Stdatomic Flag Test And Set Stdatomic Flag Test And Set Explicit Function In C++

The std::atomicflagtestandset and std::atomicflagtestandsetexplicit functions in C++ are part of the <atomic> library and are fundamental in implementing lock-free, thread-safe operations. These functions operate on a std::atomicflag, which is a simple atomic type designed explicitly for boolean flags with two possible states: set and clear. This is useful for implementing basic spinlocks.

std::atomic_flag_test_and_set:

Purpose: This function is used to check if the flag is currently set (true), and if not, it sets the flag. This all happens atomically, which means that once one thread modifies the flag, no other thread can do so simultaneously. This is essential for ensuring that the function's operations are thread-safe.

Usage: flag.testandset

Return Value: It returns the previous value of the flag (either true or false). If the flag is already set, the function will return true. If it is clear, it will set the flag to true and return false.

Example Use Case: A typical use case for std::atomicflagtestandset is in a spinlock. A thread can repeatedly call testandset in a loop, waiting until the function returns false, indicating that it successfully set the flag and "locked" the resource.

Example Code:

#include <atomic>
#include <thread>
#include <iostream>
std::atomic_flag lock_flag = ATOMIC_FLAG_INIT;
void critical_section() {
    while (lock_flag.test_and_set(std::memory_order_acquire)) {
        // Wait until the lock is acquired (spinning).
    }
    // Critical section code.
    std::cout << "In critical section\n";
    lock_flag.clear(std::memory_order_release); // Release the lock
}
int main() {
    std::thread t1(critical_section);
    std::thread t2(critical_section);
    t1.join();
    t2.join();
}

Output:

In critical section
In critical section

Explanation:

In this example, testandset is used to acquire a lock. If the flag is already set, it keeps looping (spinning) until another thread clears the flag. When the critical section finishes, lock_flag.clear releases the lock, allowing other threads to enter.

std::atomic_flag_test_and_set_explicit

Purpose: The std::atomicflagtestandsetexplicit is a more advanced version of testand_set, allowing the programmer to specify the memory ordering. Memory ordering affects how operations on the flag are perceived across different threads, which can impact performance and correctness in multithreaded programs.

Usage: flag.testandset_explicit(order)

Parameters:

• ✅ Order: This is the memory order for the operation, and it can be one of several options:
• ✅ std::memoryorderrelaxed: No synchronization or ordering constraints.
• ✅ std::memoryorderacquire: Ensures that operations after the acquire in the current thread are not moved before it.
• ✅ std::memoryorderrelease: Ensures that all operations before the release in the current thread are not moved after it.
• ✅ std::memoryorderacq_rel: Both acquire and release semantics.
• ✅ std::memoryorderseq_cst: Ensures sequential consistency among threads.

Return Value: Like testandset, it returns the previous state of the flag (true if it was already set, false if it was clear).

Example Code:

#include <atomic>
#include <thread>
#include <iostream>
std::atomic_flag lock_flag = ATOMIC_FLAG_INIT;
void critical_section() {
    while (lock_flag.test_and_set_explicit(std::memory_order_acquire)) {
        // Spin until lock is acquired.
    }
    // Critical section code.
    std::cout << "In critical section\n";
    lock_flag.clear(std::memory_order_release); // Release the lock
}
int main() {
    std::thread t1(critical_section);
    std::thread t2(critical_section);
    t1.join();
    t2.join();
}

Key differences between std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit function in C++ :

Some main differences between these functions are as follows: