LISTEN(P) POSIX Programmer's Manual LISTEN(P)
listen - listen for socket connections and limit the queue of incoming connections
int listen(int socket, int backlog);
The listen() function shall mark a connection-mode socket, specified by the socket argu-
ment, 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. Implemen-
tations 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
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 lis-
Upon successful completions, listen() shall return 0; otherwise, -1 shall be returned and
errno set to indicate the error.
The listen() function shall fail if:
EBADF The socket argument is not a valid file descriptor.
The socket is not bound to a local address, and the protocol does not support lis-
tening on an unbound socket.
EINVAL The socket is already connected.
The socket argument does not refer to a socket.
The socket protocol does not support listen().
The listen() function may fail if:
EACCES The calling process does not have the appropriate privileges.
EINVAL The socket has been shut down.
Insufficient resources are available in the system to complete the call.
The following sections are informative.
accept() , connect() , socket() , the Base Definitions volume of IEEE Std 1003.1-2001,
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 LISTEN(P)