GstBus: GStreamer 1.0 Core Reference Manual

## GstBus GstBus — Asynchronous message bus subsystem |

## Functions

gboolean | (*GstBusFunc) ()
GstBusSyncReply | (*GstBusSyncHandler) ()
GstBus * | gst_bus_new ()
gboolean | gst_bus_post ()
gboolean | gst_bus_have_pending ()
GstMessage * | gst_bus_peek ()
GstMessage * | gst_bus_pop ()
GstMessage * | gst_bus_pop_filtered ()
GstMessage * | gst_bus_timed_pop ()
GstMessage * | gst_bus_timed_pop_filtered ()
void | gst_bus_set_flushing ()
void | gst_bus_set_sync_handler ()
GstBusSyncReply | gst_bus_sync_signal_handler ()
GSource * | gst_bus_create_watch ()
guint | gst_bus_add_watch_full ()
guint | gst_bus_add_watch ()
gboolean | gst_bus_remove_watch ()
void | gst_bus_disable_sync_message_emission ()
void | gst_bus_enable_sync_message_emission ()
gboolean | gst_bus_async_signal_func ()
void | gst_bus_add_signal_watch ()
void | gst_bus_add_signal_watch_full ()
void | gst_bus_remove_signal_watch ()
GstMessage * | gst_bus_poll ()

## Properties

gboolean | enable-async | Write / Construct Only

## Signals

void | message | Has Details
void | sync-message | Has Details

## Types and Values

struct | GstBus
enum | GstBusFlags
enum | GstBusSyncReply

## Object Hierarchy

        GObject
        ╰── GInitiallyUnowned
            ╰── GstObject
                ╰── GstBus

## Includes

    #include <gst/gst.h>

## Description

The GstBus is an object responsible for delivering GstMessage packets in a first-in first-out way from the streaming threads (see GstTask) to the application.

Since the application typically only wants to deal with delivery of these messages from one thread, the GstBus will marshall the messages between different threads. This is important since the actual streaming of media is done in another thread than the application.

The GstBus provides support for GSource based notifications. This makes it possible to handle the delivery in the glib mainloop.

The GSource callback function `gst_bus_async_signal_func()` can be used to convert all bus messages into signal emissions.

A message is posted on the bus with the `gst_bus_post()` method. With the `gst_bus_peek()` and `gst_bus_pop()` methods one can look at or retrieve a previously posted message.

The bus can be polled with the `gst_bus_poll()` method. This methods blocks up to the specified timeout value until one of the specified messages types is posted on the bus. The application can then `gst_bus_pop()` the messages from the bus to handle them. Alternatively the application can register an asynchronous bus function using `gst_bus_add_watch_full()` or `gst_bus_add_watch()`. This function will install a GSource in the default glib main loop and will deliver messages a short while after they have been posted. Note that the main loop should be running for the asynchronous callbacks.

It is also possible to get messages from the bus without any thread marshalling with the `gst_bus_set_sync_handler()` method. This makes it possible to react to a message in the same thread that posted the message on the bus. This should only be used if the application is able to deal with messages from different threads.

Every GstPipeline has one bus.

Note that a GstPipeline will set its bus into flushing state when changing from READY to NULL state.

## Functions

### GstBusFunc ()

    gboolean
    (*GstBusFunc) (GstBus *bus,
                   GstMessage *message,
                   gpointer user_data);

Specifies the type of function passed to `gst_bus_add_watch()` or `gst_bus_add_watch_full()`, which is called from the mainloop when a message is available on the bus.

The message passed to the function will be unreffed after execution of this function so it should not be freed in the function.

Note that this function is used as a GSourceFunc which means that returning `FALSE` will remove the GSource from the mainloop.

#### Parameters

bus | the GstBus that sent the message |
message | the GstMessage |
user_data | user data that has been given, when registering the handler |

#### Returns

`FALSE` if the event source should be removed.

### GstBusSyncHandler ()

    GstBusSyncReply
    (*GstBusSyncHandler) (GstBus *bus,
                          GstMessage *message,
                          gpointer user_data);

Handler will be invoked synchronously, when a new message has been injected into the bus. This function is mostly used internally. Only one sync handler can be attached to a given bus.

If the handler returns GST_BUS_DROP, it should unref the message, else the message should not be unreffed by the sync handler.

#### Parameters

bus | the GstBus that sent the message |
message | the GstMessage |
user_data | user data that has been given, when registering the handler |

#### Returns

GstBusSyncReply stating what to do with the message

### gst_bus_new ()

    GstBus *
    gst_bus_new (void);

Creates a new GstBus instance.

#### Returns

a new GstBus instance.

[transfer full]

### gst_bus_post ()

    gboolean
    gst_bus_post (GstBus *bus,
                  GstMessage *message);

Post a message on the given bus. Ownership of the message is taken by the bus.

#### Parameters

bus | a GstBus to post on |
message | the GstMessage to post. | [transfer full]

#### Returns

`TRUE` if the message could be posted, `FALSE` if the bus is flushing.

MT safe.

### gst_bus_have_pending ()

    gboolean
    gst_bus_have_pending (GstBus *bus);

Check if there are pending messages on the bus that should be handled.

#### Parameters

bus | a GstBus to check |

#### Returns

`TRUE` if there are messages on the bus to be handled, `FALSE` otherwise.

MT safe.

### gst_bus_peek ()

    GstMessage *
    gst_bus_peek (GstBus *bus);

Peek the message on the top of the bus' queue. The message will remain on the bus' message queue. A reference is returned, and needs to be unreffed by the caller.

#### Parameters

bus | a GstBus |

#### Returns

the GstMessage that is on the bus, or `NULL` if the bus is empty.

MT safe.

[transfer full][nullable]

### gst_bus_pop ()

    GstMessage *
    gst_bus_pop (GstBus *bus);

Get a message from the bus.

#### Parameters

bus | a GstBus to pop |

#### Returns

the GstMessage that is on the bus, or `NULL` if the bus is empty. The message is taken from the bus and needs to be unreffed with `gst_message_unref()` after usage.

MT safe.

[transfer full][nullable]

### gst_bus_pop_filtered ()

    GstMessage *
    gst_bus_pop_filtered (GstBus *bus,
                          GstMessageType types);

Get a message matching `type` from the bus. Will discard all messages on the bus that do not match `type` and that have been posted before the first message that does match `type` . If there is no message matching `type` on the bus, all messages will be discarded. It is not possible to use message enums beyond GST_MESSAGE_EXTENDED in the `events` mask.

#### Parameters

bus | a GstBus to pop |
types | message types to take into account |

#### Returns

the next GstMessage matching `type` that is on the bus, or `NULL` if the bus is empty or there is no message matching `type` . The message is taken from the bus and needs to be unreffed with `gst_message_unref()` after usage.

MT safe.

[transfer full][nullable]

### gst_bus_timed_pop ()

    GstMessage *
    gst_bus_timed_pop (GstBus *bus,
                       GstClockTime timeout);

Get a message from the bus, waiting up to the specified timeout.

If `timeout` is 0, this function behaves like `gst_bus_pop()`. If `timeout` is GST_CLOCK_TIME_NONE, this function will block forever until a message was posted on the bus.

#### Parameters

bus | a GstBus to pop |
timeout | a timeout |

#### Returns

the GstMessage that is on the bus after the specified timeout or `NULL` if the bus is empty after the timeout expired. The message is taken from the bus and needs to be unreffed with `gst_message_unref()` after usage.

MT safe.

[transfer full][nullable]

### gst_bus_timed_pop_filtered ()

    GstMessage *
    gst_bus_timed_pop_filtered (GstBus *bus,
                                GstClockTime timeout,
                                GstMessageType types);

Get a message from the bus whose type matches the message type mask `types` , waiting up to the specified timeout (and discarding any messages that do not match the mask provided).

If `timeout` is 0, this function behaves like `gst_bus_pop_filtered()`. If `timeout` is GST_CLOCK_TIME_NONE, this function will block forever until a matching message was posted on the bus.

#### Parameters

bus | a GstBus to pop from |
timeout | a timeout in nanoseconds, or GST_CLOCK_TIME_NONE to wait forever |
types | message types to take into account, GST_MESSAGE_ANY for any type |

#### Returns

a GstMessage matching the filter in `types` , or `NULL` if no matching message was found on the bus until the timeout expired. The message is taken from the bus and needs to be unreffed with `gst_message_unref()` after usage.

MT safe.

[transfer full][nullable]

### gst_bus_set_flushing ()

    void
    gst_bus_set_flushing (GstBus *bus,
                          gboolean flushing);

If `flushing` , flush out and unref any messages queued in the bus. Releases references to the message origin objects. Will flush future messages until `gst_bus_set_flushing()` sets `flushing` to `FALSE`.

MT safe.

#### Parameters

bus | a GstBus |
flushing | whether or not to flush the bus |

### gst_bus_set_sync_handler ()

    void
    gst_bus_set_sync_handler (GstBus *bus,
                              GstBusSyncHandler func,
                              gpointer user_data,
                              GDestroyNotify notify);

Sets the synchronous handler on the bus. The function will be called every time a new message is posted on the bus. Note that the function will be called in the same thread context as the posting object. This function is usually only called by the creator of the bus. Applications should handle messages asynchronously using the gst_bus watch and poll functions.

You cannot replace an existing sync_handler. You can pass `NULL` to this function, which will clear the existing handler.

#### Parameters

bus | a GstBus to install the handler on |
func | The handler function to install. | [allow-none]
user_data | User data that will be sent to the handler function. |
notify | called when `user_data` becomes unused |

### gst_bus_sync_signal_handler ()

    GstBusSyncReply
    gst_bus_sync_signal_handler (GstBus *bus,
                                 GstMessage *message,
                                 gpointer data);

A helper GstBusSyncHandler that can be used to convert all synchronous messages into signals.

#### Parameters

bus | a GstBus |
message | the GstMessage received |
data | user data |

#### Returns

GST_BUS_PASS

### gst_bus_create_watch ()

    GSource *
    gst_bus_create_watch (GstBus *bus);

Create watch for this bus. The GSource will be dispatched whenever a message is on the bus. After the GSource is dispatched, the message is popped off the bus and unreffed.

#### Parameters

bus | a GstBus to create the watch for |

#### Returns

a GSource that can be added to a mainloop.

[transfer full]

### gst_bus_add_watch_full ()

    guint
    gst_bus_add_watch_full (GstBus *bus,
                            gint priority,
                            GstBusFunc func,
                            gpointer user_data,
                            GDestroyNotify notify);

Adds a bus watch to the default main context with the given `priority` (e.g. `G_PRIORITY_DEFAULT`). It is also possible to use a non-default main context set up using `g_main_context_push_thread_default()` (before one had to create a bus watch source and attach it to the desired main context 'manually').

This function is used to receive asynchronous messages in the main loop. There can only be a single bus watch per bus, you must remove it before you can set a new one.

The bus watch will only work if a GLib main loop is being run.

When `func` is called, the message belongs to the caller; if you want to keep a copy of it, call `gst_message_ref()` before leaving `func` .

The watch can be removed using `gst_bus_remove_watch()` or by returning `FALSE` from `func` . If the watch was added to the default main context it is also possible to remove the watch using `g_source_remove()`.

MT safe.

[rename-to gst_bus_add_watch]

#### Parameters

bus | a GstBus to create the watch for. |
priority | The priority of the watch. |
func | A function to call when a message is received. |
user_data | user data passed to `func` . |
notify | the function to call when the source is removed. |

#### Returns

The event source id or 0 if `bus` already got an event source.

### gst_bus_add_watch ()

    guint
    gst_bus_add_watch (GstBus *bus,
                       GstBusFunc func,
                       gpointer user_data);

Adds a bus watch to the default main context with the default priority (`G_PRIORITY_DEFAULT`). It is also possible to use a non-default main context set up using `g_main_context_push_thread_default()` (before one had to create a bus watch source and attach it to the desired main context 'manually').

This function is used to receive asynchronous messages in the main loop. There can only be a single bus watch per bus, you must remove it before you can set a new one.

The bus watch will only work if a GLib main loop is being run.

The watch can be removed using `gst_bus_remove_watch()` or by returning `FALSE` from `func` . If the watch was added to the default main context it is also possible to remove the watch using `g_source_remove()`.

[skip]

#### Parameters

bus | a GstBus to create the watch for |
func | A function to call when a message is received. |
user_data | user data passed to `func` . |

#### Returns

The event source id or 0 if `bus` already got an event source.

MT safe.

### gst_bus_remove_watch ()

    gboolean
    gst_bus_remove_watch (GstBus *bus);

Removes an installed bus watch from `bus` .

#### Parameters

bus | a GstBus to remove the watch from. |

#### Returns

`TRUE` on success or `FALSE` if `bus` has no event source.

Since: 1.6

### gst_bus_disable_sync_message_emission ()

    void
    gst_bus_disable_sync_message_emission (GstBus *bus);

Instructs GStreamer to stop emitting the "sync-message" signal for this bus. See `gst_bus_enable_sync_message_emission()` for more information.

In the event that multiple pieces of code have called `gst_bus_enable_sync_message_emission()`, the sync-message emissions will only be stopped after all calls to `gst_bus_enable_sync_message_emission()` were "cancelled" by calling this function. In this way the semantics are exactly the same as `gst_object_ref()` that which calls enable should also call disable.

MT safe.

#### Parameters

bus | a GstBus on which you previously called `gst_bus_enable_sync_message_emission()` |

### gst_bus_enable_sync_message_emission ()

    void
    gst_bus_enable_sync_message_emission (GstBus *bus);

Instructs GStreamer to emit the "sync-message" signal after running the bus's sync handler. This function is here so that code can ensure that they can synchronously receive messages without having to affect what the bin's sync handler is.

This function may be called multiple times. To clean up, the caller is responsible for calling `gst_bus_disable_sync_message_emission()` as many times as this function is called.

While this function looks similar to `gst_bus_add_signal_watch()`, it is not exactly the same -- this function enables synchronous emission of signals when messages arrive; `gst_bus_add_signal_watch()` adds an idle callback to pop messages off the bus asynchronously. The sync-message signal comes from the thread of whatever object posted the message; the "message" signal is marshalled to the main thread via the main loop.

MT safe.

#### Parameters

bus | a GstBus on which you want to receive the "sync-message" signal |

### gst_bus_async_signal_func ()

    gboolean
    gst_bus_async_signal_func (GstBus *bus,
                               GstMessage *message,
                               gpointer data);

A helper GstBusFunc that can be used to convert all asynchronous messages into signals.

#### Parameters

bus | a GstBus |
message | the GstMessage received |
data | user data |

#### Returns

`TRUE`

### gst_bus_add_signal_watch ()

    void
    gst_bus_add_signal_watch (GstBus *bus);

Adds a bus signal watch to the default main context with the default priority (`G_PRIORITY_DEFAULT`). It is also possible to use a non-default main context set up using `g_main_context_push_thread_default()` (before one had to create a bus watch source and attach it to the desired main context 'manually').

After calling this statement, the bus will emit the "message" signal for each message posted on the bus.

This function may be called multiple times. To clean up, the caller is responsible for calling `gst_bus_remove_signal_watch()` as many times as this function is called.

MT safe.

#### Parameters

bus | a GstBus on which you want to receive the "message" signal |

### gst_bus_add_signal_watch_full ()

    void
    gst_bus_add_signal_watch_full (GstBus *bus,
                                   gint priority);

Adds a bus signal watch to the default main context with the given `priority` (e.g. `G_PRIORITY_DEFAULT`). It is also possible to use a non-default main context set up using `g_main_context_push_thread_default()` (before one had to create a bus watch source and attach it to the desired main context 'manually').

After calling this statement, the bus will emit the "message" signal for each message posted on the bus when the main loop is running.

This function may be called multiple times. To clean up, the caller is responsible for calling `gst_bus_remove_signal_watch()` as many times as this function is called.

There can only be a single bus watch per bus, you must remove any signal watch before you can set another type of watch.

MT safe.

#### Parameters

bus | a GstBus on which you want to receive the "message" signal |
priority | The priority of the watch. |

### gst_bus_remove_signal_watch ()

    void
    gst_bus_remove_signal_watch (GstBus *bus);

Removes a signal watch previously added with `gst_bus_add_signal_watch()`.

MT safe.

#### Parameters

bus | a GstBus you previously added a signal watch to |

### gst_bus_poll ()

    GstMessage *
    gst_bus_poll (GstBus *bus,
                  GstMessageType events,
                  GstClockTime timeout);

Poll the bus for messages. Will block while waiting for messages to come. You can specify a maximum time to poll with the `timeout` parameter. If `timeout` is negative, this function will block indefinitely.

All messages not in `events` will be popped off the bus and will be ignored. It is not possible to use message enums beyond GST_MESSAGE_EXTENDED in the `events` mask

Because poll is implemented using the "message" signal enabled by `gst_bus_add_signal_watch()`, calling `gst_bus_poll()` will cause the "message" signal to be emitted for every message that poll sees. Thus a "message" signal handler will see the same messages that this function sees -- neither will steal messages from the other.

This function will run a main loop from the default main context when polling.

You should never use this function, since it is pure evil. This is especially true for GUI applications based on Gtk+ or Qt, but also for any other non-trivial application that uses the GLib main loop. As this function runs a GLib main loop, any callback attached to the default GLib main context may be invoked. This could be timeouts, GUI events, I/O events etc.; even if `gst_bus_poll()` is called with a 0 timeout. Any of these callbacks may do things you do not expect, e.g. destroy the main application window or some other resource; change other application state; display a dialog and run another main loop until the user clicks it away. In short, using this function may add a lot of complexity to your code through unexpected re-entrancy and unexpected changes to your application's state.

For 0 timeouts use `gst_bus_pop_filtered()` instead of this function; for other short timeouts use `gst_bus_timed_pop_filtered()`; everything else is better handled by setting up an asynchronous bus watch and doing things from there.

#### Parameters

bus | a GstBus |
events | a mask of GstMessageType, representing the set of message types to poll for (note special handling of extended message types below) |
timeout | the poll timeout, as a GstClockTime, or GST_CLOCK_TIME_NONE to poll indefinitely. |

#### Returns

the message that was received, or `NULL` if the poll timed out. The message is taken from the bus and needs to be unreffed with `gst_message_unref()` after usage.

[transfer full][nullable]

## Types and Values

### struct GstBus

    struct GstBus;

The opaque GstBus data structure.

### enum GstBusFlags

The standard flags that a bus may have.

#### Members

GST_BUS_FLUSHING | The bus is currently dropping all messages |
GST_BUS_FLAG_LAST | offset to define more flags |

### enum GstBusSyncReply

The result values for a GstBusSyncHandler.

#### Members

GST_BUS_DROP | drop the message |
GST_BUS_PASS | pass the message to the async queue |
GST_BUS_ASYNC | pass message to async queue, continue if message is handled |

## Property Details

### The `“enable-async”` property

      “enable-async”             gboolean

Enable async message delivery for bus watches and gst_bus_pop().

Flags: Write / Construct Only

Default value: TRUE

## Signal Details

### The `“message”` signal

    void
    user_function (GstBus     *bus,
                   GstMessage *message,
                   gpointer    user_data)

A message has been posted on the bus. This signal is emitted from a GSource added to the mainloop. this signal will only be emitted when there is a mainloop running.

#### Parameters

bus | the object which received the signal |
message | the message that has been posted asynchronously |
user_data | user data set when the signal handler was connected. |

Flags: Has Details

### The `“sync-message”` signal

    void
    user_function (GstBus     *bus,
                   GstMessage *message,
                   gpointer    user_data)

A message has been posted on the bus. This signal is emitted from the thread that posted the message so one has to be careful with locking.

This signal will not be emitted by default, you have to call `gst_bus_enable_sync_message_emission()` before.

#### Parameters

bus | the object which received the signal |
message | the message that has been posted synchronously |
user_data | user data set when the signal handler was connected. |

Flags: Has Details

## See Also

GstMessage, GstElement