A simple example of compare-and-swap shown in actual C code (which calls into assembly).
The code, in entirety, is shown here:
#include<stdio.h>intglobal=0; charcompare_and_swap(int*ptr, intold, intnew) { unsigned charret; // Note that sete sets a ’byte’ not the word __asm__ __volatile__ ( " lock\n"" cmpxchgl %2,%1\n"" sete %0\n" : "=q" (ret), "=m" (*ptr) : "r" (new), "m" (*ptr), "a" (old) : "memory"); returnret; } intmain(intargc, char*argv[]) { printf("before successful cas: %d\n", global); intsuccess=compare_and_swap(&global, 0, 100); printf("after successful cas: %d (success: %d)\n", global, success); printf("before failing cas: %d\n", global); success=compare_and_swap(&global, 0, 200); printf("after failing cas: %d (old: %d)\n", global, success); return0; }The first call to compare_and_swap() succeeds because the old value is correct; the second call does not because the old value is wrong.
To compile and run:
prompt> gcc -o compare-and-swap compare-and-swap.c -Wall prompt> ./compare-and-swap