The sem_trywait function locks a semaphore only if the semaphore
    is currently not locked. If the semaphore value is zero, the sem_
    trywait function returns without locking the semaphore.

    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.