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.\"     @(#)getsockopt.2        8.3 (Berkeley) 4/19/94

.\"

.Dd February 15, 1999

.Dt GETSOCKOPT 2

.Os

.Sh NAME

.Nm getsockopt ,

.Nm setsockopt

.Nd get and set options on sockets

.Sh SYNOPSIS

.Fd #include 

.Fd #include 

.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

.Dv NULL .

.Pp

.Fa optname

and any specified options are passed uninterpreted to the appropriate

protocol module for interpretation.

The include file

.Ao Pa sys/socket.h Ac

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

.Li 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

.Li struct linger

parameter, defined in

.Aq Pa 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

.Li struct timeval

parameter, defined in

.Aq Pa 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_OOBINLINE -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_TYPE Ta "get the type of the socket (get only)"

.It Dv SO_ERROR Ta "get and clear error on the socket (get only)"

.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.

.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.

.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

.Li 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

.Li 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

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

A 0 is returned if the call succeeds, \-1 if it fails.

.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.

.El

.Sh SEE ALSO

.Xr connect 2 ,

.Xr ioctl 2 ,

.Xr poll 2 ,

.Xr select 2 ,

.Xr poll 2 ,

.Xr socket 2 ,

.Xr getprotoent 3 ,

.Xr protocols 5

.Sh BUGS

Several of the socket options should be handled at lower levels of the system.

.Sh HISTORY

The

.Fn getsockopt

system call appeared in

.Bx 4.2 .