atomic_flag.h (1482B)
1 // Copyright 2011 The Chromium Authors 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef BASE_SYNCHRONIZATION_ATOMIC_FLAG_H_ 6 #define BASE_SYNCHRONIZATION_ATOMIC_FLAG_H_ 7 8 #include <stdint.h> 9 10 #include <atomic> 11 12 #include "base/base_export.h" 13 #include "base/sequence_checker.h" 14 15 namespace base { 16 17 // A flag that can safely be set from one thread and read from other threads. 18 // 19 // This class IS NOT intended for synchronization between threads. 20 class BASE_EXPORT AtomicFlag { 21 public: 22 AtomicFlag(); 23 24 AtomicFlag(const AtomicFlag&) = delete; 25 AtomicFlag& operator=(const AtomicFlag&) = delete; 26 27 ~AtomicFlag(); 28 29 // Set the flag. Must always be called from the same sequence. 30 void Set(); 31 32 // Returns true iff the flag was set. If this returns true, the current thread 33 // is guaranteed to be synchronized with all memory operations on the sequence 34 // which invoked Set() up until at least the first call to Set() on it. 35 bool IsSet() const { 36 // Inline here: this has a measurable performance impact on base::WeakPtr. 37 return flag_.load(std::memory_order_acquire) != 0; 38 } 39 40 // Resets the flag. Be careful when using this: callers might not expect 41 // IsSet() to return false after returning true once. 42 void UnsafeResetForTesting(); 43 44 private: 45 std::atomic<uint_fast8_t> flag_{0}; 46 SEQUENCE_CHECKER(set_sequence_checker_); 47 }; 48 49 } // namespace base 50 51 #endif // BASE_SYNCHRONIZATION_ATOMIC_FLAG_H_