GTask: GIO Reference Manual

## GTask GTask — Cancellable synchronous or asynchronous task and result |

## Functions

GTask * | g_task_new ()
void | g_task_set_task_data ()
void | g_task_set_priority ()
void | g_task_set_check_cancellable ()
gboolean | g_task_set_return_on_cancel ()
void | g_task_set_source_tag ()
void | g_task_set_name ()
void | g_task_set_static_name ()
void | g_task_report_error ()
void | g_task_report_new_error ()
gpointer | g_task_get_task_data ()
gint | g_task_get_priority ()
GCancellable * | g_task_get_cancellable ()
gboolean | g_task_get_check_cancellable ()
gboolean | g_task_get_return_on_cancel ()
GMainContext * | g_task_get_context ()
gpointer | g_task_get_source_object ()
gpointer | g_task_get_source_tag ()
const gchar * | g_task_get_name ()
void | g_task_return_boolean ()
void | g_task_return_int ()
void | g_task_return_pointer ()
void | g_task_return_value ()
void | g_task_return_error ()
void | g_task_return_new_error ()
gboolean | g_task_return_error_if_cancelled ()
gboolean | g_task_propagate_boolean ()
gssize | g_task_propagate_int ()
gpointer | g_task_propagate_pointer ()
gboolean | g_task_propagate_value ()
gboolean | g_task_had_error ()
gboolean | g_task_get_completed ()
void | g_task_run_in_thread ()
void | g_task_run_in_thread_sync ()
void | (*GTaskThreadFunc) ()
void | g_task_attach_source ()
gboolean | g_task_is_valid ()

## Properties

gboolean | completed | Read

## Types and Values

| GTask

## Object Hierarchy

        GObject
        ╰── GTask

## Implemented Interfaces

GTask implements GAsyncResult.

## Includes

    #include <gio/gio.h>

## Description

A GTask represents and manages a cancellable "task".

#### 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 in the thread-default main context where it was created (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.

Using GTask requires the thread-default GMainContext from when the GTask was constructed to be running at least until the task has completed and its data has been freed.

If a GTask has been constructed and its callback set, it is an error to not call `g_task_return_*()` on it. GLib will warn at runtime if this happens (since 2.76).

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

    static gboolean
    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);

      return G_SOURCE_REMOVE;
    }

    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, 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. When it completes, the result will be dispatched to the thread-default main context where the GTask was created.

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:

- You can save task-specific data with `g_task_set_task_data()`, and retrieve it later with `g_task_get_task_data()`. This replaces the abuse of `g_simple_async_result_set_op_res_gpointer()` for the same purpose with GSimpleAsyncResult.

- In addition to the task data, GTask also keeps track of the priority, GCancellable, and GMainContext associated with the task, so tasks that consist of a chain of simpler asynchronous operations will have easy access to those values when starting each sub-task.

- g_task_return_error_if_cancelled() provides simplified handling for cancellation. In addition, cancellation overrides any other GTask return value by default, like GSimpleAsyncResult does when `g_simple_async_result_set_check_cancellable()` is called. (You can use `g_task_set_check_cancellable()` to turn off that behavior.) On the other hand, `g_task_run_in_thread()` guarantees that it will always run your `task_func`, even if the task's GCancellable is already cancelled before the task gets a chance to run; you can start your `task_func` with a `g_task_return_error_if_cancelled()` check if you need the old behavior.

- The "return" methods (eg, `g_task_return_pointer()`) automatically cause the task to be "completed" as well, and there is no need to worry about the "complete" vs "complete in idle" distinction. (GTask automatically figures out whether the task's callback can be invoked directly, or if it needs to be sent to another GMainContext, or delayed until the next iteration of the current GMainContext.)

- The "finish" functions for GTask based operations are generally much simpler than GSimpleAsyncResult ones, normally consisting of only a single call to `g_task_propagate_pointer()` or the like. Since `g_task_propagate_pointer()` "steals" the return value from the GTask, it is not necessary to juggle pointers around to prevent it from being freed twice.

- With GSimpleAsyncResult, it was common to call `g_simple_async_result_propagate_error()` from the ``_finish()`` wrapper function, and have virtual method implementations only deal with successful returns. This behavior is deprecated, because it makes it difficult for a subclass to chain to a parent class's async methods. Instead, the wrapper function should just be a simple wrapper, and the virtual method should call an appropriate `g_task_propagate_` function. Note that wrapper methods can now use `g_async_result_legacy_propagate_error()` to do old-style GSimpleAsyncResult error-returning behavior, and `g_async_result_is_tagged()` to check if a result is tagged as having come from the ``_async()`` wrapper function (for "short-circuit" results, such as when passing 0 to `g_input_stream_read_async()`).

#### Thread-safety considerations

Due to some infelicities in the API design, there is a thread-safety concern that users of GTask have to be aware of:

If the `main` thread drops its last reference to the source object or the task data before the task is finalized, then the finalizers of these objects may be called on the worker thread.

This is a problem if the finalizers use non-threadsafe API, and can lead to hard-to-debug crashes. Possible workarounds include:

- Clear task data in a signal handler for `notify::completed`

- Keep iterating a main context in the main thread and defer dropping the reference to the source object to that main context when the task is finalized

## Functions

### 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.

#### Parameters

source_object | the GObject that owns this task, or `NULL`. | [nullable][type GObject]
cancellable | optional GCancellable object, `NULL` to ignore. | [nullable]
callback | a GAsyncReadyCallback. | [scope async]
callback_data | user data passed to `callback` . |

#### Returns

a GTask.

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).

#### Parameters

task | the GTask |
task_data | task-specific data. | [nullable]
task_data_destroy | GDestroyNotify for `task_data` . | [nullable]

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()`.

#### Parameters

task | the GTask |
priority | the priority of the request |

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`.

#### Parameters

task | the GTask |
check_cancellable | whether GTask will check the state of its GCancellable for you. |

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 uninterruptible 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.

#### Parameters

task | the GTask |
return_on_cancel | whether the task returns automatically when it is cancelled. |

#### Returns

`TRUE` if `task` 's return-on-cancel flag was changed to match `return_on_cancel` . `FALSE` if `task` has already been cancelled.

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.

A macro wrapper around this function will automatically set the task’s name to the string form of `source_tag` if it’s not already set, for convenience.

#### Parameters

task | the GTask |
source_tag | an opaque pointer indicating the source of this task |

Since: 2.36

### g_task_set_name ()

    void
    g_task_set_name (GTask *task,
                     const gchar *name);

Sets `task` ’s name, used in debugging and profiling. The name defaults to `NULL`.

The task name should describe in a human readable way what the task does. For example, ‘Open file’ or ‘Connect to network host’. It is used to set the name of the GSource used for idle completion of the task.

This function may only be called before the `task` is first used in a thread other than the one it was constructed in. It is called automatically by `g_task_set_source_tag()` if not called already.

#### Parameters

task | a GTask |
name | a human readable name for the task, or `NULL` to unset it. | [nullable]

Since: 2.60

### g_task_set_static_name ()

    void
    g_task_set_static_name (GTask *task,
                            const gchar *name);

Sets `task` ’s name, used in debugging and profiling.

This is a variant of `g_task_set_name()` that avoids copying `name` .

#### Parameters

task | a GTask |
name | a human readable name for the task. Must be a string literal. | [nullable]

Since: 2.76

### 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()`.

#### Parameters

source_object | the GObject that owns this task, or `NULL`. | [nullable][type GObject]
callback | a GAsyncReadyCallback. | [scope async][closure callback_data]
callback_data | user data passed to `callback` . |
source_tag | an opaque pointer indicating the source of this task |
error | error to report. | [transfer full]

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()`.

#### Parameters

source_object | the GObject that owns this task, or `NULL`. | [nullable][type GObject]
callback | a GAsyncReadyCallback. | [scope async][closure callback_data]
callback_data | user data passed to `callback` . |
source_tag | an opaque pointer indicating the source of this task |
domain | a GQuark. |
code | an error code. |
format | a string with format characters. |
... | a list of values to insert into `format` . |

Since: 2.36

### g_task_get_task_data ()

    gpointer
    g_task_get_task_data (GTask *task);

Gets `task` 's `task_data`.

#### Parameters

task | a GTask |

#### Returns

`task` 's `task_data`.

[transfer none]

Since: 2.36

### g_task_get_priority ()

    gint
    g_task_get_priority (GTask *task);

Gets `task` 's priority

#### Parameters

task | a GTask |

#### Returns

`task` 's priority

Since: 2.36

### g_task_get_cancellable ()

    GCancellable *
    g_task_get_cancellable (GTask *task);

Gets `task` 's GCancellable

#### Parameters

task | a GTask |

#### Returns

`task` 's GCancellable.

[nullable][transfer none]

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.

#### Parameters

task | the GTask |

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.

#### Parameters

task | the GTask |

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.

#### Parameters

task | a GTask |

#### Returns

`task` 's GMainContext.

[transfer none]

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.

#### Parameters

task | a GTask |

#### Returns

`task` 's source object, or `NULL`.

[transfer none][nullable][type GObject]

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()`.

#### Parameters

task | a GTask |

#### Returns

`task` 's source tag.

[transfer none]

Since: 2.36

### g_task_get_name ()

    const gchar *
    g_task_get_name (GTask *task);

Gets `task` ’s name. See `g_task_set_name()`.

#### Parameters

task | a GTask |

#### Returns

`task` ’s name, or `NULL`.

[nullable][transfer none]

Since: 2.60

### 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).

#### Parameters

task | a GTask. |
result | the gboolean result of a task function. |

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).

#### Parameters

task | a GTask. |
result | the integer (gssize) result of a task function. |

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.

#### Parameters

task | a GTask |
result | the pointer result of a task function. | [nullable][transfer full]
result_destroy | a GDestroyNotify function. | [nullable]

Since: 2.36

### g_task_return_value ()

    void
    g_task_return_value (GTask *task,
                         GValue *result);

Sets `task` 's result to `result` (by copying it) and completes the task.

If `result` is `NULL` then a GValue of type `G_TYPE_POINTER` with a value of `NULL` will be used for the result.

This is a very generic low-level method intended primarily for use by language bindings; for C code, `g_task_return_pointer()` and the like will normally be much easier to use.

#### Parameters

task | a GTask |
result | the GValue result of a task function. | [nullable][transfer none]

Since: 2.64

### 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()`.

#### Parameters

task | a GTask. |
error | the GError result of a task function. | [transfer full]

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()`.

#### Parameters

task | a GTask. |
domain | a GQuark. |
code | an error code. |
format | a string with format characters. |
... | a list of values to insert into `format` . |

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).

#### Parameters

task | a GTask |

#### Returns

`TRUE` if `task` has been cancelled, `FALSE` if not

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.

#### Parameters

task | a GTask. |
error | return location for a GError |

#### Returns

the task result, or `FALSE` on error

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.

#### Parameters

task | a GTask. |
error | return location for a GError |

#### Returns

the task result, or -1 on error

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.

#### Parameters

task | a GTask |
error | return location for a GError |

#### Returns

the task result, or `NULL` on error.

[transfer full]

Since: 2.36

### g_task_propagate_value ()

    gboolean
    g_task_propagate_value (GTask *task,
                            GValue *value,
                            GError **error);

Gets the result of `task` as a GValue, and transfers ownership of that value to the caller. As with `g_task_return_value()`, this is a generic low-level method; `g_task_propagate_pointer()` and the like will usually be more useful for C code.

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

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

#### Parameters

task | a GTask |
value | return location for the GValue. | [out caller-allocates]
error | return location for a GError |

#### Returns

`TRUE` if `task` succeeded, `FALSE` on error.

Since: 2.64

### g_task_had_error ()

    gboolean
    g_task_had_error (GTask *task);

Tests if `task` resulted in an error.

#### Parameters

task | a GTask. |

#### Returns

`TRUE` if the task resulted in an error, `FALSE` otherwise.

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.

#### Parameters

task | a GTask. |

#### Returns

`TRUE` if the task has completed, `FALSE` otherwise.

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.

Although GLib currently rate-limits the tasks queued via `g_task_run_in_thread()`, you should not assume that it will always do this. If you have a very large number of tasks to run (several tens of tasks), but don't want them to all run at once, you should only queue a limited number of them (around ten) at a time.

Be aware that if your task depends on other tasks to complete, use of this function could lead to a livelock if the other tasks also use this function and enough of them (around 10) execute in a dependency chain, as that will exhaust the thread pool. If this situation is possible, consider using a separate worker thread or thread pool explicitly, rather than using `g_task_run_in_thread()`.

#### Parameters

task | a GTask |
task_func | a GTaskThreadFunc. | [scope async]

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.

Although GLib currently rate-limits the tasks queued via `g_task_run_in_thread_sync()`, you should not assume that it will always do this. If you have a very large number of tasks to run, but don't want them to all run at once, you should only queue a limited number of them at a time.

#### Parameters

task | a GTask |
task_func | a GTaskThreadFunc. | [scope async]

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.

#### Parameters

task | the GTask |
source_object | `task` 's source object. | [type GObject]
task_data | `task` 's task data |
cancellable | `task` 's GCancellable, or `NULL` |

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`.

It will set the `source` ’s name to the task’s name (as set with `g_task_set_name()`), if one has been set on the task and the source doesn’t yet have a name.

This takes a reference on `task` until `source` is destroyed.

#### Parameters

task | a GTask |
source | the source to attach |
callback | the callback to invoke when `source` triggers |

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.

#### Parameters

result | A GAsyncResult. | [type Gio.AsyncResult]
source_object | the source object expected to be associated with the task. | [nullable][type GObject]

#### Returns

`TRUE` if `result` and `source_object` are valid, `FALSE` if not

Since: 2.36

## Types and Values

### GTask

    typedef struct _GTask GTask;

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

## Property Details

### 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.

Owner: GTask

Flags: Read

Default value: FALSE

Since: 2.44

## See Also

GAsyncResult