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      std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit

      From cppreference.com
      < cpp‎ | atomic
       
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      Free functions for atomic flags
      atomic_flag_test_and_setatomic_flag_test_and_set_explicit
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      Defined in header <atomic>
      bool atomic_flag_test_and_set(volatilestd::atomic_flag* obj )noexcept;
      		 (1) (since C++11)
      bool atomic_flag_test_and_set(std::atomic_flag* obj )noexcept;
      		 (2) (since C++11)
      bool atomic_flag_test_and_set_explicit(volatilestd::atomic_flag* obj,
                                              std::memory_order order )noexcept;
      		 (3) (since C++11)
      bool atomic_flag_test_and_set_explicit(std::atomic_flag* obj,
                                              std::memory_order order )noexcept;
      		 (4) (since C++11)

      Atomically changes the state of a std::atomic_flag pointed to by obj to set (true) and returns the value it held before.

      1,2) The memory synchronization order is std::memory_order_seq_cst.
      3,4) The memory synchronization order is order.

      Contents

      [edit]Parameters

      obj - pointer to std::atomic_flag to access
      order - the memory synchronization order

      [edit]Return value

      The value previously held by the flag pointed to by obj.

      [edit]Notes

      std::atomic_flag_test_and_set and std::atomic_flag_test_and_set_explicit can be implemented as obj->test_and_set() and obj->test_and_set(order) respectively.

      [edit]Example

      A spinlock mutex can be implemented in userspace using an std::atomic_flag.

      Run this code
      #include <atomic>#include <iostream>#include <thread>#include <vector>   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));// spin until the lock is acquiredstd::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();}

      Output: 

      Output from thread 2 Output from thread 6 Output from thread 7 ...<exactly 1000 lines>...

      [edit]See also

      (C++11)
      		 the lock-free boolean atomic type
      (class)[edit]
      (C++11)(C++11)
      		 atomically sets the value of the flag to false
      (function)[edit]
      (C++11)
      		 defines memory ordering constraints for the given atomic operation
      (enum)[edit]
      C documentation for atomic_flag_test_and_set, atomic_flag_test_and_set_explicit
      Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_flag_test_and_set&oldid=159553"
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