NAME¶

std::counting_semaphore,std::binary_semaphore - std::counting_semaphore,std::binary_semaphore

Synopsis¶

Defined in header <semaphore>
template< std::ptrdiff_t LeastMaxValue = /* implementation-defined
*/ > (1) (since C++20)
class counting_semaphore;
using binary_semaphore = std::counting_semaphore<1>; (2) (since C++20)

1) A counting_semaphore is a lightweight synchronization primitive that can control
access to a shared resource. Unlike a std::mutex, a counting_semaphore allows more
than one concurrent access to the same resource, for at least LeastMaxValue
concurrent accessors. The program is ill-formed if LeastMaxValue is negative.
2) binary_semaphore is an alias for specialization of std::counting_semaphore with
LeastMaxValue being 1. Implementations may implement binary_semaphore more
efficiently than the default implementation of std::counting_semaphore.

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¶

constructor constructs a counting_semaphore
(public member function)
destructor destructs the counting_semaphore
(public member function)
operator= counting_semaphore is not assignable
[deleted] (public member function)

Operations¶

release increments the internal counter and unblocks acquirers
(public member function)
acquire decrements the internal counter or blocks until it can
(public member function)
try_acquire tries to decrement the internal counter without blocking
(public member function)
tries to decrement the internal counter, blocking for up to a
try_acquire_for duration time
(public member function)
tries to decrement the internal counter, blocking until a point in
try_acquire_until time
(public member function)

Constants¶

max returns the maximum possible value of the internal counter
[static] (public static member function)

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 signaling/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.

Feature-test macro Value Std Feature
__cpp_lib_semaphore 201907L (C++20) std::counting_semaphore, std::binary_semaphore

Example¶

// Run this code

#include <chrono>
#include <iostream>
#include <semaphore>
#include <thread>

// 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();
}

Output:¶

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