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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 <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 <sys/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 argument, defined in
<sys/time.h>.
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_PROTOCOL get the protocol number for the socket (get only)
SO_PROTOTYPE SunOS alias for the Linux SO_PROTOCOL (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_USER_COOKIE set the 'so_user_cookie' value for the socket (uint32_t, set 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_USER_COOKIE can be used to set the uint32_t so_user_cookie field in the socket. The value is an
uint32_t, and can be used in the kernel code that manipulates traffic related to the socket. The default
value for the field is 0. As an example, the value can be used as the skipto target or pipe number in
ipfw/dummynet.
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, SO_PROTOCOL (and its alias SO_PROTOTYPE) 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_PROTOCOL
returns the protocol number for the socket, for AF_INET and AF_INET6 address families. 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() and setsockopt() system calls appeared in 4.2BSD.
BUGS
Several of the socket options should be handled at lower levels of the system.
Debian February 26, 2012 GETSOCKOPT(2)