listen(2) [mojave man page]
LISTEN(2) BSD System Calls Manual LISTEN(2) NAME
listen -- listen for connections on a socket SYNOPSIS
#include <sys/socket.h> int listen(int socket, int backlog); DESCRIPTION
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 listen(). 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. RETURN VALUES
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. ERRORS
listen() will fail if: [EACCES] The current process has insufficient privileges. [EBADF] The argument socket is not a valid file descriptor. [EDESTADDRREQ] The socket is not bound to a local address and the protocol does not support listening on an unbound socket. [EINVAL] socket is already connected. [ENOTSOCK] The argument socket does not reference a socket. [EOPNOTSUPP] The socket is not of a type that supports the operation listen(). SEE ALSO
accept(2), connect(2), connectx(2), socket(2) BUGS
The backlog is currently limited (silently) to 128. HISTORY
The listen() function call appeared in 4.2BSD. 4.2 Berkeley Distribution March 18, 2015 4.2 Berkeley Distribution
Check Out this Related Man Page
LISTEN(2) Linux Programmer's Manual LISTEN(2) NAME
listen - listen for connections on a socket SYNOPSIS
#include <sys/socket.h> int listen(int s, int backlog); DESCRIPTION
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 ECONNREFUSED or, if the underlying protocol supports retransmission, the request may be ignored so that retries succeed. NOTES
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. RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
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. CONFORMING TO
Single Unix, 4.4BSD, POSIX 1003.1g draft. The listen function call first appeared in 4.2BSD. BUGS
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. SEE ALSO
accept(2), connect(2), socket(2) BSD Man Page 1993-07-23 LISTEN(2)