The POSIX threads standard provides a useful synchronization primitive, the

mutex $($type pthread$\_$mutex$\_$t$).$ A mutex is used to$$wait for it$$implement mutual exclusion.

A thread locks a mutex variable by calling pthread$\_$mutex$\_$lock$(),$ and unlocks it by calling

pthread$\_$mutex$\_$unlock$().$ When a thread calls pthread$\_$mutex$\_$lock$()$ while another thread has

previously locked the same mutex but not yet unlocked it$,$ the former thread blocks until the latter

thread calls pthread$\_$mutex$\_$unlock$()$ on it$.$