| Defined in header <atomic> | ||
|---|---|---|
| (1) | (since C++11) | |
bool atomic_flag_test_and_set( volatile std::atomic_flag* p ); | ||
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 ); | ||
bool atomic_flag_test_and_set_explicit( std::atomic_flag* p,
std::memory_order order ); |
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: noexceptPossible 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.
#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
| (C++11) | the lock-free boolean atomic type (class) |
| (C++11)(C++11) | atomically sets the value of the flag to false (function) |
| (C++11) | defines memory ordering constraints for the given atomic operation (typedef) |
C documentation for atomic_flag_test_and_set, atomic_flag_test_and_set_explicit | |
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