Package ch.bailu.gtk.glib

Class Cond

java.lang.Object
ch.bailu.gtk.type.Type
ch.bailu.gtk.type.Pointer
ch.bailu.gtk.type.Record
ch.bailu.gtk.glib.Cond
All Implemented Interfaces:
PointerInterface
public class Cond extends Record
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: 
<!-- language="C" -->
   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.

https://docs.gtk.org/glib/struct.Cond.html

  • Field Details

  • Constructor Details

  • Method Details

    • getClassHandler

      public static ClassHandler getClassHandler()

    • getFieldP

      public Pointer getFieldP()

    • broadcast

      public void 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.

    • clear

      public void 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.

    • init

      public void 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.

    • signal

      public void 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.

    • wait

      public void wait(@Nonnull Mutex 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.
      Parameters:
      mutex - a #GMutex that is currently locked

    • waitUntil

      public boolean waitUntil(@Nonnull Mutex mutex, long 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): 
      <!-- language="C" -->
 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).
      Parameters:
      mutex - a #GMutex that is currently locked
      end_time - the monotonic time to wait until
      Returns:
      %TRUE on a signal, %FALSE on a timeout