atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit
Defined in header
_Bool atomic_compare_exchange_strong( volatile A* obj,
C* expected, C desired );
_Bool atomic_compare_exchange_weak( volatile A *obj,
C* expected, C desired );
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
*expected (performs load operation).
The memory models for the read-modify-write and load operations are
fail respectively. The (1-2) versions use memory_order_seq_cst by default.
The weak forms ((2) and (4)) 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.
This is a generic function defined for all atomic object types.
A is the type of an atomic object,
C is the non-atomic type corresponding to
|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 synchronization ordering for the read-modify-write operation if the comparison succeeds. All values are permitted.|
|fail||-|| the memory synchronization ordering for the load operation if the comparison fails. Cannot be memory_order_release or memory_order_acq_rel and cannot specify stronger ordering than |
 Return value
The result of the comparison: true if
*obj was equal to
*exp, false otherwise.
The behavior of
atomic_compare_exchange_strong is as if the following was executed atomically:
Per post-C11 DR 431, the standard wording which says the comparison is in terms of "value" contradicts memcmp equivalent implementation and the specification of this function in C++ (which uses bitwise comparison, not value-based comparison).
- C11 standard (ISO/IEC 9899:2011):
- 184.108.40.206 The atomic_compare_exchange generic functions (p: 283-284)
 See also
| swaps a value with the value of an atomic object |
C++ documentation for atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit