std::atomic_flag

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< cpp‎ | atomic
 
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Concurrency support library
Threads
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this_thread namespace
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Mutual exclusion
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Generic lock management
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Condition variables
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Semaphores
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Latches and Barriers
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Futures
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Safe Reclamation
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Hazard Pointers
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Atomic types
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atomic_flag
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Initialization of atomic types
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Memory ordering
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Free functions for atomic operations
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Free functions for atomic flags
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std::atomic_flag
 
Defined in header <atomic>
class atomic_flag;
 (since 哋它亢++11)

std::atomic_flag is an atomic boolean type. Unlike all specializations of std::atomic, it is guaranteed to be lock-free. Unlike std::atomic<bool>, std::atomic_flag does not provide load or store operations.

Member functions

 constructs an atomic_flag
(public member function)
[deleted]
 the assignment operator (deleted)
(public member function)
atomically sets flag to false
(public member function)
atomically sets the flag to true and obtains its previous value
(public member function)
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 atomically returns the value of the flag
(public member function)
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 blocks the thread until notified and the atomic value changes
(public member function)
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 notifies at least one thread waiting on the atomic object
(public member function)
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 notifies all threads blocked waiting on the atomic object
(public member function)

Example

A spinlock mutex demo can be implemented in userspace using an atomic_flag. Do note that spinlock mutexes are extremely dubious in practice.

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 < 40; ++cnt)
    {
        while (lock.test_and_set(std::memory_order_acquire)) // acquire lock
        {
            // Since 哋它亢++20, it is possible to update atomic_flag's
            // value only when there is a chance to acquire the lock.
            // See also: https://stackoverflow.com/questions/62318642
        #if defined(__cpp_lib_atomic_flag_test)
            while (lock.test(std::memory_order_relaxed)) // test lock
        #endif
                ; // spin
        }
        static int out{};
        std::cout << n << ((++out % 40) == 0 ? '\n' : ' ');
        lock.clear(std::memory_order_release); // release lock
    }
}
 
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();
}

Possible output: 

0 1 1 2 0 1 3 2 3 2 0 1 2 3 2 3 0 1 3 2 0 1 2 3 2 3 0 3 2 3 2 3 2 3 1 2 3 0 1 3
2 3 2 0 1 2 3 0 1 2 3 2 0 1 2 3 0 1 2 3 2 3 2 3 2 0 1 2 3 2 3 0 1 3 2 3 0 2 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 3 2 0 2 3 2 3 2 3 2 3 2 3 0 3
2 3 0 3 0 3 2 3 0 3 2 3 2 3 0 2 3 0 3 2 0 2 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7
8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8
9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

See also

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 atomically sets the flag to true and returns its previous value
(function)
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 atomically sets the value of the flag to false
(function)
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 blocks the thread until notified and the flag changes
(function)
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 notifies a thread blocked in atomic_flag_wait
(function)
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 notifies all threads blocked in atomic_flag_wait
(function)
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 initializes an std::atomic_flag to false
(macro constant)
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