NAME
getsockopt
,
setsockopt
—
get and 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_PEERCRED
- get the credentials from other side of connection
SO_RTABLE
- set the routing table used for route lookups
SO_SPLICE
- splice two sockets together or get data length
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_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 super-user. 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
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) or
connect(2) were called.
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.
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
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
4.2BSD.
BUGS
Several of the socket options should be handled at lower levels of the system.