The name that the POSIX library uses for a lock is a mutex, as it is used to provide mutual
exclusion between threads, i.e., if one thread is in the critical section, it excludes the others
from entering until it has completed the section. Thus, when you see the following POSIX
threads code, you should understand that it is doing the same thing as above (we again use
wrappers that check for errors upon lock and unlock):
pthread_mutex_t lock = PTHREAD_MUTEX_INITITALIZED;
pthread_mutex_lock(&lock);
balance = balance + 1;
pthread_mutex_unlock(&lock);
You might also notice here that the POSIX version passes a variable to lock and unlock, as we
may be using different locks to protect different variables. Doing so can increase concurrency:
instead of one big lock that is used any time any critical section is accessed (a coarse-gained
locking strategy), one will often protect different data and data structures with different locks,
thus allowing more threads to be in locked code at once (a more fine-grained approach).