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GETSOCKOPT(2) | System Calls Manual | GETSOCKOPT(2) |
NAME¶
getsockopt, setsockopt — get and set options on socketsLIBRARY¶
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 asSOL_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_ERROR |
get and clear error on the socket (get only) |
SO_SETFIB |
set the associated FIB (routing table) for the socket (set only) |
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]; };
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;
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.June 13, 2008 | Debian |