Editing Reentrancy (computing) (section)

===Reentrant and thread-safe===
An implementation of {{code|swap()}} that allocates {{code|tmp}} on the [[call stack|stack]] instead of globally and that is called only with unshared variables as parameters{{efn|If isr() called swap() with one or two global variables as parameters then swap() would not be reentrant}} is both thread-safe and reentrant. Thread-safe because the stack is local to a thread and a function acting just on local data will always produce the expected result. There is no access to shared data therefore no data race.

<syntaxhighlight lang="c">
void swap(int* x, int* y)
{
    int tmp;
    tmp = *x;
    *x = *y;
    *y = tmp;    /* Hardware interrupt might invoke isr() here. */
}

void isr()
{
    int x = 1, y = 2;
    swap(&x, &y);
}
</syntaxhighlight>