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Manual page for GETSOCKOPT(2)

getsockopt, setsockopt - get and set options on sockets


#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;


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 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.3n

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 supplied as 0.

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 (4P).

Most socket-level options take 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).

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

toggle recording of debugging information
toggle local address reuse
toggle keep connections alive
toggle routing bypass for outgoing messages
linger on close if data present
toggle permission to transmit broadcast messages
toggle reception of out-of-band data in band
set buffer size for output
set buffer size for input
get the type of the socket (get only)
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.2 call should allow reuse of local addresses. 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. A process attempting to write to the socket receives a SIGPIPE signal and the write operation returns an error. By default, a process exits when it receives SIGPIPE. A read operation on the socket returns an error but does not generate SIGPIPE. If the process is waiting in select.2 when the connection is broken, select() returns true for any read or write events selected for the socket. 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 messags are queued on socket and a close.2v 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. 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() or read() calls without the MSG_OOB flag. 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 limit on these values. 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.


getsockopt() and setsockopt() return:

on success.
on failure and set errno to indicate the error.


s is not a valid descriptor.
The address pointed to by optval is not in a valid part of the process address space.
The option is unknown at the level indicated.
s is a file, not a socket.

In addition to the above, getsockopt() may set errno to:

optlen is not in a valid part of the process address space.


ioctl.2 socket.2 getprotoent.3n


Several of the socket options should be handled at lower levels of the system.

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Created by unroff & hp-tools. © by Hans-Peter Bischof. All Rights Reserved (1997).

Last modified 21/April/97