Returns: A positive semaphore identifier on success, or
FALSE on error.
sem_get() returns an id that can
be used to access the System V semaphore with the given key.
The semaphore is created if necessary using the permission
bits specified in perm (defaults to 0666). The number of
processes that can acquire the semaphore simultaneously is
set to max_acquire (defaults to 1). Actually this value is
set only if the process finds it is the only process
currently attached to the semaphore.
A second call to sem_get() for
the same key will return a different semaphore identifier,
but both identifiers access the same underlying
semaphore.
See also: sem_acquire(),
sem_release() and ftok().
注: This function does not work on Windows systems.