The sem_wait function locks the semaphore referenced by sem by
    performing a semaphore lock operation on it. If the semaphore
    value is zero, the sem_wait function blocks until it either locks
    the semaphore or is interrupted by a signal.

    The sem_wait and sem_trywait functions help ensure that the
    resource associated with the semaphore cannot be accessed by
    other processes. The semaphore remains locked until the process
    unlocks it with a call to the sem_post function.

    Use the sem_wait function instead of the sem_trywait function if
    the process should wait for access to the semaphore.