listen - listen for connections on a socket
sockfd argument is a file descriptor that refers to a socket of type SOCK_STREAM or SOCK_SEQPACKET.
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 connection succeeds.
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
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 ip(7).
sockfd is not a valid file descriptor.
The file descriptor
sockfd does not refer to a socket.
The socket is not of a type that supports the listen() operation.
To accept connections, the following steps are performed:
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 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 syncookies are enabled there is no logical maximum length and this setting is ignored. See tcp(7) for more information.
backlog argument is greater than the value in
/proc/sys/net/core/somaxconn, 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, SOMAXCONN, with the value 128.
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