If an error condition occurs, no thread is created, the contents
    of thread are undefined, and 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 create
                another thread, or the system-imposed limit on the
                total number of threads under execution by a single
                user is exceeded.
    [EINVAL]    The value specified by attr is not a valid attributes
                block.
    [ENOMEM]    Insufficient memory exists to create a thread.
    [EPERM]     The caller does not have the appropriate permission
                to create a thread with the specified attributes.