getsockopt()

get options associated with a socket

Synopsis:

#include <sys/types.h>
#include <sys/socket.h>

int getsockopt( int s, 
                int level, 
                int optname,
                void *optval, 
                int *optlen );

Description:

The getsockopt() function gets options associated with a socket. Options may exist at multiple protocol levels; they're always present at the uppermost ``socket'' level.

When manipulating a socket option, you must specify the option's name and the level at which the option resides:

To manipulate options at the socket level, specify level as SOL_SOCKET.
To manipulate options at any other level, supply the protocol number of the appropriate protocol controlling the option. For example, to indicate that an option is to be interpreted by the TCP protocol, you should set level to the protocol number of TCP; see getprotobyname().

The optval and optlen arguments specify buffer in which the value for the requested options are to be returned. The optlen argument is a value-result parameter; you should initialize it to indicate the size of the buffer pointed to by optval. On return, optlen indicates the actual size of the value. If no option value is to be returned, optval may be NULL.

The parameter 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, which are described below. Options at other protocol levels vary in format and name.

Most socket-level options use an int parameter for optval. SO_LINGER uses a struct linger parameter, defined in <sys/socket.h>, that 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>.

Options recognized at socket level

The following options are recognized at the socket level. Except where noted, each may be examined with getsockopt() and set with setsockopt().

Option:	Description:
SO_BROADCAST	Enable permission to transmit broadcast messages.
SO_DEBUG	Enable recording of debugging information.
SO_DONTROUTE	Enable routing bypass for outgoing messages.
SO_ERROR	Get and clear error on the socket (get only).
SO_KEEPALIVE	Enable keep connections alive.
SO_LINGER	Linger on close if data present.
SO_OOBINLINE	Enable reception of out-of-band data in band.
SO_RCVBUF	Set buffer size for input.
SO_RCVLOWAT	Set minimum count for input.
SO_RCVTIMEO	Set timeout value for input.
SO_REUSEADDR	Enable local address to be reused.
SO_SNDBUF	Set buffer size for output.
SO_SNDLOWAT	Set minimum count for output.
SO_SNDTIMEO	Set timeout value for output.
SO_TYPE	Get the type of the socket (get only).

SO_BROADCAST

Requests permission to send broadcast datagrams on the socket. ``Broadcast'' was a privileged operation in earlier versions of the system.

SO_DEBUG

Enables debugging in the underlying protocol modules.

SO_DONTROUTE

Indicates that outgoing messages should bypass the standard routing facilities. The messages are directed to the appropriate network interface according to the network portion of the destination address.

SO_ERROR and SO_TYPE

SO_ERROR and SO_TYPE are used only with getsockopt() and setsockopt(). SO_TYPE, which returns the type of the socket (e.g. SOCK_STREAM), is useful for servers that inherit sockets on startup. SO_ERROR, which returns any pending error on the socket, clears the error status. You can use it to check for asynchronous errors on connected datagram sockets or for other asynchronous errors.

SO_KEEPALIVE

Enables the periodic (at least every 2 hours) 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 they attempt to send data.

SO_LINGER

Controls the action taken when unsent messages are queued on socket and a close() is performed. If the socket promises reliable delivery of data and SO_LINGER is set, the system will block the process on the close() attempt until it's able to transmit the data or until it decides it can't deliver the information (a timeout period, termed the linger interval, is specified in the setsockopt() call when SO_LINGER is requested).

If SO_LINGER is disabled and a close() is issued, the system will process the close() in a way that lets the process continue as quickly as possible.

SO_OOBINLINE

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; the data will then be accessible with recv() or read() calls without the MSG_OOB flag. Some protocols always behave as if this option is set.

SO_RCVBUF and SO_SNDBUF

SO_SNDBUF and SO_RCVBUF adjust the normal buffer sizes allocated for output and input buffers, respectively. You can increase the buffer size for high-volume connections, or decrease it to limit the possible backlog of incoming data. The system places an absolute limit on these values and defaults them to at least 8k.

SO_RCVLOWAT

The minimum count for input operations. In general, receive calls will block until any (nonzero) amount of data is received, then return with the amount available or the amount requested, whichever is smaller.

The default value for SO_RCVLOWAT is 1. If SO_RCVLOWAT is set to a larger value, blocking receive calls normally wait until they've received the low-water mark value or the requested amount, whichever is smaller. Receive calls may still return less than the low-water mark if an error occurs, if a signal is caught, or if the type of data next in the receive queue differs from that returned.

SO_RCVTIMEO

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 is received by the protocol, so 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, if no data was received, with the error EWOULDBLOCK.

SO_REUSEADDR

Indicates that the rules used in validating addresses supplied in a bind() call should allow local addresses to be reused.

SO_SNDLOWAT

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 doesn't allow the smaller of the low-water mark value or the entire request to be processed.

A select() 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 2048 bytes.

SO_SNDTIMEO

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 data weren't sent.

This timer is restarted each time additional data is 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. Timeouts are restricted to 32 seconds or under.

Returns:

0: Success.
-1: An error occurred; errno is set.

Errors:

If an error occurred, errno could contain one of the following:

EBADF: The argument s isn't a valid descriptor.
EDOM: Value was set out of range.
EFAULT: The address pointed to by optval isn't in a valid part of the process address space. For getsockopt(), this error may also be returned if optlen isn't in a valid part of the process address space.
ENOPROTOOPT: The option is unknown at the level indicated.

Classification:

POSIX 1003.1g (draft)

Safety:
Interrupt handler	No
Signal handler	No
Thread	Yes