listen - listen for connections on a socket


 #include <sys/types.h>          /* See NOTES */
#include <sys/socket.h> int listen(int sockfd, int backlog);


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() call applies only to sockets of type SOCK_STREAM or SOCK_SEQPACKET.

The backlog parameter defines the maximum length the queue of pending connections may grow to. If a connection request arrives with the queue 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 retries succeed.


On success, zero is returned. On error, -1 is returned, and errno is set appropriately.


Another socket is already listening on the same port.
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.


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 parameter on TCP sockets changed with Linux 2.2. Now it specifies 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 the tcp_max_syn_backlog sysctl. When syncookies are enabled there is no logical maximum length and this sysctl setting is ignored. See tcp(7) for more information.

If the backlog argument is greater than the value in /proc/sys/net/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 bind(2).


accept(2), bind(2), connect(2), socket(2)