| 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.
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 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.
Notes
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);
else
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).
References
- C11 standard (ISO/IEC 9899:2011):
- 7.17.7.4 The atomic_compare_exchange generic functions (p: 283-284)
See also
| swaps a value with the value of an atomic object (function) | |
C++ documentation for atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit | |
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