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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, | (3) | (since C11) |
_Bool atomic_compare_exchange_weak_explicit(volatile A*obj, C* expected, C desired, | (4) | (since C11) |
Atomically compares the contents of memory pointed to byobj with the contents of memory pointed to byexpected, and if those are bitwise equal, replaces the former withdesired (performs read-modify-write operation). Otherwise, loads the actual contents of memory pointed to byobj into*expected (performs load operation).
The memory models for the read-modify-write and load operations aresucc andfail respectively. The (1-2) versions usememory_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 ageneric function defined for allatomic object typesA. The argument is pointer to a volatile atomic type to accept addresses of both non-volatile andvolatile (e.g. memory-mapped I/O) atomic objects, and volatile semantic is preserved when applying this operation to volatile atomic objects.C is the non-atomic type corresponding toA.
It is unspecified whether the name of a generic function is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function (e.g. parenthesized like(atomic_compare_exchange)(...)), or a program defines an external identifier with the name of a generic function, the behavior is undefined.
Contents |
| 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 bememory_order_release ormemory_order_acq_rel and cannot specify stronger ordering thansucc |
The result of the comparison:true if*obj was equal to*exp,false otherwise.
The behavior ofatomic_compare_exchange_* family is as if the following was executed atomically:
| swaps a value with the value of an atomic object (function)[edit] | |
C++ documentation foratomic_compare_exchange_weak,atomic_compare_exchange_strong,atomic_compare_exchange_weak_explicit,atomic_compare_exchange_strong_explicit | |
