Editing Reentrancy (computing) (section)

===Neither reentrant nor thread-safe===
This is an example swap function that fails to be reentrant or thread-safe. Since the <code>tmp</code> variable is globally shared, without synchronization, among any concurrent instances of the function, one instance may interfere with the data relied upon by another. As such, it should not have been used in the interrupt service routine <code>isr()</code>:

<syntaxhighlight lang="c">
int tmp;

void swap(int* x, int* y)
{
    tmp = *x;
    *x = *y;
    /* Hardware interrupt might invoke isr() here. */
    *y = tmp;    
}

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