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listen - listen for connections on a socket
#include <sys/types.h> /* See NOTES */
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 incom‐
ing 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 ECON
NREFUSED 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
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() opera‐
4.4BSD, POSIX.1-2001. The listen() function call first appeared in
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 syncook‐
ies are enabled there is no logical maximum length and this sysctl set‐
ting 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.
accept(2), bind(2), connect(2), socket(2)