LISTEN(2)                     System Calls Manual                    LISTEN(2)

NAME
     listen – listen for connections on a socket

SYNOPSIS
     #include <sys/socket.h>

     int
     listen(int socket, int backlog);

DESCRIPTION
     Creation of socket-based connections requires several operations.  First,
     a socket is created with socket(2).  Next, a willingness to accept
     incoming connections and a queue limit for incoming connections are
     specified with listen().  Finally, the connections are accepted with
     accept(2).  The listen() call applies only to sockets of type
     SOCK_STREAM.

     The backlog parameter defines the maximum length for the queue of pending
     connections.  If a connection request arrives with the queue full, the
     client may receive an error with an indication of ECONNREFUSED.
     Alternatively, if the underlying protocol supports retransmission, the
     request may be ignored so that retries may succeed.

RETURN VALUES
     The listen() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     listen() will fail if:

     [EACCES]           The current process has insufficient privileges.

     [EBADF]            The argument socket is not a valid file descriptor.

     [EDESTADDRREQ]     The socket is not bound to a local address and the
                        protocol does not support listening on an unbound
                        socket.

     [EINVAL]           socket is already connected.

     [ENOTSOCK]         The argument socket does not reference a socket.

     [EOPNOTSUPP]       The socket is not of a type that supports the
                        operation listen().

SEE ALSO
     accept(2), connect(2), connectx(2), socket(2)

BUGS
     The backlog is currently limited (silently) to 128.

HISTORY
     The listen() function call appeared in 4.2BSD.

BSD 4.2                         March 18, 2015                         BSD 4.2