C++ Atomic Library - flag_test_and_set

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Description

It atomically sets the flag to true and returns its previous value.

Declaration

Following is the declaration for std::atomic_flag_test_and_set.

bool atomic_flag_test_and_set( std::atomic_flag* p );

C++11

bool atomic_flag_test_and_set( volatile std::atomic_flag* p );

Following is the declaration for std::atomic_flag_test_and_set_explicit.

bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* p, 
                                        std::memory_order order );

C++11

bool atomic_flag_test_and_set_explicit( std::atomic_flag* p, 
                                        std::memory_order order );

Parameters

• p − It is used in pointer to std::atomic_flag to access.

• order − It is used to synchronise the memory ordering for this operation.

Return Value

It returns the value previously held by the flag pointed to by p.

Exceptions

No-noexcept − this member function never throws exceptions.

Example

In below example for std::atomic_flag_test_and_set_explicit.

#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
 
std::atomic_flag lock = ATOMIC_FLAG_INIT;
 
void f(int n) {
   for (int cnt = 0; cnt < 100; ++cnt) {
      while(std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire))
         ; 
      std::cout << "Output from thread " << n << '\n';
      std::atomic_flag_clear_explicit(&lock, std::memory_order_release);
   }
}
 
int main() {
   std::vector<std::thread> v;
   for (int n = 0; n < 10; ++n) {
      v.emplace_back(f, n);
   }
   for (auto& t : v) {
      t.join();
   }
}

The sample output should be like this −

Output from thread 0
Output from thread 1
Output from thread 0
Output from thread 0
Output from thread 1
Output from thread 1
Output from thread 0
Output from thread 0
Output from thread 1
Output from thread 0
....................