std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Atomic operations library

Types

atomic (C++11)

atomic_ref (C++20)

Functions

atomic_storeatomic_store_explicit (C++11)(C++11)

atomic_loadatomic_load_explicit (C++11)(C++11)

atomic_exchangeatomic_exchange_explicit (C++11)(C++11)

atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11)

atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11)

atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11)

atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11)

atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11)

atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11)

atomic_is_lock_free (C++11)

Atomic flags

atomic_flag (C++11)

atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11)

atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11)

Initialization

atomic_init (C++11)

ATOMIC_VAR_INIT (C++11)

ATOMIC_FLAG_INIT (C++11)

Memory ordering

memory_order (C++11)

kill_dependency (C++11)

atomic_thread_fence (C++11)

atomic_signal_fence (C++11)

Defined in header <atomic>
	(1)	(since C++11)
bool atomic_flag_test_and_set( volatile std::atomic_flag* p ) noexcept;
bool atomic_flag_test_and_set( std::atomic_flag* p ) noexcept;
			(2)	(since C++11)
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* p, std::memory_order order ) noexcept;
bool atomic_flag_test_and_set_explicit( std::atomic_flag* p, std::memory_order order ) noexcept;

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

Parameters

p	-	pointer to std::atomic_flag to access
order	-	the memory synchronization order for this operation

Return value

The value previously held by the flag pointed to by p

Possible implementation

First version
bool atomic_flag_test_and_set(volatile std::atomic_flag* p) { return p->test_and_set(); }
Second version
bool atomic_flag_test_and_set(std::atomic_flag* p) { return p->test_and_set(); }
Third version
bool atomic_flag_test_and_set_explicit(volatile std::atomic_flag* p, std::memory_order order) { return p->test_and_set(order); }
Fourth version
bool atomic_flag_test_and_set_explicit(std::atomic_flag* p, std::memory_order order) { return p->test_and_set(order); }

Example

A spinlock mutex can be implemented in userspace using an atomic_flag

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 < 100; ++cnt) {
        while(std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire))
             ; // spin until the lock is acquired
        std::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>...

See also

atomic_flag (C++11)	the lock-free boolean atomic type (class)
atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11)	atomically sets the value of the flag to false (function)
memory_order (C++11)	defines memory ordering constraints for the given atomic operation (enum)