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LISTEN(2) System Calls Manual LISTEN(2)

listen for connections on a socket

#include <sys/socket.h>
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 may succeed.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

listen() will fail if:
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 operation listen().
The socket is already connected.

accept(2), connect(2), socket(2), sysctl(8)

The listen() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The listen() system call first appeared in 4.1cBSD.

The backlog is currently limited (silently) to the value of the kern.somaxconn sysctl, which defaults to 128.
September 10, 2015 OpenBSD-current