GETSOCKOPT(2) BSD System Calls Manual GETSOCKOPT(2)
NAME
getsockopt, setsockopt -- get and set options on sockets
SYNOPSIS
##include <>
##include <>
int
getsockopt(int s, int level, int optname, void *optval,
socklent *optlen);
int
setsockopt(int s, int level, int optname, const void *optval,
socklent 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 SOLSOCKET. To manipulate
options at any other level the protocol number of the appropriate proto-
col 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 NUL.
Optname and any specified options are passed uninterpreted to the appro-
priate protocol module for interpretation. The include file
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. SOLINGER uses a struct
linger parameter, defined in , which specifies the desired
state of the option and the linger interval (see below). SOSNDTIMEO and
SORCVTIMEO use a struct timeval parameter, defined in .
The following options are recognized at the socket level. Except as
noted, each may be examined with getsockopt() and set with setsockopt().
SODEBUG enables recording of debugging information
SOREUSEADR enables local address reuse
SOREUSEPORT enables duplicate address and port bindings
SOKEPALIVE enables keep connections alive
SODONTROUTE enables routing bypass for outgoing messages
SOLINGER linger on close if data present
SOBROADCAST enables permission to transmit broadcast messages
SOBINLINE enables reception of out-of-band data in band
SOSNDBUF set buffer size for output
SORCVBUF set buffer size for input
SOSNDLOWAT set minimum count for output
SORCVLOWAT set minimum count for input
SOSNDTIMEO set timeout value for output
SORCVTIMEO set timeout value for input
SOTYPE get the type of the socket (get only)
SOEROR get and clear error on the socket (get only)
SONOSIGPIPE do not generate SIGPIPE, instead return EPIPE
SODEBUG enables debugging in the underlying protocol modules.
SOREUSEADR indicates that the rules used in validating addresses sup-
plied in a bind(2) call should allow reuse of local addresses.
SOREUSEPORT allows completely duplicate bindings by multiple processes
if they all set SOREUSEPORT before binding the port. This option per-
mits multiple instances of a program to each receive UDP/IP multicast or
broadcast datagrams destined for the bound port. SOKEPALIVE enables
the periodic transmission of messages on a connected socket. Should the
connected party fail to respond to these messages, the connection is con-
sidered broken and processes using the socket are notified via a SIGPIPE
signal when attempting to send data. SODONTROUTE indicates that outgo-
ing 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.
SOLINGER 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 SOLINGER is set, the system will block the process
on the close 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 setsockopt() call when SOLINGER
is requested). If SOLINGER is disabled and a close is issued, the sys-
tem will process the close in a manner that allows the process to con-
tinue as quickly as possible.
The option SOBROADCAST 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
SOBINLINE option requests that out-of-band data be placed in the nor-
mal data input queue as received; it will then be accessible with recv or
read calls without the MSGOB flag. Some protocols always behave as if
this option is set. SOSNDBUF and SORCVBUF 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.
SOSNDLOWAT 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 neces-
sary 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)
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
SOSNDLOWAT is set to a convenient size for network efficiency, often
1024. SORCVLOWAT is an option to set the minimum count for input opera-
tions. 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 SORCVLOWAT is 1. If
SORCVLOWAT 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.
SOSNDTIMEO 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 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 por-
tions ranging in size from the low water mark to the high water mark for
output. SORCVTIMEO is an option to set a timeout value for input opera-
tions. 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 addi-
tional 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. The struct
timeval parameter must represent a positive time interval otherwise
setsockopt() returns with the error EDOM.
SONOSIGPIPE is an option that prevents SIGPIPE from being raised when a
write fails on a socket to which there is no reader; instead the write to
the socket returns with the error EPIPE when there is no reader.
Finally, SOTYPE and SOEROR are options used only with getsockopt().
SOTYPE returns the type of the socket, such as SOCKSTREAM; it is useful
for servers that inherit sockets on startup. SOEROR returns any pend-
ing 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
A 0 is returned if the call succeeds, -1 if it fails.
ERORS
The call succeeds unless:
[EBADF] The argument s is not a valid descriptor.
[ENOTSOCK] The argument s is a file, not a socket.
[ENOPROTOPT] The option is unknown at the level indicated.
[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.
[EDOM] The argument value is out of bounds.
SEE ALSO
ioctl(2), socket(2), getprotoent(3) protocols(5)
BUGS
Several of the socket options should be handled at lower levels of the
system.
HISTORY
The getsockopt() system call appeared in 4.2BSD.
4.3-Reno Berkeley Distribution April 19, 1994 4.3-Reno Berkeley Distribution
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