libc/sys/getsockopt.2

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.\"     @(#)getsockopt.2	8.4 (Berkeley) 5/2/95
.\" $FreeBSD: src/lib/libc/sys/getsockopt.2,v 1.12.2.11 2002/01/09 17:44:15 yar Exp $
.\"
.Dd August 7, 2019
.Dt GETSOCKOPT 2
.Os
.Sh NAME
.Nm getsockopt ,
.Nm setsockopt
.Nd get and set options on sockets
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.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
.Tn TCP
protocol,
.Fa level
should be set to the protocol number of
.Tn 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 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
.Fa 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
.Fa 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
.Fa 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 .
.Bl -column SO_ACCEPTFILTER -offset indent
.It Dv SO_DEBUG Ta "enables recording of debugging information"
.It Dv SO_REUSEADDR Ta "enables local address reuse"
.It Dv SO_REUSEPORT Ta "enables duplicate address and port bindings"
.It Dv SO_KEEPALIVE Ta "enables keep connections alive"
.It Dv SO_DONTROUTE Ta "enables routing bypass for outgoing messages"
.It Dv SO_LINGER Ta "linger on close if data present"
.It Dv SO_BROADCAST Ta "enables permission to transmit broadcast messages"
.It Dv SO_OOBINLINE Ta "enables reception of out-of-band data in band"
.It Dv SO_SNDBUF Ta "set buffer size for output"
.It Dv SO_RCVBUF Ta "set buffer size for input"
.It Dv SO_SNDLOWAT Ta "set minimum count for output"
.It Dv SO_RCVLOWAT Ta "set minimum count for input"
.It Dv SO_SNDTIMEO Ta "set timeout value for output"
.It Dv SO_RCVTIMEO Ta "set timeout value for input"
.It Dv SO_ACCEPTFILTER Ta "set accept filter on listening socket"
.It Dv SO_TYPE Ta "get the type of the socket (get only)"
.It Dv SO_ERROR Ta "get and clear error on the socket (get only)"
.It Dv SO_RERROR Ta "enables receive error reporting"
.El
.Pp
.Dv SO_DEBUG
enables debugging in the underlying protocol modules.
.Dv SO_REUSEADDR
indicates that the rules used in validating addresses supplied
in a
.Xr bind 2
call should allow reuse of local addresses.
.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.
.Dv 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.
.Dv 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.
.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, termed the
linger interval, is specified in seconds 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.
.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 maximum on these values, which is accessible
through the
.Xr sysctl 3
MIB variable
.Va kern.ipc.maxsockbuf .
.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
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 from that which was returned.
.Pp
.Dv SO_SNDTIMEO
is an option to set a timeout value for output operations.
It accepts a
.Fa 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 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.
.Dv SO_RCVTIMEO
is an option to set a timeout value for input operations.
It accepts a
.Fa 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
.Dv SO_ACCEPTFILTER
places an
.Xr accept_filter 9
on the socket,
which will filter incoming connections
on a listening stream socket before being presented for
.Xr accept 2 .
Once more,
.Xr listen 2
must be called on the socket before
trying to install the filter on it,
or else the
.Fn setsockopt
call will fail.
.Bd -literal
struct  accept_filter_arg {
        char    af_name[16];
        char    af_arg[256-16];
};
.Ed
.Pp
.Fa optval
should point to a
.Fa struct accept_filter_arg
that will select and configure the
.Xr accept_filter 9 .
.Fa af_name
should be filled with the name of the accept filter
that the application wishes to place on the listening socket.
.Fa af_arg
is an optional parameter that can be passed to the accept
filter specified by
.Fa af_name
to provide additional configuration options at attach time.
Passing in an
.Fa optval
of NULL will remove the filter.
.Pp
Finally,
.Dv SO_TYPE
and
.Dv SO_ERROR
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_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.
.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 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.
.It Bq Er EINVAL
Installing an
.Xr accept_filter 9
on a non-listening socket was attempted.
.El
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr getprotoent 3 ,
.Xr sysctl 3 ,
.Xr protocols 5 ,
.Xr sysctl 8 ,
.Xr accept_filter 9
.Sh HISTORY
The
.Fn getsockopt
system call appeared in
.Bx 4.2 .
.Sh BUGS
Several of the socket options should be handled at lower levels of the system.