sem_post - unlock a semaphore
#include <semaphore.h> int sem_post(sem_t *sem);
sem_post() returns 0 on success; on error, the value
of the semaphore is left unchanged, -1 is returned, and
is set to indicate the error.
sem is not a valid semaphore.
The maximum allowable value for a semaphore would be exceeded.
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