NAME
getsockopt, setsockopt - get and set options on sockets
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int
getsockopt(int s, int level, int optname, void * restrict optval,
socklen_t * restrict optlen);
int
setsockopt(int s, int level, int optname, const void *optval,
socklen_t optlen);
DESCRIPTION
The getsockopt() and setsockopt() system calls 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 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, level should be
set to the protocol number of TCP; see getprotoent(3).
The optval and optlen arguments 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 argument, 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.
The optname argument and any specified options are passed uninterpreted
to the appropriate protocol module for interpretation. The include file
#include <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 argument for optval. For
setsockopt(), the argument should be non-zero to enable a boolean option,
or zero if the option is to be disabled. SO_LINGER uses a struct linger
argument, defined in which specifies the desired state of the option and
the linger interval (see below). SO_SNDTIMEO and SO_RCVTIMEO use a
struct timeval argument, defined in
The following options are recognized at the socket level. For protocol-
specific options, see protocol manual pages, e.g. ip(4) or tcp(4).
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_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_ACCEPTFILTER set accept filter on listening socket
SO_NOSIGPIPE controls generation of SIGPIPE for the socket
SO_TIMESTAMP enables reception of a timestamp with datagrams
SO_BINTIME enables reception of a timestamp with datagrams
SO_ACCEPTCONN get listening status of the socket (get only)
SO_TYPE get the type of the socket (get only)
SO_ERROR get and clear error on the socket (get only)
SO_SETFIB set the associated FIB (routing table) for the
socket (set only)
The following options are recognized in FreeBSD:
SO_LABEL get MAC label of the socket (get only)
SO_PEERLABEL get socket’s peer’s MAC label (get only)
SO_LISTENQLIMIT get backlog limit of the socket (get only)
SO_LISTENQLEN get complete queue length of the socket (get
only)
SO_LISTENINCQLEN get incomplete queue length of the socket (get
only)
SO_DEBUG enables debugging in the underlying protocol modules.
SO_REUSEADDR indicates that the rules used in validating addresses
supplied in a bind(2) system call should allow reuse of local addresses.
SO_REUSEPORT allows completely duplicate bindings by multiple processes
if they all set 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. 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
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 appropriate 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(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 seconds in the setsockopt() system
call when SO_LINGER is requested). If SO_LINGER is disabled and a
close(2) 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 versions
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(2) or read(2) 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 maximum on these values,
which is accessible through the sysctl(3) MIB variable
“kern.ipc.maxsockbuf”.
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 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 the 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 from that which was returned.
SO_SNDTIMEO is an option to set a timeout value for output operations.
It accepts a struct timeval argument 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 set a timeout value for input
operations. It accepts a struct timeval argument 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.
SO_SETFIB can be used to over-ride the default FIB (routing table) for
the given socket. The value must be from 0 to one less than the number
returned from the sysctl net.fibs.
SO_ACCEPTFILTER places an accept_filter(9) on the socket, which will
filter incoming connections on a listening stream socket before being
presented for accept(2). Once more, listen(2) must be called on the
socket before trying to install the filter on it, or else the
setsockopt() system call will fail.
struct accept_filter_arg {
char af_name[16];
char af_arg[256-16];
};
The optval argument should point to a struct accept_filter_arg that will
select and configure the accept_filter(9). The af_name argument should
be filled with the name of the accept filter that the application wishes
to place on the listening socket. The optional argument af_arg can be
passed to the accept filter specified by af_name to provide additional
configuration options at attach time. Passing in an optval of NULL will
remove the filter.
The SO_NOSIGPIPE option controls generation of the SIGPIPE signal
normally sent when writing to a connected socket where the other end has
been closed returns with the error EPIPE.
If the SO_TIMESTAMP or SO_BINTIME option is enabled on a SOCK_DGRAM
socket, the recvmsg(2) call will return a timestamp corresponding to when
the datagram was received. The msg_control field in the msghdr structure
points to a buffer that contains a cmsghdr structure followed by a struct
timeval for SO_TIMESTAMP and struct bintime for SO_BINTIME. The cmsghdr
fields have the following values for TIMESTAMP:
cmsg_len = sizeof(struct timeval);
cmsg_level = SOL_SOCKET;
cmsg_type = SCM_TIMESTAMP;
and for SO_BINTIME:
cmsg_len = sizeof(struct bintime);
cmsg_level = SOL_SOCKET;
cmsg_type = SCM_BINTIME;
SO_ACCEPTCONN, SO_TYPE and SO_ERROR are options used only with
getsockopt(). SO_ACCEPTCONN returns whether the socket is currently
accepting connections, that is, whether or not the listen(2) system call
was invoked on the socket. 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.
Finally, SO_LABEL returns the MAC label of the socket. SO_PEERLABEL
returns the MAC label of the socket’s peer. Note that your kernel must
be compiled with MAC support. See mac(3) for more information.
SO_LISTENQLIMIT returns the maximal number of queued connections, as set
by listen(2). SO_LISTENQLEN returns the number of unaccepted complete
connections. SO_LISTENINCQLEN returns the number of unaccepted
incomplete connections.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The call succeeds unless:
[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 getsockopt(),
this error may also be returned if optlen is not in a
valid part of the process address space.
[EINVAL] Installing an accept_filter(9) on a non-listening
socket was attempted.
SEE ALSO
ioctl(2), listen(2), recvmsg(2), socket(2), getprotoent(3), mac(3),
sysctl(3), ip(4), ip6(4), sctp(4), tcp(4), protocols(5), sysctl(8),
accept_filter(9), bintime(9)
HISTORY
The getsockopt() system call appeared in 4.2BSD.
BUGS
Several of the socket options should be handled at lower levels of the
system.