atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit

< c‎ | atomic
Defined in header <stdatomic.h>
_Bool atomic_compare_exchange_strong( volatile A* obj,
                                      C* expected, C desired );
(1) (since C11)
_Bool atomic_compare_exchange_weak( volatile A *obj,
                                    C* expected, C desired );
(2) (since C11)
_Bool atomic_compare_exchange_strong_explicit( volatile A* obj,

                                               C* expected, C desired,
                                               memory_order succ,

                                               memory_order fail );
(3) (since C11)
_Bool atomic_compare_exchange_weak_explicit( volatile A *obj,

                                             C* expected, C desired,
                                             memory_order succ,

                                             memory_order fail );
(4) (since C11)

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 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. The argument is pointer to a volatile atomic type to accept addresses of both non-volatile and volatile (e.g. memory-mapped I/O) atomic variables.C is the non-atomic type corresponding to A.


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 succ

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:

if (memcmp(obj, expected, sizeof *obj) == 0)
    memcpy(obj, &desired, sizeof *obj);
    memcpy(expected, obj, sizeof *obj);

(the standard wording which says the comparison is in terms of "value" was changed to "bitwise equality" by DR 431)


  • C11 standard (ISO/IEC 9899:2011):
  • 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