LISTEN(2) Linux Programmer's Manual LISTEN(2)
listen - listen for connections on a socket
#include <sys/types.h> /* See NOTES */
int listen(int sockfd, int backlog);
listen() marks the socket referred to by sockfd as a passive socket, that is, as a socket
that will be used to accept incoming connection requests using accept(2).
The sockfd argument is a file descriptor that refers to a socket of type SOCK_STREAM or
The backlog argument defines the maximum length to which the queue of pending connections
for sockfd may grow. If a connection request arrives when the queue is 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 a later reattempt at connec-
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
Another socket is already listening on the same port.
EBADF The argument sockfd is not a valid descriptor.
The argument sockfd is not a socket.
The socket is not of a type that supports the listen() operation.
4.4BSD, POSIX.1-2001. The listen() function call first appeared in 4.2BSD.
To accept connections, the following steps are performed:
1. A socket is created with socket(2).
2. The socket is bound to a local address using bind(2), so that other sockets may be
connect(2)ed to it.
3. A willingness to accept incoming connections and a queue limit for incoming con-
nections are specified with listen().
4. Connections are accepted with accept(2).
POSIX.1-2001 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 backlog argument on TCP sockets changed with Linux 2.2. Now it speci-
fies 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 /proc/sys/net/ipv4/tcp_max_syn_backlog. When syncook-
ies are enabled there is no logical maximum length and this setting is ignored. See
tcp(7) for more information.
If the backlog argument 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.
accept(2), bind(2), connect(2), socket(2), socket(7)
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Linux 2008-11-20 LISTEN(2)