LISTEN(2) Linux Programmer's Manual LISTEN(2)
listen - listen for connections on a socket
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 incoming 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 ECONNREFUSED 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 established sockets waiting to be accepted,
instead of the number of incomplete connection requests. The maximum length of the queue
for incomplete 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.
Another socket is already listening on the same port.
EBADF The argument s is not a valid descriptor.
The argument s is not a socket.
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
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 applications 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)