NAME
getsockopt, setsockopt - get and set options on sockets

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>

getsockopt(s, level, optname, optval, optlen)
int s, level, optname;
char *optval;
int *optlen;

setsockopt(s, level, optname, optval, optlen)
int s, level, optname;
char *optval;
int optlen;

DESCRIPTION
Getsockopt and setsockopt manipulate options associated with a
socket. Options may exist at multiple protocol levels; they are
always present at the uppermost "socket" level.

When manipulating socket options the level at which the option
resides and the name of the option must be specified. To
manipulate options at the "socket" level, level is specified as
SOL_SOCKET. To manipulate options at any other level the protocol
number of the appropriate protocol controlling the option is
supplied. For example, to indicate that an option is to be
interpreted by the TCP protocol, level should be set to the
protocol number of TCP; see getprotoent(3).

The parameters optval and optlen are used to access option values
for setsockopt. For getsockopt they identify a buffer in which the
value for the requested option(s) are to be returned. For
getsockopt, optlen is a value-result parameter, initially
containing the size of the buffer pointed to by optval, and
modified on return to indicate the actual size of the value
returned. If no option value is to be supplied or returned, optval
may be supplied as 0.

Optname and any specified options are passed uninterpreted to the
appropriate protocol module for interpretation. The include file
<sys/socket.h> contains definitions for "socket" level options,
described below. Options at other protocol levels vary in format
and name; consult the appropriate entries in section (4).

Most socket-level options take an int parameter for optval. For
setsockopt, the parameter should non-zero to enable a boolean
option, or zero if the option is to be disabled. SO_LINGER uses a
struct linger parameter, defined in <sys/socket.h>, which specifies
the desired state of the option and the linger interval (see
below).

The following options are recognized at the socket level. Except
as noted, each may be examined with getsockopt and set with
setsockopt.

SO_DEBUG toggle recording of debugging information
SO_REUSEADDR toggle local address reuse
SO_KEEPALIVE toggle keep connections alive
SO_DONTROUTE toggle routing bypass for outgoing messages
SO_LINGER linger on close if data present
SO_BROADCAST toggle permission to transmit broadcast messages
SO_OOBINLINE toggle reception of out-of-band data in band
SO_SNDBUF set buffer size for output
SO_RCVBUF set buffer size for input
SO_TYPE get the type of the socket (get only)
SO_ERROR get and clear error on the socket (get only)

SO_DEBUG enables debugging in the underlying protocol modules.
SO_REUSEADDR indicates that the rules used in validating addresses
supplied in a bind(2) call should allow reuse of local addresses.
SO_KEEPALIVE enables the periodic transmission of messages on a
connected socket. Should the connected party fail to respond to
these messages, the connection is considered broken and processes
using the socket are notified via a SIGPIPE signal. SO_DONTROUTE
indicates that outgoing messages should bypass the standard routing
facilities. Instead, messages are directed to the appropriate
network interface according to the network portion of the
destination address.

SO_LINGER controls the action taken when unsent messags are queued
on socket and a close(2) is performed. If the socket promises
reliable delivery of data and SO_LINGER is set, the system will
block the process on the close attempt until it is able to transmit
the data or until it decides it is unable to deliver the
information (a timeout period, termed the linger interval, is
specified in the setsockopt call when SO_LINGER is requested). If
SO_LINGER is disabled and a close is issued, the system will
process the close in a manner that allows the process to continue
as quickly as possible.

The option SO_BROADCAST requests permission to send broadcast
datagrams on the socket. Broadcast was a privileged operation in
earlier versions of the system. With protocols that support out-
of-band data, the SO_OOBINLINE option requests that out-of-band
data be placed in the normal data input queue as received; it will
then be accessible with recv or read calls without the MSG_OOB
flag. SO_SNDBUF and SO_RCVBUF are options to adjust the normal
buffer sizes allocated for output and input buffers, respectively.
The buffer size may be increased for high-volume connections, or
may be decreased to limit the possible backlog of incoming data.
The system places an absolute limit on these values. Finally,
SO_TYPE and SO_ERROR are options used only with setsockopt.
SO_TYPE returns the type of the socket, such as SOCK_STREAM; it is
useful for servers that inherit sockets on startup. SO_ERROR
returns any pending error on the socket and clears the error
status. It may be used to check for asynchronous errors on
connected datagram sockets or for other asynchronous errors.

RETURN VALUE
A 0 is returned if the call succeeds, -1 if it fails.

ERRORS
The call succeeds unless:

[EBADF] The argument s is not a valid descriptor.

[ENOTSOCK] The argument s is a file, not a socket.

[ENOPROTOOPT] The option is unknown at the level indicated.

[EFAULT] The address pointed to by optval is not in a
valid part of the process address space. For
getsockopt, this error may also be returned if
optlen is not in a valid part of the process
address space.

SEE ALSO
ioctl(2), socket(2), getprotoent(3)

BUGS
Several of the socket options should be handled at lower levels of
the system.