std::atomic_compare_exchange_weak, std::atomic_compare_exchange_strong, std::atomic_compare_exchange_weak_explicit, std::atomic_compare_exchange_strong_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>
	(1)
template< class T > bool atomic_compare_exchange_weak( std::atomic<T>* obj,                                    T* expected, T desired );			(since C++11)
template< class T > bool atomic_compare_exchange_weak( volatile std::atomic<T>* obj,                                    T* expected, T desired );			(since C++11)
		(2)
template< class T > bool atomic_compare_exchange_strong( std::atomic<T>* obj,                                      T* expected, T desired );				(since C++11)
template< class T > bool atomic_compare_exchange_strong( volatile std::atomic<T>* obj,                                      T* expected, T desired );				(since C++11)
			(3)
template< class T > bool atomic_compare_exchange_weak_explicit( std::atomic<T>* obj,                                             T* expected, T desired,                                             std::memory_order succ,                                             std::memory_order fail );					(since C++11)
template< class T > bool atomic_compare_exchange_weak_explicit( volatile std::atomic<T>* obj,                                             T* expected, T desired,                                             std::memory_order succ,                                             std::memory_order fail );					(since C++11)
				(4)
template< class T > bool atomic_compare_exchange_strong_explicit( std::atomic<T>* obj,                                               T* expected, T desired,                                               std::memory_order succ,                                               std::memory_order fail );						(since C++11)
template< class T > bool atomic_compare_exchange_strong_explicit( volatile std::atomic<T>* obj,                                               T* expected, T desired,                                               std::memory_order succ,                                               std::memory_order fail );						(since C++11)

Atomically compares the value pointed to by obj with the value pointed to by expected, and if those are equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual value pointed to by obj into *expected (performs load operation).

The memory models for the read-modify-write and load operations are succ and fail respectively. The (1-2) versions use std::memory_order_seq_cst by default.

The weak forms ((1) and (3)) of the functions are allowed to fail spuriously, that is, act as if *obj != *expected even if they are equal. When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.

These functions are defined in terms of member functions of std::atomic:

1) obj->compare_exchange_weak(*expected, desired)

2) obj->compare_exchange_strong(*expected, desired)

3) obj->compare_exchange_weak(*expected, desired, succ, fail)

4) obj->compare_exchange_strong(*expected, desired, succ, fail)

Parameters

obj	-	pointer to the atomic object to test and modify
expected	-	pointer to the value expected to be found in the atomic object
desired	-	the value to store in the atomic object if it is as expected
succ	-	the memory sycnhronization ordering for the read-modify-write operation if the comparison succeeds. All values are permitted.
fail	-	the memory sycnhronization ordering for the load operation if the comparison fails. Cannot be std::memory_order_release or std::memory_order_acq_rel and cannot specify stronger ordering than succ

Return value

The result of the comparison: true if *obj was equal to *exp, false otherwise.

Exceptions

noexcept specification:  

noexcept

  (since C++11)

Example

compare and exchange operations are often used as basic building blocks of lockfree data structures

run this code

#include <atomic>
template<typename T>
struct node
{
    T data;
    node* next;
    node(const T& data) : data(data), next(nullptr) {}
};
 
template<typename T>
class stack
{
    std::atomic<node<T>*> head;
 public:
    void push(const T& data)
    {
        node<T>* new_node = new node<T>(data);
 
        // put the current value of head into new_node->next
        new_node->next = head.load(std::memory_order_relaxed);
 
        // now make new_node the new head, but if the head
        // is no longer what's stored in new_node->next
        // (some other thread must have inserted a node just now)
        // then put that new head into new_node->next and try again
        while(!std::atomic_compare_exchange_weak_explicit(
                                &head,
                                &new_node->next,
                                new_node,
                                std::memory_order_release,
                                std::memory_order_relaxed))
                ; // the body of the loop is empty
    }
};
int main()
{
    stack<int> s;
    s.push(1);
    s.push(2);
    s.push(3);
}

See also

compare_exchange_weak compare_exchange_strong	atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (public member function of std::atomic)
atomic_exchange atomic_exchange_explicit (C++11) (C++11)	atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic (function template)
std::atomic_compare_exchange_weak(std::shared_ptr) std::atomic_compare_exchange_strong(std::shared_ptr)	specializes atomic operations for std::shared_ptr (function template)
C documentation for atomic_compare_exchange, atomic_compare_exchange_explicit