If an error condition occurs, this routine returns an integer
    value indicating the type of error. Possible return values are as
    follows:

    Return      Description

    0           Successful completion.
    [EAGAIN]    The system lacks the necessary resources to
                initialize the read-write lock.
    [EBUSY]     The Threads Library has detected an attempt to
                reinitialize the read-write lock (a previously
                initialized, but not yet destroyed, read-write lock
                object).
    [EINVAL]    The value specified by attr is not a valid attributes
                block.
    [ENOMEM]    Insufficient memory exists to initialize the read-
                write lock.
    [EPERM]     The caller does not have privileges to perform this
                operation.