atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit

_Bool atomic_compare_exchange_strong( volatile A* obj,
                                      C* expected, C desired );

_Bool atomic_compare_exchange_weak( volatile A *obj, 
                                    C* expected, C desired );

_Bool atomic_compare_exchange_strong_explicit( volatile A* obj, 
                                               C* expected, C desired,
                                               memory_order succ, 
                                               memory_order fail );

_Bool atomic_compare_exchange_weak_explicit( volatile A *obj, 
                                             C* expected, C desired,
                                             memory_order succ, 
                                             memory_order fail );

Atomically compares the contents of memory pointed to by obj with the contents of memory pointed to by expected, and if those are bitwise equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual contents of memory 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

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);

References

• C11 standard (ISO/IEC 9899:2011):
• 7.17.7.4 The atomic_compare_exchange generic functions (p: 283-284)

See also