std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit

 

C++

Language

Standard library headers

Concepts

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)

 

Atomic operations library

Types

atomic (C++11)

atomic_is_lock_free (C++11)

Functions

atomic_store atomic_store_explicit (C++11) (C++11)

atomic_load atomic_load_explicit (C++11) (C++11)

atomic_exchange atomic_exchange_explicit (C++11) (C++11)

atomic_compare_exchange_weak atomic_compare_exchange_weak_explicit atomic_compare_exchange_strong atomic_compare_exchange_strong_explicit (C++11) (C++11) (C++11) (C++11)

atomic_fetch_add atomic_fetch_add_explicit (C++11) (C++11)

atomic_fetch_sub atomic_fetch_sub_explicit (C++11) (C++11)

atomic_fetch_and atomic_fetch_and_explicit (C++11) (C++11)

atomic_fetch_or atomic_fetch_or_explicit (C++11) (C++11)

atomic_fetch_xor atomic_fetch_xor_explicit (C++11) (C++11)

Atomic flags

atomic_flag (C++11)

atomic_flag_test_and_set atomic_flag_test_and_set_explicit (C++11) (C++11)

atomic_flag_clear atomic_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>
bool atomic_flag_test_and_set( volatile std::atomic_flag* p );	(1)	(since C++11)
bool atomic_flag_test_and_set( std::atomic_flag* p );	(2)	(since C++11)
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* p,                                         std::memory_order order );	(3)	(since C++11)
bool atomic_flag_test_and_set_explicit( std::atomic_flag* p,                                         std::memory_order order );	(4)	(since C++11)

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

Exceptions

noexcept specification:  

noexcept

  (since C++11)

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_clear atomic_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 (typedef)