listen - listen for connections on a socket
int listen(int s, int backlog);
To accept connections, a socket is first created using socket(), a
queue for incoming connections is activated using listen(), and then
connections are accepted using accept(). listen() applies only to
unconnected sockets of type SOCK_STREAM. Except for AF_VME_LINK, if
the socket has not been bound to a local port before listen() is
invoked, the system automatically binds a local port for the socket to
listen on (see inet(7F)). For sockets in the address family AF_CCITT
and AF_VME_LINK, the socket must be bound to an address by using
bind() before connection establishment can continue, otherwise an
EADDREQUIRED error is returned.
A listen queue is established for the socket specified by the s
parameter, which is a socket descriptor.
backlog defines the desirable queue length for pending connections.
The actual queue length may be greater than the specified backlog. If
a connection request arrives when the queue is full, the client will
receive an ETIMEDOUT error.
backlog is limited to the range of 0 to SOMAXCONN, which is defined in
<<<<sys/socket.h>>>>. SOMAXCONN is currently set to 4096. If any other
value is specified, the system automatically assigns the closest value
within the range. A backlog of 0 specifies only 1 pending connection
is allowed at any given time.
Call-acceptance can be controlled by the X25_CALL_ACPT_APPROVAL
ioctl() call described in RETURN VALUE . Upon successful completion,
listen() returns 0; otherwise, it returns -1 and sets errno to
indicate the error.
listen() fails if any of the following conditions are encountered:
[EBADF] s is not a valid file descriptor.
[EDESTADDRREQ] The socket s has not been bound to an address
by using bind().
[ENOTSOCK] s is a valid file descriptor but it is not a
Hewlett-Packard Company - 1 - HP-UX Release 11i: November 2000
[EOPNOTSUPP] The socket referenced by s does not support
[EINVAL] The socket has been shut down or is already
connected (see socketx25(7)).
Currently, the default behavior is the HP-UX BSD Sockets; however, it
might be changed to X/Open Sockets in a future release. At that time,
any HP-UX BSD Sockets behavior that is incompatible with X/Open
Sockets might be obsoleted. Applications that conform to the X/Open
specification now will avoid migration problems (see
The listen() system call is thread-safe. It has a cancellation point;
and it is async-cancel safe, async-signal safe, and fork-safe.
listen() was developed by HP and the University of California,
accept(2), connect(2), socket(2), socketx25(7), xopen_networking(7),
af_ccitt(7F), af_vme_link(7F), inet(7F).
Hewlett-Packard Company - 2 - HP-UX Release 11i: November 2000