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std::atomic_flag

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< cpp‎ | atomic
Revision as of 08:01, 22 November 2012 by 91.82.100.58 (Talk)

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<td>
Defined in header <atomic>
</td>

<td></td> <td></td> <tr class="t-dcl ">

<td >
class atomic_flag;
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<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> Template:ddcl list end

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

Template:cpp/atomic/atomic flag/dcl list clearTemplate:cpp/atomic/atomic flag/dcl list test and set
constructs an atomic_flag
(public member function)
the assignment operator
(public member function)

Example

A spinlock mutex can be implemented in userspace using an atomic_flag

#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 (lock.test_and_set(std::memory_order_acquire))  // acquire lock
             ; // spin
        std::cout << "Output from thread " << n << '\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();
    }
}

Output:

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

See also

Template:cpp/atomic/dcl list atomic flag test and setTemplate:cpp/atomic/dcl list atomic flag clearTemplate:cpp/atomic/dcl list ATOMIC FLAG INIT
C documentation for atomic_flag