GETSOCKOPT(2)
NAME
getsockopt, setsockopt - get and set options on sockets
SYNOPSIS
#include <<sys/types.h>>
#include <<sys/socket.h>>
int getsockopt(int s, int level, int optname, void *opt-
val, int *optlen);
int setsockopt(int s, int level, int optname, const void
*optval, int optlen);
DESCRIPTION
Getsockopt and setsockopt manipulate the options associ-
ated with a socket. Options may exist at multiple proto-
col 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 speci-
fied. To manipulate options at the socket level, level is
specified as SOL_SOCKET. 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 NULL.
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.
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. SO_LINGER uses a struct linger parameter,
defined in <linux/socket.h>, which specifies the desired
state of the option and the linger interval (see below).
SO_SNDTIMEO and SO_RCVTIMEO use a struct timeval parame-
ter, defined in <sys/time.h>.
The following options are recognized at the socket level.
Except as noted, each may be examined with getsockopt and
set with setsockopt.
SO_DEBUG
enables recording of debugging information
SO_REUSEADDR
enables local address reuse
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
get timeout value for output (get only)
SO_RCVTIMEO
get timeout value for input (get only)
SO_TYPE get the type of the socket (get only)
SO_ERROR
get and clear error on the socket (get only)
SO_DEBUG enables debugging in the underlying protocol mod-
ules. SO_REUSEADDR indicates that the rules used in vali-
dating addresses supplied in a bind(2) call should allow
reuse of local addresses. SO_KEEPALIVE enables the peri-
odic transmission of messages on a connected socket.
Should the connected party fail to respond to these
messages, the connection is considered broken and pro-
cesses using the socket are notified via a SIGPIPE signal
when attempting to send data. SO_DONTROUTE indicates that
outgoing messages should bypass the standard routing
facilities. Instead, messages are directed to the appro-
priate network interface according to the network portion
of the destination address.
SO_LINGER 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 SO_LINGER 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 SO_LINGER is requested). If
SO_LINGER is disabled and a close is issued, the system
will process the close in a manner that allows the process
to continue as quickly as possible.
The linger structure is defined in <linux/socket.h> as
follows:
struct linger {
int l_onoff; /* Linger active */
int l_linger; /* How long to linger for */
};
l_onoff indicates wether to linger or not. If it is set to
1 then l_linger contains the time in hundredths of seconds
how long the process should linger to complete the close.
If l_onoff is set to zero the process returns immediately.
The option SO_BROADCAST requests permission to send broad-
cast datagrams on the socket. Broadcast was a privileged
operation in earlier versions of the system. With proto-
cols that support out-of-band data, the 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 recv or read calls without the MSG_OOB
flag. Some protocols always behave as if this option is
set. SO_SNDBUF and 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.
SO_SNDLOWAT is an option to set the minimum count for out-
put 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 con-
trol. 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 SO_SNDLOWAT is set to a convenient size
for network efficiency, often 1024.
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 smaller of the amount available or the amount
requested. The default value for SO_RCVLOWAT is 1. If
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.
SO_SNDTIMEO is an option to get the timeout value for out-
put operations. (It can be used with getsockopt only).
It returns 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 portions ranging in size
from the low water mark to the high water mark for output.
SO_RCVTIMEO is an option to get the timeout value for
input operations. (It can be used with getsockopt only).
It returns 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 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.
Finally, also SO_TYPE and SO_ERROR are options used only
with getsockopt. SO_TYPE returns the type of the socket,
such as SOCK_STREAM; it is useful for servers that inherit
sockets on startup. 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 sock-
ets or for other asynchronous errors.
RETURN VALUE
On success, zero is returned. On error, -1 is returned,
and errno is set appropriately.
ERRORS
EBADF The argument s is not a valid descriptor.
ENOTSOCK
The argument s is a file, not a socket.
ENOPROTOOPT
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 getsock-
opt, this error may also be returned if optlen is
not in a valid part of the process address space.
CONFORMING TO
SVr4, 4.4BSD (these system calls first appeared in
4.2BSD). SVr4 documents additional ENOMEM and ENOSR error
codes, but does not document the SO_SNDLOWAT, SO_RCVLOWAT,
SO_SNDTIMEO, SO_RCVTIMEO options
BUGS
Several of the socket options should be handled at lower
levels of the system.
SEE ALSO
ioctl(2) socket(2) getprotoent(3) protocols(5)