Glib::Threads::Cond Class Reference

An opaque data structure to represent a condition. More...

#include <glibmm/threads.h>

Public Member Functions
	Cond ()
	Cond (const Cond &)=delete
Cond &	operator= (const Cond &)=delete
	~Cond ()
void	signal ()
	If threads are waiting for this Cond, exactly one of them is woken up. More...
void	broadcast ()
	If threads are waiting for this Cond, all of them are woken up. More...
void	wait (Mutex & mutex)
	Waits until this thread is woken up on this Cond. More...
bool	wait_until (Mutex & mutex, gint64 end_time)
	Waits until this thread is woken up on this Cond, but not longer than until the time specified by end_time. More...
GCond *	gobj ()

Detailed Description

An opaque data structure to represent a condition.

A Cond is an object that threads can block on, if they find a certain condition to be false. If other threads change the state of this condition they can signal the Cond, such that the waiting thread is woken up.

Deprecated:

Please use std::condition_variable instead.

Usage example:

Glib::Threads::Cond  data_cond;
Glib::Threads::Mutex data_mutex;
void* current_data = nullptr;
 
void push_data(void* data)
{
  Glib::Threads::Mutex::Lock lock(data_mutex);
 
  current_data = data;
  data_cond.signal();
}
 
void* pop_data()
{
  Glib::Threads::Mutex::Lock lock(data_mutex);
 
  while (!current_data)
    data_cond.wait(data_mutex);
 
  void* const data = current_data;
  current_data = nullptr;
 
  return data;
}

Constructor & Destructor Documentation

Cond() [1/2]

Glib::Threads::Cond::Cond

(

)

Cond() [2/2]

Glib::Threads::Cond::Cond ( const Cond &  )

delete

~Cond()

Glib::Threads::Cond::~Cond

(

)

Member Function Documentation

broadcast()

void Glib::Threads::Cond::broadcast

(

)

If threads are waiting for this Cond, all of them are woken up.

It is good practice to hold the same lock as the waiting threads, while calling this method, though not required.

gobj()

GCond * Glib::Threads::Cond::gobj ( )

inline

operator=()

Cond & Glib::Threads::Cond::operator= ( const Cond &  )

delete

signal()

void Glib::Threads::Cond::signal

(

)

If threads are waiting for this Cond, exactly one of them is woken up.

It is good practice to hold the same lock as the waiting thread, while calling this method, though not required.

wait()

void Glib::Threads::Cond::wait

(

Mutex &

mutex

)

Waits until this thread is woken up on this Cond.

The mutex is unlocked before falling asleep and locked again before resuming.

Parameters

mutex

A Mutex that is currently locked.

Note

It is important to use the wait() and wait_until() methods only inside a loop, which checks for the condition to be true as it is not guaranteed that the waiting thread will find it fulfilled, even if the signaling thread left the condition in that state. This is because another thread can have altered the condition, before the waiting thread got the chance to be woken up, even if the condition itself is protected by a Mutex.

wait_until()

bool Glib::Threads::Cond::wait_until	(	Mutex &	mutex,
		gint64	end_time
	)

Waits until this thread is woken up on this Cond, but not longer than until the time specified by end_time.

The mutex is unlocked before falling asleep and locked again before resuming.

Usage example:

Extending the example presented in the documentation of class Cond.

void* pop_data_timed()
{
  Glib::Threads::Mutex::Lock lock(data_mutex);
 
  // Wait at most 5 seconds.
  const gint64 end_time = g_get_monotonic_time() + 5 * G_TIME_SPAN_SECOND;
  while (!current_data)
    if (!data_cond.wait_until(data_mutex, end_time)
      return nullptr; // timeout
 
  void* const data = current_data;
  current_data = nullptr;
 
  return data;
}

The end time is calculated once, before entering the loop, and reused. This is the motivation behind the use of absolute time. 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 Mutex that is currently locked.
end_time	The monotonic time to wait until, in microseconds. See g_get_monotonic_time().

Returns

true if the condition variable was signalled (or in the case of a spurious wakeup), false if end_time has passed.

Note

It is important to use the wait() and wait_until() methods only inside a loop, which checks for the condition to be true as it is not guaranteed that the waiting thread will find it fulfilled, even if the signaling thread left the condition in that state. This is because another thread can have altered the condition, before the waiting thread got the chance to be woken up, even if the condition itself is protected by a Mutex.