libc/sys/getsockopt.2

.\"	$OpenBSD: getsockopt.2,v 1.62 2024/04/02 14:23:15 claudio Exp $
.\"	$NetBSD: getsockopt.2,v 1.7 1995/02/27 12:33:29 cgd Exp $
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.\"     @(#)getsockopt.2	8.3 (Berkeley) 4/19/94
.\"
.Dd $Mdocdate: April 2 2024 $
.Dt GETSOCKOPT 2
.Os
.Sh NAME
.Nm getsockopt ,
.Nm setsockopt
.Nd get or set options on sockets
.Sh SYNOPSIS
.In sys/socket.h
.Ft int
.Fn getsockopt "int s" "int level" "int optname" "void *optval" "socklen_t *optlen"
.Ft int
.Fn setsockopt "int s" "int level" "int optname" "const void *optval" "socklen_t optlen"
.Sh DESCRIPTION
.Fn getsockopt
and
.Fn setsockopt
manipulate the
.Em options
associated with a socket.
Options may exist at multiple protocol levels;
they are always present at the uppermost
.Dq socket
level.
.Pp
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,
.Fa level
is specified as
.Dv 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,
.Fa level
should be set to the protocol number of TCP; see
.Xr getprotoent 3 .
.Pp
The parameters
.Fa optval
and
.Fa optlen
are used to access option values for
.Fn setsockopt .
For
.Fn getsockopt
they identify a buffer in which the value for the
requested option(s) are to be returned.
For
.Fn getsockopt ,
.Fa optlen
is a value-result parameter, initially containing the
size of the buffer pointed to by
.Fa optval ,
and modified on return to indicate the actual size of the value returned.
If no option value is to be supplied or returned,
.Fa optval
may be
.Dv NULL .
.Pp
.Fa optname
and any specified options are passed uninterpreted to the appropriate
protocol module for interpretation.
The include file
.In 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.
.Pp
Most socket-level options utilize an
.Vt int
parameter for
.Fa optval .
For
.Fn setsockopt ,
the parameter should be non-zero to enable a boolean option,
or zero if the option is to be disabled.
.Dv SO_LINGER
uses a
.Vt struct linger
parameter, defined in
.In sys/socket.h ,
which specifies the desired state of the option and the
linger interval (see below).
.Dv SO_SNDTIMEO
and
.Dv SO_RCVTIMEO
use a
.Vt struct timeval
parameter, defined in
.In sys/time.h .
.Pp
The following options are recognized at the socket level.
Except as noted, each may be examined with
.Fn getsockopt
and set with
.Fn setsockopt .
.Pp
.Bl -tag -width SO_OOBINLINE -offset indent -compact
.It Dv SO_DEBUG
enables recording of debugging information
.It Dv SO_REUSEADDR
enables local address reuse
.It Dv SO_REUSEPORT
enables duplicate address and port bindings
.It Dv SO_KEEPALIVE
enables keep connections alive
.It Dv SO_DONTROUTE
enables routing bypass; not supported
.It Dv SO_LINGER
linger on close if data present
.It Dv SO_BROADCAST
enables permission to transmit broadcast messages
.It Dv SO_OOBINLINE
enables reception of out-of-band data in band
.It Dv SO_BINDANY
enables binding to any address
.It Dv SO_SNDBUF
set buffer size for output
.It Dv SO_RCVBUF
set buffer size for input
.It Dv SO_SNDLOWAT
set minimum count for output
.It Dv SO_RCVLOWAT
set minimum count for input
.It Dv SO_SNDTIMEO
set timeout value for output
.It Dv SO_RCVTIMEO
set timeout value for input
.It Dv SO_TIMESTAMP
enables reception of a timestamp with datagrams
.It Dv SO_RTABLE
set the routing table used for route lookups
.It Dv SO_SPLICE
splice two sockets together or get data length
.It Dv SO_ZEROIZE
clear all memory containing user supplied data
.It Dv SO_TYPE
get the type of the socket (get only)
.It Dv SO_ERROR
get and clear error on the socket (get only)
.It Dv SO_DOMAIN
get the domain of the socket (get only)
.It Dv SO_PROTOCOL
get the protocol of the socket (get only)
.It Dv SO_ACCEPTCONN
get listening status of the socket (get only)
.It Dv SO_PEERCRED
get the credentials from other side of connection (get only)
.El
.Pp
.Dv SO_DEBUG
enables debugging in the underlying protocol modules.
Transliterate the protocol trace with
.Xr trpt 8 .
.Dv SO_REUSEADDR
indicates that the rules used in validating addresses supplied in a
.Xr bind 2
call should allow reuse of local addresses
by callers with the same user ID (or the superuser).
.Dv SO_REUSEPORT
allows completely duplicate bindings by multiple processes if they all set
.Dv 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.
.Dv 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
.Dv SIGPIPE
signal when attempting to send data.
.Pp
.Dv SO_LINGER
controls the action taken when unsent messages
are queued on socket and a
.Xr close 2
is performed.
If the socket promises reliable delivery of data and
.Dv SO_LINGER
is set, the system will block the process on the
.Xr 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
.Fn setsockopt
call when
.Dv SO_LINGER
is requested).
If
.Dv SO_LINGER
is disabled and a
.Xr close 2
is issued, the system will process the close in a manner that allows
the process to continue as quickly as possible.
.Pp
The option
.Dv 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
.Dv 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
.Xr recv 2
or
.Xr read 2
calls without the
.Dv MSG_OOB
flag.
Some protocols always behave as if this option is set.
.Pp
.Dv 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,
.Dv SO_BINDANY
needs to be combined with matching outgoing
.Xr pf 4
rules with the
.Ar 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:
.Pp
.Dl pass out inet from 192.168.0.10 divert-reply
.Pp
.Dv SO_SNDBUF
and
.Dv 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.
.Pp
.Dv 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
.Xr select 2
or
.Xr 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
.Dv SO_SNDLOWAT
is set to a convenient size for network efficiency, often 1024.
.Dv 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
.Dv SO_RCVLOWAT
is 1.
If
.Dv 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.
.Pp
.Dv SO_SNDTIMEO
is an option to set a timeout value for output operations.
It accepts a
.Vt 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
.Er 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.
.Dv SO_RCVTIMEO
is an option to set a timeout value for input operations.
It accepts a
.Vt 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
.Er EWOULDBLOCK
if no data were received.
.Pp
If the
.Dv SO_TIMESTAMP
option is enabled on a
.Dv SOCK_DGRAM
socket, the
.Xr 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:
.Bd -literal -offset indent
cmsg_len = CMSG_LEN(sizeof(struct timeval))
cmsg_level = SOL_SOCKET
cmsg_type = SCM_TIMESTAMP
.Ed
.Pp
The
.Dv 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
.Er 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
.Xr setrtable 2 .
.Pp
.Dv 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,
.Fn setsockopt
is called with the source socket
.Fa s
and the drain socket's
.Vt int
file descriptor as
.Fa optval .
In the second form,
.Fa optval
is a
.Vt struct splice
with the drain socket in
.Va sp_fd ,
a positive maximum number of bytes or 0 in
.Va sp_max
and an idle timeout
.Va sp_idle
in the form of a
.Vt struct timeval .
If \-1 is given as drain socket, the source socket
.Fa 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
.Xr select 2 ,
.Xr poll 2 ,
or
.Xr 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
.Dv SO_ERROR
at the source socket.
The
.Er ELOOP
error is set if userland created a loop by splicing sockets connected
to localhost.
The
.Er ETIMEDOUT
error is set if there was no data transferred between two sockets
during the
.Va sp_idle
period of time.
The
.Er EFBIG
error is set after exactly
.Va 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
.Dv SO_SPLICE
option with
.Fn getsockopt
and an
.Vt off_t
value as
.Fa optval
can be used to retrieve the number of bytes transferred so far from the
source socket
.Fa s .
A successful new splice resets this number.
.Pp
Userland may write sensitive data into a socket.
If
.Dv SO_ZEROIZE
is set, overwrite kernel memory after sending data.
.Pp
Finally,
.Dv SO_TYPE ,
.Dv SO_DOMAIN ,
.Dv SO_PROTOCOL ,
.Dv SO_ERROR ,
.Dv SO_ACCEPTCONN ,
and
.Dv SO_PEERCRED
are options used only with
.Fn getsockopt .
.Dv SO_TYPE
returns the type of the socket, such as
.Dv SOCK_STREAM ;
it is useful for servers that inherit sockets on startup.
.Dv SO_DOMAIN
returns the domain of the socket, such as
.Dv AF_INET .
.Dv SO_PROTOCOL
returns the protocol of the socket such as
.Dv IPPROTO_TCP .
.Dv 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.
.Dv SO_ACCEPTCONN
returns whether the socket is currently accepting connections, that is,
whether or not
.Xr listen 2
was called.
.Dv SO_PEERCRED
fetches the
.Va struct sockpeercred
credentials from the other side of the connection
(currently only possible on
.Dv AF_UNIX
sockets).
These credentials are from the time that
.Xr bind 2 ,
.Xr connect 2
or
.Xr socketpair 2
were called.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The call succeeds unless:
.Bl -tag -width Er
.It Bq Er EBADF
The argument
.Fa s
is not a valid descriptor.
.It Bq Er ENOTSOCK
The argument
.Fa s
is a file, not a socket.
.It Bq Er ENOPROTOOPT
The option is unknown at the level indicated.
.It Bq Er EOPNOTSUPP
The option is unsupported.
.It Bq Er EFAULT
The address pointed to by
.Fa optval
is not in a valid part of the process address space.
For
.Fn getsockopt ,
this error may also be returned if
.Fa optlen
is not in a valid part of the process address space.
.El
.Sh SEE ALSO
.Xr connect 2 ,
.Xr getrtable 2 ,
.Xr ioctl 2 ,
.Xr poll 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr getprotoent 3 ,
.Xr divert 4 ,
.Xr pf.conf 5 ,
.Xr protocols 5 ,
.Xr sosplice 9
.Sh STANDARDS
The
.Fn getsockopt
and
.Fn setsockopt
functions conform to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn getsockopt
system call appeared in
.Bx 4.1c .
.Sh BUGS
Several of the socket options should be handled at lower levels of the system.