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Linux man pages : listen (2)
LISTEN(2)		   Linux Programmer's Manual		     LISTEN(2)


listen - listen for connections on a socket


#include <sys/socket.h> int listen(int s, int backlog);


To accept connections, a socket is first created with socket(2), a willingness to accept incoming connections and a queue limit for incom- ing connections are specified with listen, and then the connections are accepted with accept(2). The listen call applies only to sockets of type SOCK_STREAM or SOCK_SEQPACKET. The backlog parameter defines the maximum length the queue of pending connections may grow to. If a connection request arrives with the queue full the client may receive an error with an indication of ECON- NREFUSED or, if the underlying protocol supports retransmission, the request may be ignored so that retries succeed.


The behaviour of the backlog parameter on TCP sockets changed with Linux 2.2. Now it specifies the queue length for completely estab- lished sockets waiting to be accepted, instead of the number of incom- plete connection requests. The maximum length of the queue for incom- plete sockets can be set using the tcp_max_syn_backlog sysctl. When syncookies are enabled there is no logical maximum length and this sysctl setting is ignored. See tcp(7) for more information.


On success, zero is returned. On error, -1 is returned, and errno is set appropriately.


EADDRINUSE Another socket is already listening on the same port. EBADF The argument s is not a valid descriptor. ENOTSOCK The argument s is not a socket. EOPNOTSUPP The socket is not of a type that supports the listen operation.


Single Unix, 4.4BSD, POSIX 1003.1g draft. The listen function call first appeared in 4.2BSD.


If the socket is of type AF_INET, and the backlog argument is greater than the constant SOMAXCONN (128 in Linux 2.0 & 2.2), it is silently truncated to SOMAXCONN. Don't rely on this value in portable applica- tions since BSD (and some BSD-derived systems) limit the backlog to 5.


accept(2), connect(2), socket(2) BSD Man Page 1993-07-23 LISTEN(2)