table of contents
GETSOCKOPT(2) | System Calls Manual | GETSOCKOPT(2) |
NAME¶
getsockopt
, setsockopt
—
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¶
Thegetsockopt
() 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 = CMSG_LEN(sizeof(struct timeval)); cmsg_level = SOL_SOCKET; cmsg_type = SCM_TIMESTAMP;
and for SO_BINTIME
:
cmsg_len = 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¶
Thegetsockopt
() and
setsockopt
() system calls appeared in
4.2BSD.
BUGS¶
Several of the socket options should be handled at lower levels of the system.April 5, 2013 | Linux 4.9.0-9-amd64 |