The C Standard Library

C++ Library - <semaphore>

Quiz

The <semaphore> header in C++20, is used to control access to a shared resource by multiple threads. It is used in scenarios where to limit the number of threads that can access a section or shared resources simultaneously.

The <semaphore> can be in two states; binary semaphore or counting semaphore. Where a binary_semaphore allows a single thread to access a resource at a time, while counting_semaphore allows up to a certain number of threads. When the count reaches zero, the additional threads becomes blocked until the count gets greater than zero again.

The counting_semaphore holds an integer value, while binary_semaphore is limited to either 0 or 1

Including <semaphore> Header

To include the <semaphore> header in your C++ program, you can use the following syntax.

#include <semaphore>

Functions of <semaphore> Header

Below is list of all functions from <semaphore> header.

Sr.No	Functions & Description
1	release It increments the internal counter and unblocks acquirers.
2	acquire It decrements the internal counter or blocks until it can.
3	try_acquire It tries to decrement the internal counter without blocking.
4	try_acquire_for It tries to decrement the internal counter, blocking for up to a duration time.
5	try_acquire_until It tries to decrement the internal counter, blocking until a point in time.
5	max It returns the maximum possible value of the internal counter.

Controlling the Access

In the following example, we are going to use the counting semaphore to limit access to a shared resource to a maximum of two threads at a time.

#include <iostream>
#include <semaphore>
#include <thread>
std::counting_semaphore < 2 > sem(2);
void a(int b) {
   sem.acquire();
   std::cout << "A" << b << " Accessing Resource.\n";
   std::this_thread::sleep_for(std::chrono::seconds(1));
   std::cout << "A" << b << " Releasing Resource.\n";
   sem.release();
}
int main() {
   std::thread x1(a, 1);
   std::thread x2(a, 2);
   std::thread x3(a, 3);
   x1.join();
   x2.join();
   x3.join();
   return 0;
}

Output

Output of the above code is as follows −

A1 Accessing Resource.
A2 Accessing Resource.
A1 Releasing Resource.
A3 Accessing Resource.
A2 Releasing Resource.
A3 Releasing Resource.

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