Compare-and-swap - Implementation in C

Implementation in C

The following C function shows the basic behavior of a compare-and-swap variant that returns the old value of the specified memory location; however, this version does not provide the crucial guarantees of atomicity that a real compare-and-swap operation would:

int compare_and_swap (int* reg, int oldval, int newval) { int old_reg_val = *reg; if (old_reg_val == oldval) *reg = newval; return old_reg_val; }

old_reg_val is always returned, but it can be tested following the compare_and_swap operation to see if it matches oldval, as it may be different, meaning that another process has managed to succeed in a competing compare_and_swap to change the reg value from oldval.

For example, an election protocol can be done, where every process compares the result of compare_and_swap with its say PID (=newval). The winning process finds the compare_and_swap returning the initial non-PID value (e.g., zero). For the losers it will return the winning PID.

bool compare_and_swap (int *accum, int *dest, int newval) { if ( *accum == *dest ) { *dest = newval; return true; } else { *accum = *dest; return false; } }

This is the logic in the Intel Software Manual Vol 2A.