class: GCond

[56:7] extends: object

The #GCond struct is an opaque data structure that represents a condition. Threads can block on a #GCond if they find a certain condition to be false. If other threads change the state of this condition they signal the #GCond, and that causes the waiting threads to be woken up. Consider the following example of a shared variable. One or more threads can wait for data to be published to the variable and when another thread publishes the data, it can signal one of the waiting threads to wake up to collect the data. Here is an example for using GCond to block a thread until a condition is satisfied: |[ gpointer current_data = NULL; GMutex data_mutex; GCond data_cond; void push_data (gpointer data) { g_mutex_lock (&data_mutex); current_data = data; g_cond_signal (&data_cond); g_mutex_unlock (&data_mutex); } gpointer pop_data (void) { gpointer data; g_mutex_lock (&data_mutex); while (!current_data) g_cond_wait (&data_cond, &data_mutex); data = current_data; current_data = NULL; g_mutex_unlock (&data_mutex); return data; } ]| Whenever a thread calls pop_data() now, it will wait until current_data is non-%NULL, i.e. until some other thread has called push_data(). The example shows that use of a condition variable must always be paired with a mutex. Without the use of a mutex, there would be a race between the check of @current_data by the while loop in pop_data() and waiting. Specifically, another thread could set @current_data after the check, and signal the cond (with nobody waiting on it) before the first thread goes to sleep. #GCond is specifically useful for its ability to release the mutex and go to sleep atomically. It is also important to use the g_cond_wait() and g_cond_wait_until() functions only inside a loop which checks for the condition to be true. See g_cond_wait() for an explanation of why the condition may not be true even after it returns. If a #GCond is allocated in static storage then it can be used without initialisation. Otherwise, you should call g_cond_init() on it and g_cond_clear() when done. A #GCond should only be accessed via the g_cond_ functions.

Members

handleObj
lib
retainedCallbacks
signalHandlerNames
signalSetterHandlers

Methods

GCond (Handle = null)

Creates a new Cond by wrapping a native handle or another wrapper.
- @p Handle is the native handle or another wrapper whose handle to adopt.
toNativeHandle (Source)

Normalizes a constructor argument into a raw pointer carrier. Accepts a raw NativeHandle, a raw NativeBuffer returned from fn.call(...), another generated wrapper exposing handle(), or null. Returns null when the argument carries no pointer.
- @p Source is the raw handle, raw buffer, wrapper, or null.
- @r A raw pointer carrier or null when no pointer is present.
getLib ()

Returns the opened native library for this generated wrapper.
- @r The opened native library.
handle ()

Returns the wrapped NativeHandle.
- @r The wrapped NativeHandle.
isNull ()

Returns true when the wrapped handle is null.
- @r A bool.
describe ()

Returns a small string for debugging generated wrappers.
- @r A string.
broadcast ()

If threads are waiting for @cond, all of them are unblocked. If no threads are waiting for @cond, this function has no effect. It is good practice to lock the same mutex as the waiting threads while calling this function, though not required.
- @r None.
clear ()

Frees the resources allocated to a #GCond with g_cond_init(). This function should not be used with a #GCond that has been statically allocated. Calling g_cond_clear() for a #GCond on which threads are blocking leads to undefined behaviour.
- @r None.
free ()

Destroys a #GCond that has been created with g_cond_new(). Calling g_cond_free() for a #GCond on which threads are blocking leads to undefined behaviour.
- @r None.
init ()

Initialises a #GCond so that it can be used. This function is useful to initialise a #GCond that has been allocated as part of a larger structure. It is not necessary to initialise a #GCond that has been statically allocated. To undo the effect of g_cond_init() when a #GCond is no longer needed, use g_cond_clear(). Calling g_cond_init() on an already-initialised #GCond leads to undefined behaviour.
- @r None.
signal ()

If threads are waiting for @cond, at least one of them is unblocked. If no threads are waiting for @cond, this function has no effect. It is good practice to hold the same lock as the waiting thread while calling this function, though not required.
- @r None.
timed_wait (object mutex, object abs_time)

Waits until this thread is woken up on @cond, but not longer than until the time specified by @abs_time. The @mutex is unlocked before falling asleep and locked again before resuming. If @abs_time is %NULL, g_cond_timed_wait() acts like g_cond_wait(). This function can be used even if g_thread_init() has not yet been called, and, in that case, will immediately return %TRUE. To easily calculate @abs_time a combination of g_get_real_time() and g_time_val_add() can be used.
- @p mutex is a #GMutex that is currently locked.
- @p abs_time is a #GTimeVal, determining the final time.
- @r %TRUE if @cond was signalled, or %FALSE on timeout.
wait (object mutex)

Atomically releases @mutex and waits until @cond is signalled. When this function returns, @mutex is locked again and owned by the calling thread. When using condition variables, it is possible that a spurious wakeup may occur (ie: g_cond_wait() returns even though g_cond_signal() was not called). It's also possible that a stolen wakeup may occur. This is when g_cond_signal() is called, but another thread acquires @mutex before this thread and modifies the state of the program in such a way that when g_cond_wait() is able to return, the expected condition is no longer met. For this reason, g_cond_wait() must always be used in a loop. See the documentation for #GCond for a complete example.
- @p mutex is a #GMutex that is currently locked.
- @r None.
wait_until (object mutex, int end_time)

Waits until either @cond is signalled or @end_time has passed. As with g_cond_wait() it is possible that a spurious or stolen wakeup could occur. For that reason, waiting on a condition variable should always be in a loop, based on an explicitly-checked predicate. %TRUE is returned if the condition variable was signalled (or in the case of a spurious wakeup). %FALSE is returned if @end_time has passed. The following code shows how to correctly perform a timed wait on a condition variable (extending the example presented in the documentation for #GCond): |[ gpointer pop_data_timed (void) { gint64 end_time; gpointer data; g_mutex_lock (&data_mutex); end_time = g_get_monotonic_time () + 5 * G_TIME_SPAN_SECOND; while (!current_data) if (!g_cond_wait_until (&data_cond, &data_mutex, end_time)) { // timeout has passed. g_mutex_unlock (&data_mutex); return NULL; } // there is data for us data = current_data; current_data = NULL; g_mutex_unlock (&data_mutex); return data; } ]| Notice that the end time is calculated once, before entering the loop and reused. This is the motivation behind the use of absolute time on this API -- if a relative time of 5 seconds were passed directly to the call and a spurious wakeup occurred, the program would have to start over waiting again (which would lead to a total wait time of more than 5 seconds).
- @p mutex is a #GMutex that is currently locked.
- @p end_time is the monotonic time to wait until.
- @r %TRUE on a signal, %FALSE on a timeout.

class: GCondLayout

[12:14] static extends: object

Generated struct layout helper for GIR record Cond.

Methods

layout ()

Returns the Panama struct layout for Cond.