documents: listen
LISTEN(2) System Calls Manual LISTEN(2)

listen — listen for connections on a socket

#include <sys/socket.h>

int
listen(int socket, int backlog);

Creation of socket-based connections requires several operations. First, a socket is created with socket(2). Next, a willingness to accept incoming connections and a queue limit for incoming connections are specified with (). Finally, the connections are accepted with accept(2). The listen() call applies only to sockets of type SOCK_STREAM.

The backlog parameter defines the maximum length for the queue of pending connections. If a connection request arrives with the queue full, the client may receive an error with an indication of ECONNREFUSED. Alternatively, if the underlying protocol supports retransmission, the request may be ignored so that retries may succeed.

The listen() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

listen() will fail if:

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The current process has insufficient privileges.
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The argument socket is not a valid file descriptor.
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The socket is not bound to a local address and the protocol does not support listening on an unbound socket.
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socket is already connected.
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The argument socket does not reference a socket.
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The socket is not of a type that supports the operation listen().

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

The backlog is currently limited (silently) to 128.

The listen() function call appeared in 4.2BSD.

March 18, 2015 BSD 4.2