atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit

 

C

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Atomic operations (C11)

Technical Specifications

 

Atomic operations library

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atomic_compare_exchange

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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,                                                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 is the type of an atomic object, 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, object, 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

atomic_exchangeatomic_exchange_explicit (C11)	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