NAME

getsockopt, setsockopt – get and set options on sockets

SYNOPSIS

#include <sys/socket.h>

int getsockopt(int s ,

int level ,

int optname ,

char *optval ,

int *optlen );

int setsockopt(int s ,

int level ,

int optname ,

const char *optval ,

int optlen );

DESCRIPTION

getsockopt() and setsockopt() manipulate 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, level is the protocol number of the protocol that controls the option. For example, to indicate that an option is to be interpreted by the TCP protocol, level is set to the TCP protocol number. Look up these protocol levels by name in #include <sys/socket.h>

The parameters optval and optlen are used to access option values for setsockopt(). For getsockopt(), they identify a buffer in which the value(s) 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. Use a 0 (zero) for optval, if no option value is to be supplied or returned.

optname and any specified options are passed uninterpreted to the appropriate protocol module for interpretation. The include file <sys/socket.h> contains definitions for the socket-level options described below. Options at other protocol levels vary in format and name.

Most socket-level options take an int for optval . For setsockopt(), the optval 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 that specifies the desired state of the option and the linger interval (see below). struct linger is defined in <sys/socket.h>. struct linger contains the following members:

        l_onoff         on = 1/off = 0
        l_linger        linger time in seconds

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

SO_DEBUG

toggle recording of debugging information

SO_LINGER

linger on close if data is present

SO_REUSEADDR

toggle local address reuse

SO_KEEPALIVE

toggle keep connections alive

SO_BROADCAST

toggle permission to transmit broadcast messages

SO_OOBINLINE

toggle reception of out-of-band data in band

SO_SNDBUF

set buffer size for output

SO_RCVBUF

set buffer size for input

SO_TYPE

get the type of the socket (get only)

SO_SNDLOWAT

set minimum count for output

SO_RCVLOWAT

set minimum count for input

SO_ERROR

get and clear error on 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() call should allow reuse of local addresses. SO_KEEPALIVE enables the periodic transmission of messages on a connected socket. If the connected party fails to respond to these messages, the connection is considered broken and processes using the socket are notified using a SIGPIPE signal. 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 a 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 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 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 manner that allows the process to continue as quickly as possible.

The option SO_BROADCAST requests permission to send broadcast datagrams on the socket. 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() or read() calls without the MSG_OOB flag.

SO_SNDBUF and SO_RCVBUF are options that 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 absolute limits on these values are 16K bytes.

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() or poll() 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 (nonzero) 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 he 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.

Finally, SO_TYPE and SO_ERROR are options used only with getsockopt() . . SO_TYPE returns the type of the socket (for example, 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.

Options for use with getsockopt and setsockopt at the IP level.

IP_OPTIONS

Specifies IP options to be inserted into outgoing packets. Setting new options overwrites all previously specified options. Setting optval to zero removes all previously specified options. IP_OPTIONS support is not required; to check whether IP_OPTIONS is supported, use getsockopt to get current options. If getsockopt fails, IP_OPTIONS is not supported.

IP_TOS

Type of Service (TOS) settings should only be set using the Quality of Service API.

IP_TTL

Changes the default value set by the TCP/IP service provider in the TTL field of the IP header in outgoing datagrams. IP_TTL support is not required; to check whether IP_TTL is supported, use getsockopt to get current options. If getsockopt fails, IP_TTL is not supported.

IP_RECVOPTS

Enables a SOCK_DGRAM socket to receive the IPv4 options for a UDP datagram.

IP_RECVRETOPTS

Receive IP opts for response

IP_RECVDSTADDR

Enables a SOCK_DGRAM socket to receive the destination IPv4 address for a UDP datagram.

IP_MULTICAST_IF

Gets or sets the outgoing interface for sending IPv4 multicast traffic. This option does not change the default interface for receiving IPv4 multicast traffic. The input value for setting this option is a 4-byte IPv4 address in network byte order. The interface index can be used to specify the default interface for multicast traffic for IPv4. If optval is zero , the default interface for receiving multicast is specified for sending multicast traffic. When getting this option, the optval returns the current default interface index for sending multicast IPv4 traffic in host byte order.

IP_MULTICAST_TTL

Sets/gets the TTL value associated with IP multicast traffic on the socket.

IP_MULTICAST_LOOP

Controls whether data sent by an application on the local computer (not necessarily by the same socket) in a multicast session will be received by a socket joined to the multicast destination group on the loopback interface. A value of TRUE causes multicast data sent by an application on the local computer to be delivered to a listening socket on the loopback interface. A value of FALSE prevents multicast data sent by an application on the local computer from being delivered to a listening socket on the loopback interface. By default, IP_MULTICAST_LOOPBACK is enabled.

IP_ADD_MEMBERSHIP

Join the socket to the supplied multicast group on the specified interface.

RETURN VALUES

If successful, getsockopt() returns 0; otherwise it returns -1 and sets errno to indicate the error.

ERRORS

These functiona are a member of Unison’s IOLIB family of functions. IOLIB is implemented as a message passing and generalized interface layer. Each Unison I/O server is responsible for its own error reporting.

For an exact list of error codes returned by a particular server, refer to that server’s documentation in the Unison Programmer’s Guide for each specific platform.

Servers may implement these errors codes in response to this function.

The call succeeds unless:

EBADF

The argument s is not a valid file descriptor.

ENOMEM

There was insufficient memory available for the operation to complete.

ENOPROTOOPT

The option is unknown at the level indicated.

ENOTSOCK

The argument s is not a socket.

SEE ALSO

close(), ioctl(), bind(), socket()