# listen

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           listen — listen for socket connections and limit the queue of
           incoming connections
    


## SYNOPSIS



           #include <sys/socket.h>
    
           int listen(int _socket_ , int _backlog_);
    


## DESCRIPTION



           The _listen_() function shall mark a connection-mode socket,
           specified by the _socket_ argument, as accepting connections.
    
           The _backlog_ argument provides a hint to the implementation which
           the implementation shall use to limit the number of outstanding
           connections in the socket's listen queue. Implementations may
           impose a limit on _backlog_ and silently reduce the specified value.
           Normally, a larger _backlog_ argument value shall result in a larger
           or equal length of the listen queue. Implementations shall support
           values of _backlog_ up to SOMAXCONN, defined in _< sys/socket.h>_.
    
           The implementation may include incomplete connections in its
           listen queue. The limits on the number of incomplete connections
           and completed connections queued may be different.
    
           The implementation may have an upper limit on the length of the
           listen queue—either global or per accepting socket. If _backlog_
           exceeds this limit, the length of the listen queue is set to the
           limit.
    
           If _listen_() is called with a _backlog_ argument value that is less
           than 0, the function behaves as if it had been called with a
           _backlog_ argument value of 0.
    
           A _backlog_ argument of 0 may allow the socket to accept
           connections, in which case the length of the listen queue may be
           set to an implementation-defined minimum value.
    
           The socket in use may require the process to have appropriate
           privileges to use the _listen_() function.
    


## RETURN VALUE



           Upon successful completions, _listen_() shall return 0; otherwise,
           -1 shall be returned and _errno_ set to indicate the error.
    


## ERRORS



           The _listen_() function shall fail if:
    
           **EBADF** The _socket_ argument 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** The _socket_ is already connected.
    
           **ENOTSOCK**
                  The _socket_ argument does not refer to a socket.
    
           **EOPNOTSUPP**
                  The socket protocol does not support _listen_().
    
           The _listen_() function may fail if:
    
           **EACCES** The calling process does not have appropriate privileges.
    
           **EINVAL** The _socket_ has been shut down.
    
           **ENOBUFS**
                  Insufficient resources are available in the system to
                  complete the call.
    
           _The following sections are informative._
    


## EXAMPLES



           None.
    


## APPLICATION USAGE



           None.
    


## RATIONALE



           None.
    


## FUTURE DIRECTIONS



           None.
    


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