GETSOCKOPT(2) UNIX Programmer's Manual GETSOCKOPT(2)
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;
DESCRIPTIONGetsockopt 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 con-
trolling 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(3N).
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 (4P).
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 structlinger parameter, defined in
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GETSOCKOPT(2) UNIX Programmer's Manual GETSOCKOPT(2)
<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 con-
nection 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 appropri-
ate 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 broad-
cast 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
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GETSOCKOPT(2) UNIX Programmer's Manual GETSOCKOPT(2)
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 descrip-
tor.
[ENOTSOCK] The argument s is a file, not a socket.
[ENOPROTOOPT] The option is unknown at the level indi-
cated.
[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 ALSOioctl(2), socket(2), getprotoent(3N)BUGS
Several of the socket options should be handled at lower
levels of the system.
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