## Main loop and Events

Main loop and Events — Library initialization, main event loop, and events

## Synopsis

    #include <gtk/gtk.h>

    void                gtk_disable_setlocale               (void);
    PangoLanguage *     gtk_get_default_language            (void);
    gboolean            gtk_parse_args                      (int *argc,
                                                             char ***argv);
    void                gtk_init                            (int *argc,
                                                             char ***argv);
    gboolean            gtk_init_check                      (int *argc,
                                                             char ***argv);
    gboolean            gtk_init_with_args                  (gint *argc,
                                                             gchar ***argv,
                                                             const gchar *parameter_string,
                                                             const GOptionEntry *entries,
                                                             const gchar *translation_domain,
                                                             GError **error);
    GOptionGroup *      gtk_get_option_group                (gboolean open_default_display);
    gboolean            gtk_events_pending                  (void);
    void                gtk_main                            (void);
    guint               gtk_main_level                      (void);
    void                gtk_main_quit                       (void);
    gboolean            gtk_main_iteration                  (void);
    gboolean            gtk_main_iteration_do               (gboolean blocking);
    void                gtk_main_do_event                   (GdkEvent *event);
    void                (*GtkModuleInitFunc)                (gint *argc,
                                                             gchar ***argv);
    void                (*GtkModuleDisplayInitFunc)         (GdkDisplay *display);

    gboolean            gtk_true                            (void);
    gboolean            gtk_false                           (void);

    void                gtk_grab_add                        (GtkWidget *widget);
    GtkWidget *         gtk_grab_get_current                (void);
    void                gtk_grab_remove                     (GtkWidget *widget);
    void                gtk_device_grab_add                 (GtkWidget *widget,
                                                             GdkDevice *device,
                                                             gboolean block_others);
    void                gtk_device_grab_remove              (GtkWidget *widget,
                                                             GdkDevice *device);

    #define             GTK_PRIORITY_RESIZE

    guint               gtk_key_snooper_install             (GtkKeySnoopFunc snooper,
                                                             gpointer func_data);
    gint                (*GtkKeySnoopFunc)                  (GtkWidget *grab_widget,
                                                             GdkEventKey *event,
                                                             gpointer func_data);
    void                gtk_key_snooper_remove              (guint snooper_handler_id);

    GdkEvent *          gtk_get_current_event               (void);
    guint32             gtk_get_current_event_time          (void);
    gboolean            gtk_get_current_event_state         (GdkModifierType *state);
    GdkDevice *         gtk_get_current_event_device        (void);
    GtkWidget *         gtk_get_event_widget                (GdkEvent *event);
    void                gtk_propagate_event                 (GtkWidget *widget,
                                                             GdkEvent *event);

## Description

Before using GTK+, you need to initialize it; initialization connects to the window system display, and parses some standard command line arguments. The `gtk_init()` macro initializes GTK+. `gtk_init()` exits the application if errors occur; to avoid this, use `gtk_init_check()`. `gtk_init_check()` allows you to recover from a failed GTK+ initialization - you might start up your application in text mode instead.

Like all GUI toolkits, GTK+ uses an event-driven programming model. When the user is doing nothing, GTK+ sits in the main loop and waits for input. If the user performs some action - say, a mouse click - then the main loop "wakes up" and delivers an event to GTK+. GTK+ forwards the event to one or more widgets.

When widgets receive an event, they frequently emit one or more signals. Signals notify your program that "something interesting happened" by invoking functions you've connected to the signal with `g_signal_connect()`. Functions connected to a signal are often termed callbacks.

When your callbacks are invoked, you would typically take some action - for example, when an Open button is clicked you might display a GtkFileChooserDialog. After a callback finishes, GTK+ will return to the main loop and await more user input.

Example 6. Typical ``main()`` function for a GTK+ application

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24

    int
    main (int argc, char **argv)
    {
      /* Initialize i18n support */
      gtk_set_locale ();

      /* Initialize the widget set */
      gtk_init (&argc, &argv);

      /* Create the main window */
      mainwin = gtk_window_new (GTK_WINDOW_TOPLEVEL);

      /* Set up our GUI elements */
      ...

      /* Show the application window */
      gtk_widget_show_all (mainwin);

      /* Enter the main event loop, and wait for user interaction */
      gtk_main ();

      /* The user lost interest */
      return 0;
    }

It's OK to use the GLib main loop directly instead of `gtk_main()`, though it involves slightly more typing. See GMainLoop in the GLib documentation.

## Details

### gtk_disable_setlocale ()

    void                gtk_disable_setlocale               (void);

Prevents `gtk_init()`, `gtk_init_check()`, `gtk_init_with_args()` and `gtk_parse_args()` from automatically calling `setlocale (LC_ALL, "")`. You would want to use this function if you wanted to set the locale for your program to something other than the user's locale, or if you wanted to set different values for different locale categories.

Most programs should not need to call this function.

### gtk_get_default_language ()

    PangoLanguage *     gtk_get_default_language            (void);

Returns the PangoLanguage for the default language currently in effect. (Note that this can change over the life of an application.) The default language is derived from the current locale. It determines, for example, whether GTK+ uses the right-to-left or left-to-right text direction.

This function is equivalent to `pango_language_get_default()`. See that function for details.

Returns : | the default language as a PangoLanguage, must not be freed

### gtk_parse_args ()

    gboolean            gtk_parse_args                      (int *argc,
                                                             char ***argv);

Parses command line arguments, and initializes global attributes of GTK+, but does not actually open a connection to a display. (See `gdk_display_open()`, `gdk_get_display_arg_name()`)

Any arguments used by GTK+ or GDK are removed from the array and `argc` and `argv` are updated accordingly.

There is no need to call this function explicitely if you are using `gtk_init()`, or `gtk_init_check()`.

`argc` : | a pointer to the number of command line arguments. [inout]
`argv` : | a pointer to the array of command line arguments. [array length=argc][inout]
Returns : | `TRUE` if initialization succeeded, otherwise `FALSE`

### gtk_init ()

    void                gtk_init                            (int *argc,
                                                             char ***argv);

Call this function before using any other GTK+ functions in your GUI applications. It will initialize everything needed to operate the toolkit and parses some standard command line options.

Although you are expected to pass the `argc`, `argv` parameters from `main()` to this function, it is possible to pass `NULL` if `argv` is not available or commandline handling is not required.

`argc` and `argv` are adjusted accordingly so your own code will never see those standard arguments.

Note that there are some alternative ways to initialize GTK+: if you are calling `gtk_parse_args()`, `gtk_init_check()`, `gtk_init_with_args()` or `g_option_context_parse()` with the option group returned by `gtk_get_option_group()`, you don't have to call `gtk_init()`.

### Note

This function will terminate your program if it was unable to initialize the windowing system for some reason. If you want your program to fall back to a textual interface you want to call `gtk_init_check()` instead.

### Note

Since 2.18, GTK+ calls `signal (SIGPIPE, SIG_IGN)` during initialization, to ignore SIGPIPE signals, since these are almost never wanted in graphical applications. If you do need to handle SIGPIPE for some reason, reset the handler after `gtk_init()`, but notice that other libraries (e.g. libdbus or gvfs) might do similar things.

`argc` : | Address of the `argc` parameter of your `main()` function (or 0 if `argv` is `NULL`). This will be changed if any arguments were handled. [inout]
`argv` : | Address of the `argv` parameter of `main()`, or `NULL`. Any options understood by GTK+ are stripped before return. [array length=argc][inout][allow-none]

### gtk_init_check ()

    gboolean            gtk_init_check                      (int *argc,
                                                             char ***argv);

This function does the same work as `gtk_init()` with only a single change: It does not terminate the program if the windowing system can't be initialized. Instead it returns `FALSE` on failure.

This way the application can fall back to some other means of communication with the user - for example a curses or command line interface.

`argc` : | Address of the `argc` parameter of your `main()` function (or 0 if `argv` is `NULL`). This will be changed if any arguments were handled. [inout]
`argv` : | Address of the `argv` parameter of `main()`, or `NULL`. Any options understood by GTK+ are stripped before return. [array length=argc][inout][allow-none]
Returns : | `TRUE` if the windowing system has been successfully initialized, `FALSE` otherwise

### gtk_init_with_args ()

    gboolean            gtk_init_with_args                  (gint *argc,
                                                             gchar ***argv,
                                                             const gchar *parameter_string,
                                                             const GOptionEntry *entries,
                                                             const gchar *translation_domain,
                                                             GError **error);

This function does the same work as `gtk_init_check()`. Additionally, it allows you to add your own commandline options, and it automatically generates nicely formatted `--help` output. Note that your program will be terminated after writing out the help output.

`argc` : | Address of the `argc` parameter of your `main()` function (or 0 if `argv` is `NULL`). This will be changed if any arguments were handled. [inout]
`argv` : | Address of the `argv` parameter of `main()`, or `NULL`. Any options understood by GTK+ are stripped before return. [array length=argc][inout][allow-none]
`parameter_string` : | a string which is displayed in the first line of `--help` output, after ``programname` [OPTION...]`
`entries` : | a `NULL`-terminated array of GOptionEntrys describing the options of your program. [array zero-terminated=1]
`translation_domain` : | a translation domain to use for translating the `--help` output for the options in `entries` and the `parameter_string` with `gettext()`, or `NULL`
`error` : | a return location for errors
Returns : | `TRUE` if the windowing system has been successfully initialized, `FALSE` otherwise

Since 2.6

### gtk_get_option_group ()

    GOptionGroup *      gtk_get_option_group                (gboolean open_default_display);

Returns a GOptionGroup for the commandline arguments recognized by GTK+ and GDK.

You should add this group to your GOptionContext with `g_option_context_add_group()`, if you are using `g_option_context_parse()` to parse your commandline arguments.

`open_default_display` : | whether to open the default display when parsing the commandline arguments
Returns : | a GOptionGroup for the commandline arguments recognized by GTK+

Since 2.6

### gtk_events_pending ()

    gboolean            gtk_events_pending                  (void);

Checks if any events are pending.

This can be used to update the UI and invoke timeouts etc. while doing some time intensive computation.

Example 7. Updating the UI during a long computation

    1
    2
    3
    4
    5
    6

    /* computation going on... */

    while (gtk_events_pending ())
      gtk_main_iteration ();

    /* ...computation continued */

Returns : | `TRUE` if any events are pending, `FALSE` otherwise

### gtk_main ()

    void                gtk_main                            (void);

Runs the main loop until `gtk_main_quit()` is called.

You can nest calls to `gtk_main()`. In that case `gtk_main_quit()` will make the innermost invocation of the main loop return.

### gtk_main_level ()

    guint               gtk_main_level                      (void);

Asks for the current nesting level of the main loop.

Returns : | the nesting level of the current invocation of the main loop

### gtk_main_quit ()

    void                gtk_main_quit                       (void);

Makes the innermost invocation of the main loop return when it regains control.

### gtk_main_iteration ()

    gboolean            gtk_main_iteration                  (void);

Runs a single iteration of the mainloop.

If no events are waiting to be processed GTK+ will block until the next event is noticed. If you don't want to block look at `gtk_main_iteration_do()` or check if any events are pending with `gtk_events_pending()` first.

Returns : | `TRUE` if `gtk_main_quit()` has been called for the innermost mainloop

### gtk_main_iteration_do ()

    gboolean            gtk_main_iteration_do               (gboolean blocking);

Runs a single iteration of the mainloop. If no events are available either return or block depending on the value of `blocking`.

`blocking` : | `TRUE` if you want GTK+ to block if no events are pending
Returns : | `TRUE` if `gtk_main_quit()` has been called for the innermost mainloop

### gtk_main_do_event ()

    void                gtk_main_do_event                   (GdkEvent *event);

Processes a single GDK event.

This is public only to allow filtering of events between GDK and GTK+. You will not usually need to call this function directly.

While you should not call this function directly, you might want to know how exactly events are handled. So here is what this function does with the event:

1. Compress enter/leave notify events. If the event passed build an enter/leave pair together with the next event (peeked from GDK), both events are thrown away. This is to avoid a backlog of (de-)highlighting widgets crossed by the pointer.

2. Find the widget which got the event. If the widget can't be determined the event is thrown away unless it belongs to a INCR transaction. In that case it is passed to `gtk_selection_incr_event()`.

3. Then the event is pushed onto a stack so you can query the currently handled event with `gtk_get_current_event()`.

4. The event is sent to a widget. If a grab is active all events for widgets that are not in the contained in the grab widget are sent to the latter with a few exceptions:

   - Deletion and destruction events are still sent to the event widget for obvious reasons.

   - Events which directly relate to the visual representation of the event widget.

   - Leave events are delivered to the event widget if there was an enter event delivered to it before without the paired leave event.

   - Drag events are not redirected because it is unclear what the semantics of that would be.

   Another point of interest might be that all key events are first passed through the key snooper functions if there are any. Read the description of `gtk_key_snooper_install()` if you need this feature.

5. After finishing the delivery the event is popped from the event stack.

`event` : | An event to process (normally passed by GDK)

### GtkModuleInitFunc ()

    void                (*GtkModuleInitFunc)                (gint *argc,
                                                             gchar ***argv);

Each GTK+ module must have a function `gtk_module_init()` with this prototype. This function is called after loading the module.

`argc` : | GTK+ always passes `NULL` for this argument. [allow-none]
`argv` : | GTK+ always passes `NULL` for this argument. [allow-none][array length=argc]

### GtkModuleDisplayInitFunc ()

    void                (*GtkModuleDisplayInitFunc)         (GdkDisplay *display);

A multihead-aware GTK+ module may have a `gtk_module_display_init()` function with this prototype. GTK+ calls this function for each opened display.

`display` : | an open GdkDisplay

Since 2.2

### gtk_true ()

    gboolean            gtk_true                            (void);

All this function does it to return `TRUE`.

This can be useful for example if you want to inhibit the deletion of a window. Of course you should not do this as the user expects a reaction from clicking the close icon of the window...

Example 8. A persistent window

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26

    #include <gtk/gtk.h><

    int
    main (int argc, char **argv)
    {
      GtkWidget *win, *but;

      gtk_init (&argc, &argv);

      win = gtk_window_new (GTK_WINDOW_TOPLEVEL);
      g_signal_connect (win, "delete-event",
                        G_CALLBACK (gtk_true), NULL);
      g_signal_connect (win, "destroy",
                        G_CALLBACK (gtk_main_quit), NULL);

      but = gtk_button_new_with_label ("Close yourself. I mean it!");
      g_signal_connect_swapped (but, "clicked",
                                G_CALLBACK (gtk_object_destroy), win);
      gtk_container_add (GTK_CONTAINER (win), but);

      gtk_widget_show_all (win);

      gtk_main ();

      return 0;
    }

Returns : | `TRUE`

### gtk_false ()

    gboolean            gtk_false                           (void);

Analogical to `gtk_true()`, this function does nothing but always returns `FALSE`.

Returns : | `FALSE`

### gtk_grab_add ()

    void                gtk_grab_add                        (GtkWidget *widget);

Makes `widget` the current grabbed widget.

This means that interaction with other widgets in the same application is blocked and mouse as well as keyboard events are delivered to this widget.

If `widget` is not sensitive, it is not set as the current grabbed widget and this function does nothing.

`widget` : | The widget that grabs keyboard and pointer events

### gtk_grab_get_current ()

    GtkWidget *         gtk_grab_get_current                (void);

Queries the current grab of the default window group.

Returns : | The widget which currently has the grab or `NULL` if no grab is active. [transfer none]

### gtk_grab_remove ()

    void                gtk_grab_remove                     (GtkWidget *widget);

Removes the grab from the given widget.

You have to pair calls to `gtk_grab_add()` and `gtk_grab_remove()`.

If `widget` does not have the grab, this function does nothing.

`widget` : | The widget which gives up the grab

### gtk_device_grab_add ()

    void                gtk_device_grab_add                 (GtkWidget *widget,
                                                             GdkDevice *device,
                                                             gboolean block_others);

Adds a GTK+ grab on `device`, so all the events on `device` and its associated pointer or keyboard (if any) are delivered to `widget`. If the `block_others` parameter is `TRUE`, any other devices will be unable to interact with `widget` during the grab.

`widget` : | a GtkWidget
`device` : | a GtkDevice to grab on.
`block_others` : | `TRUE` to prevent other devices to interact with `widget`.

Since 3.0

### gtk_device_grab_remove ()

    void                gtk_device_grab_remove              (GtkWidget *widget,
                                                             GdkDevice *device);

Removes a device grab from the given widget.

You have to pair calls to `gtk_device_grab_add()` and `gtk_device_grab_remove()`.

`widget` : | a GtkWidget
`device` : | a GdkDevice

Since 3.0

### GTK_PRIORITY_RESIZE

    #define GTK_PRIORITY_RESIZE (G_PRIORITY_HIGH_IDLE + 10)

Use this priority for functionality related to size allocation.

It is used internally by GTK+ to compute the sizes of widgets. This priority is higher than `GDK_PRIORITY_REDRAW` to avoid resizing a widget which was just redrawn.

### gtk_key_snooper_install ()

    guint               gtk_key_snooper_install             (GtkKeySnoopFunc snooper,
                                                             gpointer func_data);

Installs a key snooper function, which will get called on all key events before delivering them normally.

`snooper` : | a GtkKeySnoopFunc
`func_data` : | data to pass to `snooper`
Returns : | a unique id for this key snooper for use with `gtk_key_snooper_remove()`.

### GtkKeySnoopFunc ()

    gint                (*GtkKeySnoopFunc)                  (GtkWidget *grab_widget,
                                                             GdkEventKey *event,
                                                             gpointer func_data);

Key snooper functions are called before normal event delivery. They can be used to implement custom key event handling.

`grab_widget` : | the widget to which the event will be delivered
`event` : | the key event
`func_data` : | data supplied to `gtk_key_snooper_install()`
Returns : | `TRUE` to stop further processing of `event`, `FALSE` to continue.

### gtk_key_snooper_remove ()

    void                gtk_key_snooper_remove              (guint snooper_handler_id);

Removes the key snooper function with the given id.

`snooper_handler_id` : | Identifies the key snooper to remove

### gtk_get_current_event ()

    GdkEvent *          gtk_get_current_event               (void);

Obtains a copy of the event currently being processed by GTK+.

For example, if you are handling a "clicked" signal, the current event will be the GdkEventButton that triggered the ::clicked signal.

Returns : | a copy of the current event, or `NULL` if there is no current event. The returned event must be freed with `gdk_event_free()`. [transfer full]

### gtk_get_current_event_time ()

    guint32             gtk_get_current_event_time          (void);

If there is a current event and it has a timestamp, return that timestamp, otherwise return `GDK_CURRENT_TIME`.

Returns : | the timestamp from the current event, or `GDK_CURRENT_TIME`.

### gtk_get_current_event_state ()

    gboolean            gtk_get_current_event_state         (GdkModifierType *state);

If there is a current event and it has a state field, place that state field in `state` and return `TRUE`, otherwise return `FALSE`.

`state` : | a location to store the state of the current event. [out]
Returns : | `TRUE` if there was a current event and it had a state field

### gtk_get_current_event_device ()

    GdkDevice *         gtk_get_current_event_device        (void);

If there is a current event and it has a device, return that device, otherwise return `NULL`.

Returns : | a GdkDevice, or `NULL`. [transfer none]

### gtk_get_event_widget ()

    GtkWidget *         gtk_get_event_widget                (GdkEvent *event);

If `event` is `NULL` or the event was not associated with any widget, returns `NULL`, otherwise returns the widget that received the event originally.

`event` : | a GdkEvent
Returns : | the widget that originally received `event`, or `NULL`. [transfer none]

### gtk_propagate_event ()

    void                gtk_propagate_event                 (GtkWidget *widget,
                                                             GdkEvent *event);

Sends an event to a widget, propagating the event to parent widgets if the event remains unhandled.

Events received by GTK+ from GDK normally begin in `gtk_main_do_event()`. Depending on the type of event, existence of modal dialogs, grabs, etc., the event may be propagated; if so, this function is used.

`gtk_propagate_event()` calls `gtk_widget_event()` on each widget it decides to send the event to. So `gtk_widget_event()` is the lowest-level function; it simply emits the "event" and possibly an event-specific signal on a widget. `gtk_propagate_event()` is a bit higher-level, and `gtk_main_do_event()` is the highest level.

All that said, you most likely don't want to use any of these functions; synthesizing events is rarely needed. There are almost certainly better ways to achieve your goals. For example, use `gdk_window_invalidate_rect()` or `gtk_widget_queue_draw()` instead of making up expose events.

`widget` : | a GtkWidget
`event` : | an event

## See Also

See the GLib manual, especially GMainLoop and signal-related functions such as `g_signal_connect()`