HP-UX Reference (11i v2 04/09) - 2 System Calls (vol 5)

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listen(2) listen(2)

NAME

listen - listen for connections on a socket

SYNOPSIS

#include <sys/socket.h>

int listen(int s, int backlog);

DESCRIPTION

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, oth-

erwise 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.

DEPENDENCIES

AF_CCITT:

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.

ERRORS

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 socket.

[EOPNOTSUPP] The socket referenced by s does not support listen().

[EINVAL] The socket has been shut down or is already connected.

FUTURE DIRECTION

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 xopen_networking(7)).

AUTHOR

listen() was developed by HP and the University of California, Berkeley.