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listen(2) [freebsd man page]

LISTEN(2)						      BSD System Calls Manual							 LISTEN(2)

listen -- listen for connections on a socket LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/types.h> #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 incoming connections are specified with listen(), and then the connections are accepted with accept(2). The listen() system call applies only to sockets of type SOCK_STREAM or SOCK_SEQPACKET. The backlog argument defines the maximum length the queue of pending connections may grow to. The real maximum queue length will be 1.5 times more than the value specified in the backlog argument. A subsequent listen() system call on the listening socket allows the caller to change the maximum queue length using a new backlog argument. If a connection request arrives with the queue full the client may receive an error with an indication of ECONNREFUSED, or, in the case of TCP, the connection will be silently dropped. Current queue lengths of listening sockets can be queried using netstat(1) command. Note that before FreeBSD 4.5 and the introduction of the syncache, the backlog argument also determined the length of the incomplete connec- tion queue, which held TCP sockets in the process of completing TCP's 3-way handshake. These incomplete connections are now held entirely in the syncache, which is unaffected by queue lengths. Inflated backlog values to help handle denial of service attacks are no longer neces- sary. The sysctl(3) MIB variable kern.ipc.soacceptqueue specifies a hard limit on backlog; if a value greater than kern.ipc.soacceptqueue or less than zero is specified, backlog is silently forced to kern.ipc.soacceptqueue. INTERACTION WITH ACCEPT FILTERS
When accept filtering is used on a socket, a second queue will be used to hold sockets that have connected, but have not yet met their accept filtering criteria. Once the criteria has been met, these sockets will be moved over into the completed connection queue to be accept(2)ed. If this secondary queue is full and a new connection comes in, the oldest socket which has not yet met its accept filter criteria will be terminated. This secondary queue, like the primary listen queue, is sized according to the backlog argument. 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
The listen() system call will fail if: [EBADF] The argument s is not a valid descriptor. [EDESTADDRREQ] The socket is not bound to a local address, and the protocol does not support listening on an unbound socket. [EINVAL] The socket is already connected, or in the process of being connected. [ENOTSOCK] The argument s is not a socket. [EOPNOTSUPP] The socket is not of a type that supports the operation listen(). SEE ALSO
netstat(1), accept(2), connect(2), socket(2), sysctl(3), sysctl(8), accept_filter(9) HISTORY
The listen() system call appeared in 4.2BSD. The ability to configure the maximum backlog at run-time, and to use a negative backlog to request the maximum allowable value, was introduced in FreeBSD 2.2. The kern.ipc.somaxconn sysctl(3) has been replaced with kern.ipc.soacceptqueue in FreeBSD 10.0 to prevent confusion about its actual functionality. The original sysctl(3) kern.ipc.somaxconn is still available but hidden from a sysctl(3) -a output so that existing applications and scripts continue to work. BSD
July 15, 2014 BSD

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listen(3XNET)					   X/Open Networking Services Library Functions 				     listen(3XNET)

listen - listen for socket connections and limit the queue of incoming connections SYNOPSIS
cc [ flag ... ] file ... -lxnet [ library ... ] #include <sys/socket.h> int listen(int socket, int backlog); DESCRIPTION
The listen() function marks a connection-mode socket, specified by the socket argument, as accepting connections, and limits the number of outstanding connections in the socket's listen queue to the value specified by the backlog argument. If listen() is called with a backlog argument value that is less than 0, the function sets the length of the socket's listen queue to 0. The implementation may include incomplete connections in the queue subject to the queue limit. The implementation may also increase the specified queue limit internally if it includes such incomplete connections in the queue subject to this limit. Implementations may limit the length of the socket's listen queue. If backlog exceeds the implementation-dependent maximum queue length, the length of the socket's listen queue will be set to the maximum supported value. The socket in use may require the process to have appropriate privileges to use the listen() function. RETURN VALUES
Upon successful completions, listen() returns 0. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The listen() function will fail if: EBADF The socket argument 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 The socket is already connected. ENOTSOCK The socket argument does not refer to a socket. EOPNOTSUPP The socket protocol does not support listen(). The listen() function may fail if: EACCES The calling process does not have the appropriate privileges. EINVAL The socket has been shut down. ENOBUFS Insufficient resources are available in the system to complete the call. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ SEE ALSO
accept(3XNET), connect(3XNET), socket(3XNET), attributes(5), standards(5) SunOS 5.11 10 Jun 2002 listen(3XNET)
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