NAME
getsockopt,
setsockopt —
get and set options on sockets
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/socket.h>
int
getsockopt(
int
s,
int level,
int optname,
void * restrict optval,
socklen_t * restrict
optlen);
int
setsockopt(
int
s,
int level,
int optname,
const void *optval,
socklen_t optlen);
DESCRIPTION
getsockopt() and
setsockopt() manipulate the
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
NULL
.
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 of the manual,
including:
clnp(4),
faith(4),
icmp6(4),
ip(4),
ip6(4),
ipsec(4),
multicast(4),
pim(4),
route(4),
tcp(4),
tp(4), and
unix(4).
Most socket-level options use an
int parameter for
optval. For
setsockopt(), the
parameter should be 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).
SO_SNDTIMEO
and
SO_RCVTIMEO
use a
struct timeval parameter, defined in
⟨
sys/time.h⟩.
The following options are recognized at the socket level. Except as noted, each
may be examined with
getsockopt() and set with
setsockopt().
SO_DEBUG |
enables recording of debugging information |
SO_REUSEADDR |
enables local address reuse |
SO_REUSEPORT |
enables duplicate address and port bindings |
SO_KEEPALIVE |
enables keep connections alive |
SO_DONTROUTE |
enables routing bypass for outgoing messages |
SO_LINGER |
linger on close if data present |
SO_BROADCAST |
enables permission to transmit broadcast messages |
SO_OOBINLINE |
enables reception of out-of-band data in band |
SO_SNDBUF |
set buffer size for output |
SO_RCVBUF |
set buffer size for input |
SO_SNDLOWAT |
set minimum count for output |
SO_RCVLOWAT |
set minimum count for input |
SO_SNDTIMEO |
set timeout value for output |
SO_RCVTIMEO |
set timeout value for input |
SO_TIMESTAMP |
enables reception of a timestamp with datagrams |
SO_ACCEPTFILTER |
set accept filter on listening socket |
SO_NOSIGPIPE |
controls generation of SIGPIPE
for the socket |
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_REUSEPORT
allows completely
duplicate bindings by multiple processes if they all set
SO_REUSEPORT
before binding the port. This option
permits multiple instances of a program to each receive UDP/IP multicast or
broadcast datagrams destined for the bound port.
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 when
attempting to send data.
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 messages
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(2) attempt until it is
able to transmit the data or until it decides it is unable to deliver the
information (a timeout period, measured in seconds, termed the linger
interval, is specified in the
setsockopt() call when
SO_LINGER
is requested). If
SO_LINGER
is disabled and a
close(2) 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(2) or
read(2) calls without the
MSG_OOB
flag. Some protocols always behave as if this
option is set.
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.
SO_SNDLOWAT
is an option to set the minimum count for
output operations. Most output operations process all of the data supplied by
the call, delivering data to the protocol for transmission and blocking as
necessary for flow control. Nonblocking output operations will process as much
data as permitted subject to flow control without blocking, but will process
no data if flow control does not allow the smaller of the low water mark value
or the entire request to be processed. A
select(2) or
poll(2) operation testing the
ability to write to a socket will return true only if the low water mark
amount could be processed. The default value for
SO_SNDLOWAT
is set to a convenient size for network
efficiency, often 1024.
SO_RCVLOWAT
is an option to
set the minimum count for input operations. In general, receive calls will
block until any (non-zero) amount of data is received, then return with the
smaller of the amount available or the amount requested. The default value for
SO_RCVLOWAT
is 1. If
SO_RCVLOWAT
is set to a larger value, blocking receive
calls normally wait until they have received the smaller of the low water mark
value or the requested amount. Receive calls may still return less than the
low water mark if an error occurs, a signal is caught, or the type of data
next in the receive queue is different than that returned.
SO_SNDTIMEO
is an option to set a timeout value for
output operations. It accepts a
struct timeval parameter
with the number of seconds and microseconds used to limit waits for output
operations to complete. If a send operation has blocked for this much time, it
returns with a partial count or with the error
EAGAIN
if no data were sent. In the current implementation, this timer is restarted
each time additional data are delivered to the protocol, implying that the
limit applies to output portions ranging in size from the low water mark to
the high water mark for output.
SO_RCVTIMEO
is an
option to set a timeout value for input operations. It accepts a
struct timeval parameter with the number of seconds and
microseconds used to limit waits for input operations to complete. In the
current implementation, this timer is restarted each time additional data are
received by the protocol, and thus the limit is in effect an inactivity timer.
If a receive operation has been blocked for this much time without receiving
additional data, it returns with a short count or with the error
EAGAIN
if no data were received.
If the
SO_TIMESTAMP
option is enabled on a
SOCK_DGRAM
socket, the
recvmsg(2) call will return a
timestamp corresponding to when the datagram was received. The msg_control
field in the msghdr structure points to a buffer that contains a cmsghdr
structure followed by a struct timeval. The cmsghdr fields have the following
values:
cmsg_len = sizeof(struct timeval)
cmsg_level = SOL_SOCKET
cmsg_type = SCM_TIMESTAMP
SO_ACCEPTFILTER
places an
accept_filter(9) on the
socket, which will filter incoming connections on a listening socket before
being presented for
accept(2).
The
setsockopt() system call will fail if the socket already
has a filter set, and
listen(2)
must be called on the socket before trying to install a filter. The
optval argument should point to a
struct
accept_filter_arg that will select and configure the
accept_filter(9), defined
as follows:
struct accept_filter_arg {
char af_name[16];
char af_arg[256-16];
};
The
af_name argument should be filled with the name of the
accept filter that the application wishes to place on the listening socket.
The optional argument
af_arg can be passed to the accept
filter specified by
af_name to provide additional
configuration options at attach time. Passing in an
optval of
NULL
will remove the
filter.
Finally,
SO_TYPE
and
SO_ERROR
are options used only with
getsockopt().
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 VALUES
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.
-
-
- [
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.
-
-
- [
EINVAL
]
- The socket s was not suitable for
installing an
accept_filter(9).
-
-
- [
ENOPROTOOPT
]
- The option is unknown at the level indicated.
-
-
- [
ENOTSOCK
]
- The argument s is a file, not a
socket.
SEE ALSO
ioctl(2),
poll(2),
select(2),
socket(2),
getprotoent(3),
clnp(4),
faith(4),
icmp6(4),
ip(4),
ip6(4),
ipsec(4),
multicast(4),
pim(4),
route(4),
tcp(4),
tp(4),
unix(4),
protocols(5),
accept_filter(9)
HISTORY
The
getsockopt() system call appeared in
4.2BSD.
BUGS
Several of the socket options should be handled at lower levels of the
system.