VMS Help POSIX Threads, PTHREAD routines, pthread_setconcurrency, Return Values  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    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 value specified by new_level would cause a system
                resource to be exceeded.
    [EINVAL]    The value specified by new_level is negative.