DragonFly On-Line Manual Pages
GETSOCKOPT(2) DragonFly System Calls Manual GETSOCKOPT(2)
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 *optval,
socklen_t *optlen);
int
setsockopt(int s, int level, int optname, const void *optval,
socklen_t optlen);
DESCRIPTION
Getsockopt() and setsockopt() 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 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 getsockopt(), 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 NULL.
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 4 of the manual.
Most socket-level options utilize an int parameter for optval. For
setsockopt(), 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). SO_SNDTIMEO and
SO_RCVTIMEO use a struct timeval parameter, defined in <sys/time.h>.
The following options are recognized at the socket level. 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_USER_COOKIE set the user cookie (uint32_t value)
SO_TYPE get the type of the socket (get only)
SO_ERROR get and clear error on the socket (get only)
SO_RERROR enables receive error reporting
SO_DEBUG enables debugging in the underlying protocol modules.
SO_REUSEADDR indicates that the rules used in validating addresses
supplied in a bind(2) 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_RERROR indicates that receive buffer overflows should be handled as
errors. Historically receive buffer overflows have been ignored and
programs could not tell if they missed messages or messages had been
truncated because of overflows. Since programs historically do not
expect to get receive overflow errors, this behavior is not the default.
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() 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 parameter 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 parameter 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_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() call will fail.
struct accept_filter_arg {
char af_name[16];
char af_arg[256-16];
};
optval should point to a struct accept_filter_arg that will select and
configure the accept_filter(9). af_name should be filled with the name
of the accept filter that the application wishes to place on the
listening socket. af_arg is an optional parameter that 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.
SO_USER_COOKIE can be used to attach an arbitrary uint32_t value to the
socket. The value acts like a user-defined cookie and can be used in the
kernel code that manipulates traffic related to the socket.
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.
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), socket(2), getprotoent(3), sysctl(3), protocols(5), sysctl(8),
accept_filter(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.
DragonFly 6.5-DEVELOPMENT October 30, 2023 DragonFly 6.5-DEVELOPMENT