NAME
getsockopt
,
setsockopt
—
get or set options on
sockets
SYNOPSIS
#include
<sys/socket.h>
int
getsockopt
(int
s, int level,
int optname,
void *optval,
socklen_t *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.
Most socket-level options utilize 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; not supported
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_BINDANY
- enables binding to any address
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_RTABLE
- set the routing table used for route lookups
SO_SPLICE
- splice two sockets together or get data length
SO_ZEROIZE
- clear all memory containing user supplied data
SO_TYPE
- get the type of the socket (get only)
SO_ERROR
- get and clear error on the socket (get only)
SO_DOMAIN
- get the domain of the socket (get only)
SO_PROTOCOL
- get the protocol of the socket (get only)
SO_PEERCRED
- get the credentials from other side of connection (get only)
SO_DEBUG
enables debugging in the
underlying protocol modules. Transliterate the protocol trace with
trpt(8).
SO_REUSEADDR
indicates that the rules used in
validating addresses supplied in a bind(2) call should allow reuse of local addresses by callers with
the same user ID (or the superuser). 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_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_BINDANY
allows the socket to be bound
to addresses which are not local to the machine, so it can be used to make a
transparent proxy. Note that this option is limited to the superuser. In
order to receive packets for these addresses,
SO_BINDANY
needs to be combined with matching
outgoing pf(4) rules with the
divert-reply parameter. For example, with the
following rule the socket receives packets for 192.168.0.10 even if it is
not a local address:
pass out inet from 192.168.0.10
divert-reply
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 EWOULDBLOCK
if no data was 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
EWOULDBLOCK
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 = CMSG_LEN(sizeof(struct timeval)) cmsg_level = SOL_SOCKET cmsg_type = SCM_TIMESTAMP
The SO_RTABLE
option gets or sets the
routing table which will be used by the socket for address lookups. If a
protocol family of the socket doesn't support switching routing tables, the
ENOPROTOOPT
error is returned. Only the superuser is
allowed to change the routing table if it is already set to a non-zero
value. A socket's chosen routing table is initialized from the process's
configuration, previously selected using
setrtable(2).
SO_SPLICE
can splice
together two TCP or UDP sockets for unidirectional zero-copy data transfers.
Splice also the other way around to get bidirectional data flow. Both
sockets must be of the same type. In the first form,
setsockopt
()
is called with the source socket s and the drain
socket's int file descriptor as
optval. In the second form,
optval is a struct splice with
the drain socket in sp_fd, a positive maximum number
of bytes or 0 in sp_max and an idle timeout
sp_idle in the form of a struct
timeval. If -1 is given as drain socket, the source socket
s gets unspliced. Otherwise the spliced data transfer
continues within the kernel until the optional maximum is reached, one of
the connections terminates, idle timeout expires or an error occurs. A
successful select(2),
poll(2), or
kqueue(2) operation testing
the ability to read from the source socket indicates that the splicing has
terminated. When one of the sockets gets closed, splicing ends. The error
status can be examined with SO_ERROR
at the source
socket. The ELOOP
error is set if userland created a
loop by splicing sockets connected to localhost. The
ETIMEDOUT
error is set if there was no data
transferred between two sockets during the sp_idle
period of time. The EFBIG
error is set after exactly
sp_max bytes have been transferred. Note that if a
maximum is given, it is only guaranteed that no more bytes are transferred.
A short splice can happen, but then a second call to splice will transfer
the remaining data immediately. The SO_SPLICE
option
with getsockopt
() and an off_t
value as optval can be used to retrieve the number of
bytes transferred so far from the source socket s. A
successful new splice resets this number.
Userland may write sensitive data into a socket. If
SO_ZEROIZE
is set, overwrite kernel memory after
sending data.
Finally, SO_TYPE
,
SO_DOMAIN
, SO_PROTOCOL
,
SO_ERROR
, and SO_PEERCRED
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_DOMAIN
returns the domain of
the socket, such as AF_INET
.
SO_PROTOCOL
returns the protocol of the socket such
as IPPROTO_TCP
. 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. SO_PEERCRED
fetches
the struct sockpeercred credentials from the other
side of the connection (currently only possible on
AF_UNIX
sockets). These credentials are from the
time that bind(2),
connect(2) or
socketpair(2) were
called.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
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.
- [
EOPNOTSUPP
] - The option is unsupported.
- [
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
connect(2), getrtable(2), ioctl(2), poll(2), select(2), socket(2), getprotoent(3), divert(4), pf.conf(5), protocols(5), sosplice(9)
STANDARDS
The getsockopt
() and
setsockopt
() functions conform to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
The getsockopt
() system call appeared in
4.1cBSD.
BUGS
Several of the socket options should be handled at lower levels of the system.