NAME
getsockopt
,
setsockopt
, getsockopt2
— get and set options on
sockets
LIBRARY
library “libc”
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);
int
getsockopt2
(int
s, int level,
int optname,
void * restrict optval,
socklen_t * restrict
optlen);
DESCRIPTION
getsockopt
(),
setsockopt
() and getsockopt2
()
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
() and
getsockopt2
() they identify a buffer in which the
value for the requested option(s) are to be returned. For
getsockopt2
() they are also used to provide an extra
argument to select which value to return. For
getsockopt
() and
getsockopt2
(), 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 the size of the requested option value
to be stored in optval is greater than the size
indicated in optlen then only
optlen bytes will be stored in
optval and the result will be silently truncated. 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: faith(4), icmp6(4), ip(4), ip6(4), ipsec(4), multicast(4), pim(4), route(4), tcp(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
()
or
getsockopt2
()
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_RERROR |
enables receive size error reporting |
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_RERROR
indicates that
receive buffer overflows should be handled as errors. Historically receive
buffer overflows have been ignored and programs could not tell if they
missed messages or messages had been truncated because of overflows. Since
programs historically do not expect to get receive overflow errors, this
behavior is not the default, but the default can be changed by setting the
SO_RERROR
flag using
sysctl(8) and kern.sooptions
.
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
()
or
getsockopt2
().
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), faith(4), icmp6(4), ip(4), ip6(4), ipsec(4), multicast(4), pim(4), route(4), tcp(4), unix(4), protocols(5), accept_filter(9)
HISTORY
The getsockopt
() system call appeared in
4.2BSD.
The getsockopt2
() system call appeared in
NetBSD 9.0.
BUGS
Several of the socket options should be handled at lower levels of the system.