getsockopt(3N)

get and set options on sockets

From the original Amiga UNIX 2.1 manual. Some things it mentions may not be in this release.

NAME

getsockopt, setsockopt - get and set options on sockets

SYNOPSIS

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

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

int setsockopt(s, level, optname, optval, optlen)
int s, level, optname;
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 [see getprotoent(3N)].

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. If no option value is to be supplied or returned, a 0 optval may be supplied.

optname and any specified options are passed uninterpreted to the appropriate protocol module for interpretation. The include file /usr/include/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 /usr/include/sys/socket.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            toggle   recording   of   debugging
                    information
SO_REUSEADDR        toggle local address reuse
SO_KEEPALIVE        toggle keep connections alive
SO_DONTROUTE        toggle routing bypass for  outgoing
                    messages
SO_LINGER           linger on close if data is present
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_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(3N) 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(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() 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 system places an absolute limit on these values. 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.

RETURN VALUE

A 0 is returned if the call succeeds, -1 if it fails.

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.

ENOMEM There was insufficient user memory available for the operation to complete.