Manpage of LISTEN
LISTENSection: Linux Programmer's Manual (2)
NAMElisten - listen for connections on a socket
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>int listen(int sockfd, int backlog);
DESCRIPTIONlisten() marks the socket referred to by sockfdas a passive socket, that is, as a socket that will be used to accept incoming connection requests using accept(2).
The sockfdargument is a file descriptor that refers to a socket of type SOCK_STREAMor SOCK_SEQPACKET.
The backlogargument defines the maximum length to which the queue of pending connections for sockfdmay grow. If a connection request arrives when the queue is full, the client may receive an error with an indication of ECONNREFUSEDor, if the underlying protocol supports retransmission, the request may be ignored so that a later reattempt at connection succeeds.
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errnois set appropriately.
- Another socket is already listening on the same port.
- (Internet domain sockets) The socket referred to by sockfdhad not ip(7). ly been bound to an address and, upon attempting to bind it to an ephemeral port, it was determined that all port numbers in the ephemeral port range are currently in use. See the discussion of /proc/sys/net/ipv4/ip_local_port_rangein
- The argument sockfdis not a valid file descriptor.
- The file descriptor sockfddoes not refer to a socket.
- The socket is not of a type that supports the listen() operation.
CONFORMING TOPOSIX.1-2001, POSIX.1-2008, 4.4BSD (listen() first appeared in 4.2BSD).
NOTESTo accept connections, the following steps are performed:
- A socket is created with socket(2).
- The socket is bound to a local address using bind(2), so that other sockets may be connect(2)ed to it.
- A willingness to accept incoming connections and a queue limit for incoming connections are specified with listen().
- Connections are accepted with accept(2).
POSIX.1 does not require the inclusion of <sys/types.h>, and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it.
The behavior of the backlogargument on TCP sockets changed with Linux 2.2. Now it specifies the queue length for completelyestablished 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 /proc/sys/net/ipv4/tcp_max_syn_backlog. When syncookies are enabled there is no logical maximum length and this setting is ignored. See tcp(7) for more information.
If the backlogargument is greater than the value in /proc/sys/net/core/somaxconn, then it is silently truncated to that value; the default value in this file is 128. In kernels before 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128.
SEE ALSOaccept(2), bind(2), connect(2), socket(2), socket(7)
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