Difference between revisions of "cpp/thread/counting semaphore"

Revision as of 06:51, 9 October 2022

A counting_semaphore contains an internal counter initialized by the constructor. This counter is decremented by calls to acquire() and related methods, and is incremented by calls to release(). When the counter is zero, acquire() blocks until the counter is incremented, but try_acquire() does not block; try_acquire_for() and try_acquire_until() block until the counter is incremented or a timeout is reached.

Similar to std::condition_variable::wait(), counting_semaphore's try_acquire() can spuriously fail.

Specializations of std::counting_semaphore are not DefaultConstructible, CopyConstructible, MoveConstructible, CopyAssignable, or MoveAssignable.

Member functions

Notes

As its name indicates, the LeastMaxValue is the minimum max value, not the actual max value. Thus max() can yield a number larger than LeastMaxValue.

Unlike std::mutex a counting_semaphore is not tied to threads of execution - acquiring a semaphore can occur on a different thread than releasing the semaphore, for example. All operations on counting_semaphore can be performed concurrently and without any relation to specific threads of execution, with the exception of the destructor which cannot be performed concurrently but can be performed on a different thread.

Semaphores are also often used for the semantics of signalling/notifying rather than mutual exclusion, by initializing the semaphore with ​0​ and thus blocking the receiver(s) that try to acquire(), until the notifier "signals" by invoking release(n). In this respect semaphores can be considered alternatives to std::condition_variables, often with better performance.

Example

#include <iostream>
#include <thread>
#include <chrono>
#include <semaphore> // Note: This does not work on Linux, so don't bother trying
 
 
// global binary semaphore instances
// object counts are set to zero
// objects are in non-signaled state
std::binary_semaphore
	smphSignalMainToThread{0},
	smphSignalThreadToMain{0};
 
void ThreadProc()
{	
	// wait for a signal from the main proc
	// by attempting to decrement the semaphore
	smphSignalMainToThread.acquire();
 
	// this call blocks until the semaphore's count
	// is increased from the main proc
 
	std::cout << "[thread] Got the signal\n"; // response message
 
	// wait for 3 seconds to imitate some work
	// being done by the thread
	using namespace std::literals;
	std::this_thread::sleep_for(3s);
 
	std::cout << "[thread] Send the signal\n"; // message
 
	// signal the main proc back
	smphSignalThreadToMain.release();
}
 
int main()
{
	// create some worker thread
	std::thread thrWorker(ThreadProc);
 
	std::cout << "[main] Send the signal\n"; // message
 
	// signal the worker thread to start working
	// by increasing the semaphore's count
	smphSignalMainToThread.release();
 
	// wait until the worker thread is done doing the work
	// by attempting to decrement the semaphore's count
	smphSignalThreadToMain.acquire();
 
	std::cout << "[main] Got the signal\n"; // response message
	thrWorker.join();
}

[main] Send the signal
[thread] Got the signal
[thread] Send the signal
[main] Got the signal