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.
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
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.
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 listening 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 appropriate privileges.
EINVAL The socket has been shut down.
Insufficient resources are available in the system to complete
The following sections are informative.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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 original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 LISTEN(3P)