InterlockedCompareExchangePointer

Atomically compares a pointer-sized value at a memory location with a comparand and replaces it with a new value if they match. Returns the original value either way.

Syntax: InterlockedCompareExchangePointer( Target , NewValue , OldValueCompare ) As LongPtr

Target

required LongPtr. The pointer-sized variable to update, passed by reference.

NewValue

required LongPtr. The value to write into Target if the compare succeeds.

OldValueCompare

required LongPtr. The expected current value of Target.

The compare-and-swap happens as one atomic operation. The return value is the value that was in Target at the start of the call — equal to OldValueCompare on success, anything else on failure (in which case Target is left unchanged). Wraps the Win32 InterlockedCompareExchangePointer intrinsic.

Example

' Atomically claim ownership of a slot.
Dim Slot As LongPtr = 0
Dim NewObj As LongPtr = ObjPtr(New Collection)
If InterlockedCompareExchangePointer(Slot, NewObj, 0) = 0 Then
    ' Won the race — Slot now holds NewObj.
End If

See Also

• InterlockedExchangePointer function
• InterlockedCompareExchange32, InterlockedCompareExchange64 functions