39.7 Mutexes: use of static keyword

[happysalada]

The first time you define a mutex you use

#include <stdio.h>
#include <threads.h>

static mtx_t serial_mtx;     // <-- MUTEX VARIABLE

no need for the static here right?
Maybe i'm missing something

[beejjorgensen]

It just makes it local to that file (opposite of extern), so it's basically harmless in that regard. But I think it might just be confusing because we use static in its other sense elsewhere in the chapter. I'll remove it.