C++ barrier::arrive() Function

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The std::barrier::arrive() function in C++, is a synchronization mechanism introduced in C++20 that helps to coordinate multiple threads. It provides a way for threads to reach a specific point in execution.

When a thread calls arrive(), it signals that it has reached the barrier. The barrier waits until a predefined number of threads have called arrive(), then proceeds to release all waiting threads simultaneously, allowing them to continue execution.

Syntax

Following is the syntax for std::barrier::arrive() function.

 arrival_token arrive( std::ptrdiff_t n = 1 ); 

Parameters

• n − It indicates the value by which the expected count is decreased.

Return value

This function returns the constructed arrival_token object.

Exception

It throws std::system_error with an error code allowed for mutex types on error.

Example 1

In the following example, we are going to use the arrive() to signal the completion of each thread work before they proceed to next phase.

 #include <iostream> #include <thread> #include <barrier> #include <vector> void x(std::barrier < > & sync_point, int y) { std::cout << "Thread " << y << " started working.\n"; std::this_thread::sleep_for(std::chrono::milliseconds(200 * y)); std::cout << "Thread " << y << " completed work and arrives.\n"; sync_point.arrive(); } int main() { const int a = 2; std::barrier sync_point(a); std::vector < std::thread > b; for (int i = 0; i < a; ++i) { b.emplace_back(x, std::ref(sync_point), i + 1); } for (auto & t: b) { t.join(); } std::cout << "All threads arrived at the barrier.\n"; return 0; } 

Output

Output of the above code is as follows −

 Thread 2 started working. Thread 1 started working. Thread 1 completed work and arrives. Thread 2 completed work and arrives. All threads arrived at the barrier. 

Example 2

Consider the following example, where we are going to split the task into phases and each phase requires threads to arrive at the barrier before proceeding.

 #include <iostream> #include <thread> #include <barrier> void a(std::barrier < > & sync_point, int b) { std::cout << "Starting phase " << b << "\n"; sync_point.arrive(); std::cout << "Finished phase " << b << "\n"; } int main() { std::barrier sync_point(2); for (int i = 0; i < 2; ++i) { std::thread(a, std::ref(sync_point), i + 1).detach(); } std::this_thread::sleep_for(std::chrono::seconds(1)); return 0; } 

Output

Following is the output of the above code −

 Starting phase 1 Finished phase 1 Starting phase 2 Finished phase 2