atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit

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 objects, and volatile semantic is preserved when applying this operation to volatile atomic objects. C is the non-atomic type corresponding to A .

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.

Parameters

Return value

The result of the comparison: true if *obj was equal to *exp , false otherwise.

Notes

The behavior of atomic_compare_exchange_* family is as if the following was executed atomically:

if (memcmp(obj, expected, sizeof *obj) == 0) {
    memcpy(obj, &desired, sizeof *obj);
    return true;
} else {
    memcpy(expected, obj, sizeof *obj);
    return false;
}

References

See also