GETSOCKOPT(2) System Calls Manual GETSOCKOPT(2)
getsockopt, setsockopt - get and set options on sockets
int getsockopt(s, level, optname, optval, optlen)
int s, level, optname;
int setsockopt(s, level, optname, optval, optlen)
int s, level, optname;
getsockopt() and setsockopt() manipulate 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 SOL_SOCKET. To manipulate
options at any other level the protocol number of the appropriate pro-
tocol 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(3N).
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 getsock-
opt(), 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 supplied as 0.
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 (4P).
Most socket-level options take an int parameter for optval. For set-
sockopt(), 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 <<sys/socket.h>>, which specifies the
desired state of the option and the linger interval (see below).
The following options are recognized at the socket level. Except as
noted, each may be examined with getsockopt() and set with setsock-
SO_DEBUG toggle recording of debugging information
SO_REUSEADDR toggle local address reuse
SO_KEEPALIVE toggle keep connections alive
SO_DONTROUTE toggle routing bypass for outgoing messages
SO_LINGER linger on close if data present
SO_BROADCAST toggle permission to transmit broadcast mes-
SO_OOBINLINE toggle reception of out-of-band data in band
SO_SNDBUF set buffer size for output
SO_RCVBUF set buffer size for input
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 modules.
SO_REUSEADDR indicates that the rules used in validating addresses sup-
plied in a bind(2) call should allow reuse of local addresses.
SO_KEEPALIVE enables the periodic transmission of messages on a con-
nected socket. Should the connected party fail to respond to these
messages, the connection is considered broken. A process attempting to
write to the socket receives a SIGPIPE signal and the write operation
returns an error. By default, a process exits when it receives SIG-
PIPE. A read operation on the socket returns an error but does not
generate SIGPIPE. If the process is waiting in select(2) when the con-
nection is broken, select() returns true for any read or write events
selected for the socket. 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.
SO_LINGER controls the action taken when unsent messags are queued on
socket and a close(2V) 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 option SO_BROADCAST requests permission to send broadcast datagrams
on the socket. Broadcast was a privileged operation in earlier ver-
sions of the system. With protocols 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. 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. Finally, 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 sockets or for other asynchronous errors.
getsockopt() and setsockopt() return:
0 on success.
-1 on failure and set errno to indicate the error.
EBADF s is not a valid descriptor.
EFAULT The address pointed to by optval is not in a valid
part of the process address space.
ENOPROTOOPT The option is unknown at the level indicated.
ENOTSOCK s is a file, not a socket.
In addition to the above, getsockopt() may set errno to:
EFAULT optlen is not in a valid part of the process
ioctl(2), socket(2), getprotoent(3N)
Several of the socket options should be handled at lower levels of the
21 January 1990 GETSOCKOPT(2)