Editing Compare-and-swap (section)

===Implementation in C===
Many C compilers support using compare-and-swap either with the [[C11 (C standard revision)|C11]] <code><stdatomic.h></code> functions,<ref>{{Cite web |title=atomic_compare_exchange_weak, atomic_compare_exchange_strong, atomic_compare_exchange_weak_explicit, atomic_compare_exchange_strong_explicit |url=https://en.cppreference.com/w/c/atomic/atomic_compare_exchange |access-date= |website=en.cppreference.com}}</ref> or some non-standard C extension of that particular C compiler,<ref>[https://gcc.gnu.org/onlinedocs/gcc-4.3.5/gcc/Atomic-Builtins.html "GNU C Extensions to the C Language Family: Built-in functions for atomic memory access"]</ref> or by calling a function written directly in assembly language using the compare-and-swap instruction.

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:

<syntaxhighlight lang="cpp">
int compare_and_swap(int* reg, int oldval, int newval)
{
    ATOMIC();
    int old_reg_val = *reg;
    if (old_reg_val == oldval)
        *reg = newval;
    END_ATOMIC();
    return old_reg_val;
}
</syntaxhighlight>

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

For example, an election protocol can be implemented such that every process checks the result of <code>compare_and_swap</code> against its own PID (= newval). The winning process finds the <code>compare_and_swap</code> returning the initial non-PID value (e.g., zero).  For the losers it will return the winning PID.

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

<syntaxhighlight lang="cpp">
bool compare_and_swap(int *accum, int *dest, int newval)
{
    if (*accum == *dest) {
        *dest = newval;
        return true;
    } else {
        *accum = *dest;
        return false;
    }
}
</syntaxhighlight>