Memory management of signal handlers

If you are connecting handlers to signals and using a GObject instance as your signal handler user data, you should remember to pair calls to g_signal_connect() with calls to g_signal_handler_disconnect() or g_signal_handlers_disconnect_by_func(). While signal handlers are automatically disconnected when the object emitting the signal is finalised, they are not automatically disconnected when the signal handler user data is destroyed. If this user data is a GObject instance, using it from a signal handler after it has been finalised is an error.

There are two strategies for managing such user data. The first is to disconnect the signal handler (using g_signal_handler_disconnect() or g_signal_handlers_disconnect_by_func()) when the user data (object) is finalised; this has to be implemented manually. For non-threaded programs, g_signal_connect_object() can be used to implement this automatically. Currently, however, it is unsafe to use in threaded programs.

The second is to hold a strong reference on the user data until after the signal is disconnected for other reasons. This can be implemented automatically using g_signal_connect_data().

The first approach is recommended, as the second approach can result in effective memory leaks of the user data if the signal handler is never disconnected for some reason.

GSignalAccumulator ()

gboolean
(*GSignalAccumulator) (GSignalInvocationHint *ihint,
                       GValue *return_accu,
                       const GValue *handler_return,
                       gpointer data);

The signal accumulator is a special callback function that can be used to collect return values of the various callbacks that are called during a signal emission. The signal accumulator is specified at signal creation time, if it is left NULL, no accumulation of callback return values is performed. The return value of signal emissions is then the value returned by the last callback.

GSignalEmissionHook ()

gboolean
(*GSignalEmissionHook) (GSignalInvocationHint *ihint,
                        guint n_param_values,
                        const GValue *param_values,
                        gpointer data);

A simple function pointer to get invoked when the signal is emitted. This allows you to tie a hook to the signal type, so that it will trap all emissions of that signal, from any object.

You may not attach these to signals created with the G_SIGNAL_NO_HOOKS flag.

g_signal_new ()

guint
g_signal_new (const gchar *signal_name,
              GType itype,
              GSignalFlags signal_flags,
              guint class_offset,
              GSignalAccumulator accumulator,
              gpointer accu_data,
              GSignalCMarshaller c_marshaller,
              GType return_type,
              guint n_params,
              ...);

Creates a new signal. (This is usually done in the class initializer.)

A signal name consists of segments consisting of ASCII letters and digits, separated by either the '-' or '_' character. The first character of a signal name must be a letter. Names which violate these rules lead to undefined behaviour of the GSignal system.

When registering a signal and looking up a signal, either separator can be used, but they cannot be mixed.

If 0 is used for class_offset subclasses cannot override the class handler in their class_init method by doing super_class->signal_handler = my_signal_handler. Instead they will have to use g_signal_override_class_handler().

If c_marshaller is NULL, g_cclosure_marshal_generic() will be used as the marshaller for this signal.

g_signal_newv ()

guint
g_signal_newv (const gchar *signal_name,
               GType itype,
               GSignalFlags signal_flags,
               GClosure *class_closure,
               GSignalAccumulator accumulator,
               gpointer accu_data,
               GSignalCMarshaller c_marshaller,
               GType return_type,
               guint n_params,
               GType *param_types);

Creates a new signal. (This is usually done in the class initializer.)

See g_signal_new() for details on allowed signal names.

If c_marshaller is NULL, g_cclosure_marshal_generic() will be used as the marshaller for this signal.

g_signal_new_valist ()

guint
g_signal_new_valist (const gchar *signal_name,
                     GType itype,
                     GSignalFlags signal_flags,
                     GClosure *class_closure,
                     GSignalAccumulator accumulator,
                     gpointer accu_data,
                     GSignalCMarshaller c_marshaller,
                     GType return_type,
                     guint n_params,
                     va_list args);

Creates a new signal. (This is usually done in the class initializer.)

See g_signal_new() for details on allowed signal names.

If c_marshaller is NULL, g_cclosure_marshal_generic() will be used as the marshaller for this signal.

g_signal_set_va_marshaller ()

void
g_signal_set_va_marshaller (guint signal_id,
                            GType instance_type,
                            GSignalCVaMarshaller va_marshaller);

Change the GSignalCVaMarshaller used for a given signal. This is a specialised form of the marshaller that can often be used for the common case of a single connected signal handler and avoids the overhead of GValue. Its use is optional.

Since: 2.32

g_signal_query ()

void
g_signal_query (guint signal_id,
                GSignalQuery *query);

Queries the signal system for in-depth information about a specific signal. This function will fill in a user-provided structure to hold signal-specific information. If an invalid signal id is passed in, the signal_id member of the GSignalQuery is 0. All members filled into the GSignalQuery structure should be considered constant and have to be left untouched.

g_signal_lookup ()

guint
g_signal_lookup (const gchar *name,
                 GType itype);

Given the name of the signal and the type of object it connects to, gets the signal's identifying integer. Emitting the signal by number is somewhat faster than using the name each time.

Also tries the ancestors of the given type.

See g_signal_new() for details on allowed signal names.

g_signal_name ()

const gchar *
g_signal_name (guint signal_id);

Given the signal's identifier, finds its name.

Two different signals may have the same name, if they have differing types.

g_signal_list_ids ()

guint *
g_signal_list_ids (GType itype,
                   guint *n_ids);

Lists the signals by id that a certain instance or interface type created. Further information about the signals can be acquired through g_signal_query().

g_signal_emit ()

void
g_signal_emit (gpointer instance,
               guint signal_id,
               GQuark detail,
               ...);

Emits a signal.

Note that g_signal_emit() resets the return value to the default if no handlers are connected, in contrast to g_signal_emitv().

g_signal_emit_by_name ()

void
g_signal_emit_by_name (gpointer instance,
                       const gchar *detailed_signal,
                       ...);

Emits a signal.

Note that g_signal_emit_by_name() resets the return value to the default if no handlers are connected, in contrast to g_signal_emitv().

g_signal_emitv ()

void
g_signal_emitv (const GValue *instance_and_params,
                guint signal_id,
                GQuark detail,
                GValue *return_value);

Emits a signal.

Note that g_signal_emitv() doesn't change return_value if no handlers are connected, in contrast to g_signal_emit() and g_signal_emit_valist().

g_signal_emit_valist ()

void
g_signal_emit_valist (gpointer instance,
                      guint signal_id,
                      GQuark detail,
                      va_list var_args);

Emits a signal.

Note that g_signal_emit_valist() resets the return value to the default if no handlers are connected, in contrast to g_signal_emitv().

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g_signal_connect()

#define             g_signal_connect(instance, detailed_signal, c_handler, data)

Connects a GCallback function to a signal for a particular object.

The handler will be called before the default handler of the signal.

See memory management of signal handlers for details on how to handle the return value and memory management of data .

g_signal_connect_after()

#define             g_signal_connect_after(instance, detailed_signal, c_handler, data)

Connects a GCallback function to a signal for a particular object.

The handler will be called after the default handler of the signal.

g_signal_connect_swapped()

#define             g_signal_connect_swapped(instance, detailed_signal, c_handler, data)

Connects a GCallback function to a signal for a particular object.

The instance on which the signal is emitted and data will be swapped when calling the handler. This is useful when calling pre-existing functions to operate purely on the data , rather than the instance : swapping the parameters avoids the need to write a wrapper function.

For example, this allows the shorter code:

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g_signal_connect_swapped (button, "clicked",
                          (GCallback) gtk_widget_hide, other_widget);

Rather than the cumbersome:

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static void
button_clicked_cb (GtkButton *button, GtkWidget *other_widget)
{
    gtk_widget_hide (other_widget);
}

...

g_signal_connect (button, "clicked",
                  (GCallback) button_clicked_cb, other_widget);

g_signal_connect_object ()

gulong
g_signal_connect_object (gpointer instance,
                         const gchar *detailed_signal,
                         GCallback c_handler,
                         gpointer gobject,
                         GConnectFlags connect_flags);

This is similar to g_signal_connect_data(), but uses a closure which ensures that the gobject stays alive during the call to c_handler by temporarily adding a reference count to gobject .

When the gobject is destroyed the signal handler will be automatically disconnected. Note that this is not currently threadsafe (ie: emitting a signal while gobject is being destroyed in another thread is not safe).

[skip]

g_signal_connect_data ()

gulong
g_signal_connect_data (gpointer instance,
                       const gchar *detailed_signal,
                       GCallback c_handler,
                       gpointer data,
                       GClosureNotify destroy_data,
                       GConnectFlags connect_flags);

Connects a GCallback function to a signal for a particular object. Similar to g_signal_connect(), but allows to provide a GClosureNotify for the data which will be called when the signal handler is disconnected and no longer used. Specify connect_flags if you need ..._after() or ..._swapped() variants of this function.

g_signal_connect_closure ()

gulong
g_signal_connect_closure (gpointer instance,
                          const gchar *detailed_signal,
                          GClosure *closure,
                          gboolean after);

Connects a closure to a signal for a particular object.

g_signal_connect_closure_by_id ()

gulong
g_signal_connect_closure_by_id (gpointer instance,
                                guint signal_id,
                                GQuark detail,
                                GClosure *closure,
                                gboolean after);

Connects a closure to a signal for a particular object.

g_signal_handler_block ()

void
g_signal_handler_block (gpointer instance,
                        gulong handler_id);

Blocks a handler of an instance so it will not be called during any signal emissions unless it is unblocked again. Thus "blocking" a signal handler means to temporarily deactive it, a signal handler has to be unblocked exactly the same amount of times it has been blocked before to become active again.

The handler_id has to be a valid signal handler id, connected to a signal of instance .

g_signal_handler_unblock ()

void
g_signal_handler_unblock (gpointer instance,
                          gulong handler_id);

Undoes the effect of a previous g_signal_handler_block() call. A blocked handler is skipped during signal emissions and will not be invoked, unblocking it (for exactly the amount of times it has been blocked before) reverts its "blocked" state, so the handler will be recognized by the signal system and is called upon future or currently ongoing signal emissions (since the order in which handlers are called during signal emissions is deterministic, whether the unblocked handler in question is called as part of a currently ongoing emission depends on how far that emission has proceeded yet).

The handler_id has to be a valid id of a signal handler that is connected to a signal of instance and is currently blocked.

g_signal_handler_disconnect ()

void
g_signal_handler_disconnect (gpointer instance,
                             gulong handler_id);

Disconnects a handler from an instance so it will not be called during any future or currently ongoing emissions of the signal it has been connected to. The handler_id becomes invalid and may be reused.

The handler_id has to be a valid signal handler id, connected to a signal of instance .

g_signal_handler_find ()

gulong
g_signal_handler_find (gpointer instance,
                       GSignalMatchType mask,
                       guint signal_id,
                       GQuark detail,
                       GClosure *closure,
                       gpointer func,
                       gpointer data);

Finds the first signal handler that matches certain selection criteria. The criteria mask is passed as an OR-ed combination of GSignalMatchType flags, and the criteria values are passed as arguments. The match mask has to be non-0 for successful matches. If no handler was found, 0 is returned.

g_signal_handlers_block_matched ()

guint
g_signal_handlers_block_matched (gpointer instance,
                                 GSignalMatchType mask,
                                 guint signal_id,
                                 GQuark detail,
                                 GClosure *closure,
                                 gpointer func,
                                 gpointer data);

Blocks all handlers on an instance that match a certain selection criteria. The criteria mask is passed as an OR-ed combination of GSignalMatchType flags, and the criteria values are passed as arguments. Passing at least one of the G_SIGNAL_MATCH_CLOSURE, G_SIGNAL_MATCH_FUNC or G_SIGNAL_MATCH_DATA match flags is required for successful matches. If no handlers were found, 0 is returned, the number of blocked handlers otherwise.

g_signal_handlers_unblock_matched ()

guint
g_signal_handlers_unblock_matched (gpointer instance,
                                   GSignalMatchType mask,
                                   guint signal_id,
                                   GQuark detail,
                                   GClosure *closure,
                                   gpointer func,
                                   gpointer data);

Unblocks all handlers on an instance that match a certain selection criteria. The criteria mask is passed as an OR-ed combination of GSignalMatchType flags, and the criteria values are passed as arguments. Passing at least one of the G_SIGNAL_MATCH_CLOSURE, G_SIGNAL_MATCH_FUNC or G_SIGNAL_MATCH_DATA match flags is required for successful matches. If no handlers were found, 0 is returned, the number of unblocked handlers otherwise. The match criteria should not apply to any handlers that are not currently blocked.

g_signal_handlers_disconnect_matched ()

guint
g_signal_handlers_disconnect_matched (gpointer instance,
                                      GSignalMatchType mask,
                                      guint signal_id,
                                      GQuark detail,
                                      GClosure *closure,
                                      gpointer func,
                                      gpointer data);

Disconnects all handlers on an instance that match a certain selection criteria. The criteria mask is passed as an OR-ed combination of GSignalMatchType flags, and the criteria values are passed as arguments. Passing at least one of the G_SIGNAL_MATCH_CLOSURE, G_SIGNAL_MATCH_FUNC or G_SIGNAL_MATCH_DATA match flags is required for successful matches. If no handlers were found, 0 is returned, the number of disconnected handlers otherwise.

g_signal_handler_is_connected ()

gboolean
g_signal_handler_is_connected (gpointer instance,
                               gulong handler_id);

Returns whether handler_id is the ID of a handler connected to instance .

g_signal_handlers_block_by_func()

#define             g_signal_handlers_block_by_func(instance, func, data)

Blocks all handlers on an instance that match func and data .

g_signal_handlers_unblock_by_func()

#define             g_signal_handlers_unblock_by_func(instance, func, data)

Unblocks all handlers on an instance that match func and data .

g_signal_handlers_disconnect_by_func()

#define             g_signal_handlers_disconnect_by_func(instance, func, data)

Disconnects all handlers on an instance that match func and data .

g_signal_handlers_disconnect_by_data()

#define             g_signal_handlers_disconnect_by_data(instance, data)

Disconnects all handlers on an instance that match data .

Since: 2.32

g_signal_has_handler_pending ()

gboolean
g_signal_has_handler_pending (gpointer instance,
                              guint signal_id,
                              GQuark detail,
                              gboolean may_be_blocked);

Returns whether there are any handlers connected to instance for the given signal id and detail.

If detail is 0 then it will only match handlers that were connected without detail. If detail is non-zero then it will match handlers connected both without detail and with the given detail. This is consistent with how a signal emitted with detail would be delivered to those handlers.

Since 2.46 this also checks for a non-default class closure being installed, as this is basically always what you want.

One example of when you might use this is when the arguments to the signal are difficult to compute. A class implementor may opt to not emit the signal if no one is attached anyway, thus saving the cost of building the arguments.

g_signal_stop_emission ()

void
g_signal_stop_emission (gpointer instance,
                        guint signal_id,
                        GQuark detail);

Stops a signal's current emission.

This will prevent the default method from running, if the signal was G_SIGNAL_RUN_LAST and you connected normally (i.e. without the "after" flag).

Prints a warning if used on a signal which isn't being emitted.

g_signal_stop_emission_by_name ()

void
g_signal_stop_emission_by_name (gpointer instance,
                                const gchar *detailed_signal);

Stops a signal's current emission.

This is just like g_signal_stop_emission() except it will look up the signal id for you.

g_signal_override_class_closure ()

void
g_signal_override_class_closure (guint signal_id,
                                 GType instance_type,
                                 GClosure *class_closure);

Overrides the class closure (i.e. the default handler) for the given signal for emissions on instances of instance_type . instance_type must be derived from the type to which the signal belongs.

See g_signal_chain_from_overridden() and g_signal_chain_from_overridden_handler() for how to chain up to the parent class closure from inside the overridden one.

g_signal_chain_from_overridden ()

void
g_signal_chain_from_overridden (const GValue *instance_and_params,
                                GValue *return_value);

Calls the original class closure of a signal. This function should only be called from an overridden class closure; see g_signal_override_class_closure() and g_signal_override_class_handler().

g_signal_new_class_handler ()

guint
g_signal_new_class_handler (const gchar *signal_name,
                            GType itype,
                            GSignalFlags signal_flags,
                            GCallback class_handler,
                            GSignalAccumulator accumulator,
                            gpointer accu_data,
                            GSignalCMarshaller c_marshaller,
                            GType return_type,
                            guint n_params,
                            ...);

Creates a new signal. (This is usually done in the class initializer.)

This is a variant of g_signal_new() that takes a C callback instead off a class offset for the signal's class handler. This function doesn't need a function pointer exposed in the class structure of an object definition, instead the function pointer is passed directly and can be overriden by derived classes with g_signal_override_class_closure() or g_signal_override_class_handler()and chained to with g_signal_chain_from_overridden() or g_signal_chain_from_overridden_handler().

See g_signal_new() for information about signal names.

If c_marshaller is NULL, g_cclosure_marshal_generic() will be used as the marshaller for this signal.

Since: 2.18

g_signal_override_class_handler ()

void
g_signal_override_class_handler (const gchar *signal_name,
                                 GType instance_type,
                                 GCallback class_handler);

Overrides the class closure (i.e. the default handler) for the given signal for emissions on instances of instance_type with callback class_handler . instance_type must be derived from the type to which the signal belongs.

See g_signal_chain_from_overridden() and g_signal_chain_from_overridden_handler() for how to chain up to the parent class closure from inside the overridden one.

Since: 2.18

g_signal_chain_from_overridden_handler ()

void
g_signal_chain_from_overridden_handler
                               (gpointer instance,
                                ...);

Calls the original class closure of a signal. This function should only be called from an overridden class closure; see g_signal_override_class_closure() and g_signal_override_class_handler().

[skip]

Since: 2.18

g_signal_add_emission_hook ()

gulong
g_signal_add_emission_hook (guint signal_id,
                            GQuark detail,
                            GSignalEmissionHook hook_func,
                            gpointer hook_data,
                            GDestroyNotify data_destroy);

Adds an emission hook for a signal, which will get called for any emission of that signal, independent of the instance. This is possible only for signals which don't have G_SIGNAL_NO_HOOKS flag set.

g_signal_remove_emission_hook ()

void
g_signal_remove_emission_hook (guint signal_id,
                               gulong hook_id);

Deletes an emission hook.

g_signal_parse_name ()

gboolean
g_signal_parse_name (const gchar *detailed_signal,
                     GType itype,
                     guint *signal_id_p,
                     GQuark *detail_p,
                     gboolean force_detail_quark);

Internal function to parse a signal name into its signal_id and detail quark.

g_signal_get_invocation_hint ()

GSignalInvocationHint *
g_signal_get_invocation_hint (gpointer instance);

Returns the invocation hint of the innermost signal emission of instance.

g_signal_type_cclosure_new ()

GClosure *
g_signal_type_cclosure_new (GType itype,
                            guint struct_offset);

Creates a new closure which invokes the function found at the offset struct_offset in the class structure of the interface or classed type identified by itype .

g_signal_accumulator_first_wins ()

gboolean
g_signal_accumulator_first_wins (GSignalInvocationHint *ihint,
                                 GValue *return_accu,
                                 const GValue *handler_return,
                                 gpointer dummy);

A predefined GSignalAccumulator for signals intended to be used as a hook for application code to provide a particular value. Usually only one such value is desired and multiple handlers for the same signal don't make much sense (except for the case of the default handler defined in the class structure, in which case you will usually want the signal connection to override the class handler).

This accumulator will use the return value from the first signal handler that is run as the return value for the signal and not run any further handlers (ie: the first handler "wins").

Since: 2.28

g_signal_accumulator_true_handled ()

gboolean
g_signal_accumulator_true_handled (GSignalInvocationHint *ihint,
                                   GValue *return_accu,
                                   const GValue *handler_return,
                                   gpointer dummy);

A predefined GSignalAccumulator for signals that return a boolean values. The behavior that this accumulator gives is that a return of TRUE stops the signal emission: no further callbacks will be invoked, while a return of FALSE allows the emission to continue. The idea here is that a TRUE return indicates that the callback handled the signal, and no further handling is needed.

Since: 2.4

struct GSignalInvocationHint

struct GSignalInvocationHint {
  guint		signal_id;
  GQuark detail;
  GSignalFlags run_type;
};

The GSignalInvocationHint structure is used to pass on additional information to callbacks during a signal emission.

GSignalCMarshaller

typedef GClosureMarshal			 GSignalCMarshaller;

This is the signature of marshaller functions, required to marshall arrays of parameter values to signal emissions into C language callback invocations. It is merely an alias to GClosureMarshal since the GClosure mechanism takes over responsibility of actual function invocation for the signal system.

GSignalCVaMarshaller

typedef GVaClosureMarshal		 GSignalCVaMarshaller;

This is the signature of va_list marshaller functions, an optional marshaller that can be used in some situations to avoid marshalling the signal argument into GValues.

enum GSignalFlags

The signal flags are used to specify a signal's behaviour, the overall signal description outlines how especially the RUN flags control the stages of a signal emission.

enum GSignalMatchType

The match types specify what g_signal_handlers_block_matched(), g_signal_handlers_unblock_matched() and g_signal_handlers_disconnect_matched() match signals by.

struct GSignalQuery

struct GSignalQuery {
  guint		signal_id;
  const gchar  *signal_name;
  GType		itype;
  GSignalFlags signal_flags;
  GType		return_type; /* mangled with G_SIGNAL_TYPE_STATIC_SCOPE flag */
  guint		n_params;
  const GType  *param_types; /* mangled with G_SIGNAL_TYPE_STATIC_SCOPE flag */
};

A structure holding in-depth information for a specific signal. It is filled in by the g_signal_query() function.

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@return_type callback (#gpointer     data1,
[param_types param_names,]
gpointer     data2);

G_SIGNAL_TYPE_STATIC_SCOPE

#define G_SIGNAL_TYPE_STATIC_SCOPE (G_TYPE_FLAG_RESERVED_ID_BIT)

This macro flags signal argument types for which the signal system may assume that instances thereof remain persistent across all signal emissions they are used in. This is only useful for non ref-counted, value-copy types.

To flag a signal argument in this way, add | G_SIGNAL_TYPE_STATIC_SCOPE to the corresponding argument of g_signal_new().

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g_signal_new ("size_request",
  G_TYPE_FROM_CLASS (gobject_class),
	 G_SIGNAL_RUN_FIRST,
	 G_STRUCT_OFFSET (GtkWidgetClass, size_request),
	 NULL, NULL,
	 _gtk_marshal_VOID__BOXED,
	 G_TYPE_NONE, 1,
	 GTK_TYPE_REQUISITION | G_SIGNAL_TYPE_STATIC_SCOPE);

G_SIGNAL_MATCH_MASK

#define G_SIGNAL_MATCH_MASK  0x3f

A mask for all GSignalMatchType bits.

G_SIGNAL_FLAGS_MASK

#define G_SIGNAL_FLAGS_MASK  0x1ff

A mask for all GSignalFlags bits.

enum GConnectFlags

The connection flags are used to specify the behaviour of a signal's connection.