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

 s
    A socket descriptor of type SOCK_STREAM created using the
    socket() function.
 backlog
    The maximum number of pending connections that can be queued
    on the socket at any given time. The maximum number of pending
    connections can be set using the sysconfig utility to set the
    value of somaxconn. The default value for the maximum number of
    pending connections is 1024.