Legacy Document

Inside Macintosh: Networking With Open Transport / Part 2 - Open Transport Reference

Chapter 27 - Utilities Reference / Functions

Atomic Operations /

OTCompareAndSwapPtr

Atomically compares the value of a pointer at a memory location and atomically swaps it with a second pointer value if the compare is successful.

C INTERFACE

Boolean OTCompareAndSwapPtr(
                     void* oldVal,
                     void* newVal,
                     void** where)

C++ INTERFACE

None. C++ applications use the C interface to this function.

PARAMETERS

oldVal

The pointer being compared to the pointer referenced by the where parameter.

newVal

The value to be assigned to the pointer referenced by the where parameter.

where

A reference to the pointer being compared to the pointer specified by the oldVal parameter.

function result

Returns true if the swap is done.

DISCUSSION

This function compares the value specified by the oldVal parameter with the value referenced by the where parameter. If the two values are the same, the function replaces the values referenced by the where parameter with the value specified by the newVal parameter and it returns true. If the two values are not the same, no swap occurs and the function returns false.

The entire compare and swap operation is atomic.

WARNING

The pointer where must be on a 4-byte boundary, or calling this function may hang the machine.

Use the OTCompareAndSwap32 function to compare two 32-bit values and set one if they are the same.