std::atomic::compare_exchange_weak, std::atomic::compare_exchange_strong

 

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)

Filesystem library (C++17)

Technical Specifications

 

Atomic operations library

Types

atomic (C++11)

atomic_is_lock_free (C++11)

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 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)

 

std::atomic

Member functions

atomic::atomic

atomic::operator=

atomic::is_lock_free

atomic::store

atomic::load

atomic::operator T

atomic::exchange

atomic::compare_exchange

Specialized member functions

atomic::fetch_add

atomic::fetch_sub

atomic::fetch_and

atomic::fetch_or

atomic::fetch_xor

atomic::operator++atomic::operator++(int)atomic::operator--atomic::operator--(int)

atomic::operator+=atomic::operator-=atomic::operator&=atomic::operator|=atomic::operator^=

 

Defined in header <atomic>
	(1)	(since C++11)
bool compare_exchange_weak( T& expected, T desired,                             std::memory_order success,                             std::memory_order failure );
bool compare_exchange_weak( T& expected, T desired,                             std::memory_order success,                             std::memory_order failure ) volatile;
			(2)	(since C++11)
bool compare_exchange_weak( T& expected, T desired,                             std::memory_order order =                                 std::memory_order_seq_cst );
bool compare_exchange_weak( T& expected, T desired,                             std::memory_order order =                                 std::memory_order_seq_cst ) volatile;
					(3)	(since C++11)
bool compare_exchange_strong( T& expected, T desired,                               std::memory_order success,                               std::memory_order failure );
bool compare_exchange_strong( T& expected, T desired,                               std::memory_order success,                               std::memory_order failure ) volatile;
							(4)	(since C++11)
bool compare_exchange_strong( T& expected, T desired,                               std::memory_order order =                                   std::memory_order_seq_cst );
bool compare_exchange_strong( T& expected, T desired,                               std::memory_order order =                                   std::memory_order_seq_cst ) volatile;

Atomically compares the object representation of *this with the object representation of expected, as if by std::memcmp, and if those are bitwise-equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual value stored in *this into expected (performs load operation). Copying is performed as if by std::memcpy.

The memory models for the read-modify-write and load operations are success and failure respectively. In the (2) and (4) versions order is used for both read-modify-write and load operations, except that std::memory_order_acquire and std::memory_order_relaxed are used for the load operation if order == std::memory_order_acq_rel, or order == std::memory_order_release respectively.

Parameters

expected	-	reference 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
success	-	the memory synchronization ordering for the read-modify-write operation if the comparison succeeds. All values are permitted.
failure	-	the memory synchronization 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 success (until C++17)
order	-	the memory synchronization ordering for both operations

Return value

true if the underlying atomic value was successfully changed, false otherwise.

Exceptions

noexcept specification:  

noexcept

  

Notes

The weak forms (1-2) of the functions are allowed to fail spuriously, that is, act as if *this != 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 unless the object representation of T may include padding bits, trap bits, or offers multiple object representations for the same value (e.g. floating-point NaN). In those cases, weak compare-and-exchange typically works because it quickly converges on some stable object representation.

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(!head.compare_exchange_weak(new_node->next, new_node,
                                        std::memory_order_release,
                                        std::memory_order_relaxed))
          ; // the body of the loop is empty
 
// Note: the above use is not thread-safe in at least 
// GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
// MSVC prior to 2014-03-17 (bug 819819). The following is a workaround:
//      node<T>* old_head = head.load(std::memory_order_relaxed);
//      do {
//          new_node->next = old_head;
//       } while(!head.compare_exchange_weak(old_head, new_node,
//                                           std::memory_order_release,
//                                           std::memory_order_relaxed));
    }
};
int main()
{
    stack<int> s;
    s.push(1);
    s.push(2);
    s.push(3);
}

Demonstrates how compare_exchange_strong either changes the value of the atomic variable or the variable used for comparison.

Run this code

#include <atomic>
#include <iostream>
 
std::atomic<int>  ai;
 
int  tst_val= 4;
int  new_val= 5;
bool exchanged= false;
 
void valsout()
{
    std::cout << "ai= " << ai
	      << "  tst_val= " << tst_val
	      << "  new_val= " << new_val
	      << "  exchanged= " << std::boolalpha << exchanged
	      << "\n";
}
 
int main()
{
    ai= 3;
    valsout();
 
    // tst_val != ai   ==>  tst_val is modified
    exchanged= ai.compare_exchange_strong( tst_val, new_val );
    valsout();
 
    // tst_val == ai   ==>  ai is modified
    exchanged= ai.compare_exchange_strong( tst_val, new_val );
    valsout();
}

Output:

ai= 3  tst_val= 4  new_val= 5  exchanged= false
ai= 3  tst_val= 3  new_val= 5  exchanged= false
ai= 5  tst_val= 3  new_val= 5  exchanged= true

See also

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

atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (function template)