Section: Linux Programmer's Manual (2)
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
as a passive socket, that is, as a socket that will
be used to accept incoming connection requests using
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
If a connection request arrives when the queue is full, the client
may receive an error with an indication of
or, if the underlying protocol supports retransmission, the request may be
ignored so that a later reattempt at connection succeeds.
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
Another socket is already listening on the same port.
(Internet domain sockets)
The socket referred to by
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
is not a valid file descriptor.
The file descriptor
does not refer to a socket.
The socket is not of a type that supports the
POSIX.1-2001, POSIX.1-2008, 4.4BSD
first appeared in 4.2BSD).
To accept connections, the following steps are performed:
A socket is created with
The socket is bound to a local address using
so that other sockets may be
A willingness to accept incoming connections and a queue limit for incoming
connections are specified with
Connections are accepted with
POSIX.1 does not require the inclusion of
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
argument on TCP sockets changed with Linux 2.2.
Now it specifies the queue length for
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
When syncookies are enabled there is no logical maximum
length and this setting is ignored.
for more information.
argument is greater than the value in
then it is silently truncated to that value.
Since Linux 5.4, the default in this file is 4096;
in earlier kernels, the default value is 128.
In kernels before 2.4.25, this limit was a hard coded value,
with the value 128.
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