gboolean
g_task_get_completed (GTask *task);

Gets the value of “completed”. This changes from FALSE to TRUE after the task’s callback is invoked, and will return FALSE if called from inside the callback.

Since 2.44

g_task_run_in_thread ()

void
g_task_run_in_thread (GTask *task,
                      GTaskThreadFunc task_func);

Runs task_func in another thread. When task_func returns, task 's GAsyncReadyCallback will be invoked in task 's GMainContext.

This takes a ref on task until the task completes.

See GTaskThreadFunc for more details about how task_func is handled.

Since 2.36

g_task_run_in_thread_sync ()

void
g_task_run_in_thread_sync (GTask *task,
                           GTaskThreadFunc task_func);

Runs task_func in another thread, and waits for it to return or be cancelled. You can use g_task_propagate_pointer(), etc, afterward to get the result of task_func .

See GTaskThreadFunc for more details about how task_func is handled.

Normally this is used with tasks created with a NULL callback, but note that even if the task does have a callback, it will not be invoked when task_func returns. “completed” will be set to TRUE just before this function returns.

Since 2.36

GTaskThreadFunc ()

void
(*GTaskThreadFunc) (GTask *task,
                    gpointer source_object,
                    gpointer task_data,
                    GCancellable *cancellable);

The prototype for a task function to be run in a thread via g_task_run_in_thread() or g_task_run_in_thread_sync().

If the return-on-cancel flag is set on task , and cancellable gets cancelled, then the GTask will be completed immediately (as though g_task_return_error_if_cancelled() had been called), without waiting for the task function to complete. However, the task function will continue running in its thread in the background. The function therefore needs to be careful about how it uses externally-visible state in this case. See g_task_set_return_on_cancel() for more details.

Other than in that case, task will be completed when the GTaskThreadFunc returns, not when it calls a g_task_return_ function.

Since 2.36

g_task_attach_source ()

void
g_task_attach_source (GTask *task,
                      GSource *source,
                      GSourceFunc callback);

A utility function for dealing with async operations where you need to wait for a GSource to trigger. Attaches source to task 's GMainContext with task 's priority, and sets source 's callback to callback , with task as the callback's user_data.

This takes a reference on task until source is destroyed.

Since 2.36

g_task_is_valid ()

gboolean
g_task_is_valid (gpointer result,
                 gpointer source_object);

Checks that result is a GTask, and that source_object is its source object (or that source_object is NULL and result has no source object). This can be used in g_return_if_fail() checks.

Since 2.36

GTask

typedef struct _GTask GTask;

The opaque object representing a synchronous or asynchronous task and its result.

The “completed” property

  “completed”                gboolean

Whether the task has completed, meaning its callback (if set) has been invoked. This can only happen after g_task_return_pointer(), g_task_return_error() or one of the other return functions have been called on the task.

This property is guaranteed to change from FALSE to TRUE exactly once.

The “notify” signal for this change is emitted in the same main context as the task’s callback, immediately after that callback is invoked.

Flags: Read

Default value: FALSE

Since 2.44