VMS Help TCPIP Services, Programming Interfaces, Sockets API, select(), Return Values  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    n                  The number of sockets ready for I/O or pending
                       exceptions. This value matches the number
                       of returned bits that are set in all output
                       masks.
    0                  The select() function timed out before any
                       socket became ready for I/O.
    -1                 Error; errno is set to indicate the error.