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and manipulate the associated with a socket. Options may exist at multiple
protocol levels; they are always present at the uppermost 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, is specified as 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 protocol,
should be set to the protocol number of see The parameters and are
used to access option values for For they identify a buffer in which
the value for the requested option(s) are to be returned. For is a value-result
parameter, initially containing the size of the buffer pointed to by and
modified on return to indicate the actual size of the value returned. If
no option value is to be supplied or returned, may be NULL. and any specified
options are passed uninterpreted to the appropriate protocol module for
interpretation. The include file 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 parameter for For the parameter should
be non-zero to enable a boolean option, or zero if the option is to be disabled.
uses a parameter, defined in which specifies the desired state of the
option and the linger interval (see below). and use a parameter, defined
in The following options are recognized at the socket level. Except as
noted, each may be examined with and set with enables
debugging in the underlying protocol modules. indicates that the rules
used in validating addresses supplied in a call should allow reuse of
local addresses. allows completely duplicate bindings by multiple processes
if they all set 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. 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 signal when attempting to send data. 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. controls the action taken when unsent
messages are queued on socket and a is performed. If the socket promises
reliable delivery of data and the system will block the process on the
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 the call when is requested). If is disabled
and a is issued, the system will process the close in a manner that allows
the process to continue as quickly as possible. The option 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 option requests that out-of-band data be placed in the normal
data input queue as received; it will then be accessible with or calls
without the flag. Some protocols always behave as if this option is set.
and 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 limit on these values. 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 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 is set to a convenient size for network efficiency, often 1024. 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 is 1. If 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 than that returned. is an option to set
a timeout value for output operations. It accepts a 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 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. is an
option to set a timeout value for input operations. It accepts a 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 if no data were received. Finally, and
are options used only with returns the type of the socket, such as
it is useful for servers that inherit sockets on startup. 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.
A 0 is returned if the call succeeds, -1 if it fails.
The call succeeds unless: The argument is not a valid descriptor.
The argument is a file, not a socket. The option is unknown at the level
indicated. The address pointed to by is not in a valid part of the process
address space. For this error may also be returned if is not in a valid
part of the process address space.
Several of the socket
options should be handled at lower levels of the system.
The system
call appeared in
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