listen - listen for connections on a socket


  1. 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 incoming connections are specified with listen(2), and then the connections are accepted with accept(2). The listen(2) 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.
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(2) operation.


Single Unix, 4.4BSD, POSIX 1003.1g draft. The listen(2) 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 applications since BSD (and some BSD-derived systems) limit the backlog to 5.


accept(2), connect(2), socket(2)


To use listen(2), you need a socket fd created with socket(2), and you probably also want to bind(2) it to a local port. So the sequence of actions you want are probably socket(2), bind(2) (optional, but recommended), listen(2), select(2) (optional), accept(2), close(2)

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