Editing Compare-and-swap (section)

===Example application: atomic adder===
As an example use case of compare-and-swap, here is an algorithm for [[fetch-and-add|atomically incrementing or decrementing an integer]]. This is useful in a variety of applications that use counters. The function {{mono|add}} performs the action {{mono|*p ← *p + a}}, atomically (again denoting pointer indirection by {{mono|*}}, as in C) and returns the final value stored in the counter. Unlike in the {{mono|cas}} pseudocode above, there is no requirement that any sequence of operations is atomic except for {{mono|cas}}.

 '''function''' add(p: pointer to int, a: int) returns int
     done ← false
     '''while''' not done
         value ← *p  // Even this operation doesn't need to be atomic.
         done ← cas(p, value, value + a)
 
     '''return''' value + a

In this algorithm, if the value of {{mono|*p}} changes after (or while!) it is fetched and before the CAS does the store, CAS will notice and report this fact, causing the algorithm to retry.<ref>{{cite web |url=https://www.ibm.com/developerworks/library/j-jtp11234/index.html |title=Java theory and practice: Going atomic |website=IBM developerWorks |date=23 November 2004 |first=Brian |last=Goetz}}</ref>