gno/usr.man/man2/getsockopt.2

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.\"     @(#)getsockopt.2	8.3 (Berkeley) 4/19/94
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.TH GETSOCKOPT 2 "16 January 1997" GNO "System Calls"
.SH NAME
.BR getsockopt ,
.BR setsockopt
\- get and set options on sockets
.SH SYNOPSIS
#include <sys/types.h>
.br
#include <sys/socket.h>
.sp 1
int
\fBgetsockopt\fR (int \fIs\fR, int \fIlevel\fR, int \fIoptname\fR, 
void *\fIoptval\fR, int *\fIoptlen\fR);
.sp 1
int
\fBsetsockopt\fR (int \fIs\fR, int \fIlevel\fR, int \fIoptname\fR,
const void *\fIoptval\fR, int \fIoptlen\fR);
.SH DESCRIPTION
.BR Getsockopt 
and
.BR setsockopt 
manipulate the
.IR options 
associated with a socket.  Options may exist at multiple
protocol levels; they are always present at the uppermost
socket level.
.LP
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,
.I level
is specified as
.BR 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,
.I level
should be set to the protocol number of TCP; see
.BR getprotoent (3).
.LP
The parameters
.I optval
and
.I optlen
are used to access option values for
.BR setsockopt .
For
.BR getsockopt 
they identify a buffer in which the value for the
requested option(s) are to be returned.  For
.BR getsockopt ,
.I optlen
is a value-result parameter, initially containing the
size of the buffer pointed to by
.IR optval ,
and modified on return to indicate the actual size of
the value returned.  If no option value is
to be supplied or returned,
.I optval
may be NULL.
.LP
.I 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.
.LP
Most socket-level options utilize an
.I int
parameter for
.IR optval .
For
.BR setsockopt ,
the parameter should be non-zero to enable a boolean option,
or zero if the option is to be disabled.
.BR SO_LINGER
uses a
.B "struct linger"
parameter, defined in <sys/socket.h>,
which specifies the desired state of the option and the
linger interval (see below).
.BR SO_SNDTIMEO
and
.BR SO_RCVTIMEO
use a
.B "struct timeval"
parameter, defined in <sys/time.h>.
.LP
The following options are recognized at the socket level.
Except as noted, each may be examined with
.BR getsockopt 
and set with
.BR setsockopt .
.RS
.nf
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 for outgoing messages
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_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_TYPE		get the type of the socket (get only)
SO_ERROR		get and clear error on the socket (get only)
.fi
.RE
.LP
.BR SO_DEBUG
enables debugging in the underlying protocol modules.
.BR SO_REUSEADDR
indicates that the rules used in validating addresses supplied
in a
.BR bind (2)
call should allow reuse of local addresses.
.BR SO_REUSEPORT
allows completely duplicate bindings by multiple processes
if they all set
.BR 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.
.BR 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
.BR SIGPIPE
signal when attempting to send data.
.BR 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.
.LP
.BR SO_LINGER
controls the action taken when unsent messages
are queued on socket and a 
.BR close (2)
is performed.
If the socket promises reliable delivery of data and
.BR SO_LINGER is set,
the system will block the process on the 
.BR 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 the
.BR setsockopt 
call when
.BR SO_LINGER
is requested). 
If
.BR SO_LINGER
is disabled and a 
.BR close (2)
is issued, the system will process the close in a manner that allows
the process to continue as quickly as possible.
.LP
The option
.BR 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
.BR 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
.BR recv (2)
or
.BR read (2)
calls without the
.BR MSG_OOB
flag.
Some protocols always behave as if this option is set.
.BR SO_SNDBUF
and
.BR 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.
.LP
.BR 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
.BR 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
.BR SO_SNDLOWAT
is set to a convenient size for network efficiency, often 1024.
.BR 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
.BR SO_RCVLOWAT
is 1.
If 
.BR 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.
.LP
.BR SO_SNDTIMEO
is an option to set a timeout value for output operations.
It accepts a
.B "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 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.
.BR SO_RCVTIMEO
is an option to set a timeout value for input operations.
It accepts a
.B "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.
.LP
Finally,
.BR SO_TYPE
and
.BR SO_ERROR
are options used only with
.BR getsockopt .
.BR SO_TYPE
returns the type of the socket, such as
.BR SOCK_STREAM ;
it is useful for servers that inherit sockets on startup.
.BR 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:
.RS
.IP \fBEBADF\fR
The argument
.I s
is not a valid descriptor.
.IP \fBENOTSOCK\fR
The argument
.I s
is a file, not a socket.
.IP \fBENOPROTOOPT\fR
The option is unknown at the level indicated.
.IP \fBEFAULT\fR
The address pointed to by 
.I optval
is not in a valid part of the process address space.
For
.BR getsockopt ,
this error may also be returned if
.I optlen
is not in a valid part of the process address space.
.RE
.SH SEE ALSO
.BR ioctl (2),
.BR socket (2),
.BR getprotoent (3)
.BR protocols (5)
.SH BUGS
Several of the socket options should be handled at lower levels of the system.
.SH HISTORY
The
.BR setsockopt
system call appeared in 4.2BSD.