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Difference between revisions of "cpp/atomic/atomic flag"

From cppreference.com
< cpp‎ | atomic
(missed that this was refactored in december. back to non-degenerate spinlock use)
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−
{{dsc mem fun  | cpp/atomic/atomic_flag/operator{{=}} | the assignment operator }}
+
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{{dsc inc | cpp/atomic/atomic_flag/dsc clear}}
 
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Revision as of 09:10, 15 August 2023

 
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Free functions for atomic flags
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std::atomic_flag
 
Defined in header <atomic>
class atomic_flag;
 (since C++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)
the assignment operator (deleted)
(public member function)
atomically sets flag to false
(public member function) [edit]
atomically sets the flag to true and obtains its previous value
(public member function) [edit]
(C++20)
 atomically returns the value of the flag
(public member function) [edit]
(C++20)
 blocks the thread until notified and the atomic value changes
(public member function) [edit]
(C++20)
 notifies at least one thread waiting on the atomic object
(public member function) [edit]
(C++20)
 notifies all threads blocked waiting on the atomic object
(public member function) [edit]

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 <thread>
#include <vector>
#include <iostream>
#include <atomic>
 
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 C++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

(C++11)(C++11)
 atomically sets the flag to true and returns its previous value
(function) [edit]
(C++11)(C++11)
 atomically sets the value of the flag to false
(function) [edit]
(C++20)(C++20)
 blocks the thread until notified and the flag changes
(function) [edit]
(C++20)
 notifies a thread blocked in atomic_flag_wait
(function) [edit]
(C++20)
 notifies all threads blocked in atomic_flag_wait
(function) [edit]
(C++11)
 initializes an std::atomic_flag to false
(macro constant) [edit]
C documentation for atomic_flag
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