std::counting_semaphore, std::binary_semaphore

< cpp‎ | thread
Concurrency support library
this_thread namespace
Mutual exclusion
Generic lock management
Condition variables
Latches and Barriers
Safe Reclamation
Hazard Pointers

Atomic types
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
Defined in header <semaphore>
template< std::ptrdiff_t LeastMaxValue = /* implementation-defined */ >
class counting_semaphore;
(1) (since C++20)
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.


[edit] Data Members

Member name Definition
counter (private) The internal counter of type std::ptrdiff_t.
(exposition-only member object*)

[edit] Member functions

constructs a counting_semaphore
(public member function) [edit]
destructs the counting_semaphore
(public member function) [edit]
counting_semaphore is not assignable
(public member function) [edit]
increments the internal counter and unblocks acquirers
(public member function) [edit]
decrements the internal counter or blocks until it can
(public member function) [edit]
tries to decrement the internal counter without blocking
(public member function) [edit]
tries to decrement the internal counter, blocking for up to a duration time
(public member function) [edit]
tries to decrement the internal counter, blocking until a point in time
(public member function) [edit]
returns the maximum possible value of the internal counter
(public static member function) [edit]

[edit] 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

[edit] Example

#include <chrono>
#include <iostream>
#include <semaphore>
#include <thread>
// global binary semaphore instances
// object counts are set to zero
// objects are in non-signaled state
void ThreadProc()
    // wait for a signal from the main proc
    // by attempting to decrement the semaphore
    // 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::cout << "[thread] Send the signal\n"; // message
    // signal the main proc back
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
    // wait until the worker thread is done doing the work
    // by attempting to decrement the semaphore's count
    std::cout << "[main] Got the signal\n"; // response message


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