listen - listen for connections on a socket
#include <sys/types.h> /* See NOTES */
int listen(int sockfd, int backlog);
() 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)
argument is a file descriptor that refers to a socket of type
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
On success, zero is returned. On error, -1 is returned, and errno
- Another socket is already listening on the same port.
- (Internet domain sockets) The socket referred to by
sockfd had not previously 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_range in
- The argument sockfd is not a valid file
- The file descriptor sockfd does not refer to a
- The socket is not of a type that supports the
POSIX.1-2001, POSIX.1-2008, 4.4BSD (listen
() first appeared in 4.2BSD).
To 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
- 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 backlog
argument on TCP sockets changed with Linux
2.2. Now it specifies the queue length for completely
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
. When syncookies are
enabled there is no logical maximum length and this setting is ignored. See
for more information.
If the backlog
argument is greater than the value in
, 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.
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