#include <sys/types.h> #include <sys/socket.h>
int getsockopt(int s, int level, int optname, void *optval, size_t *optlen);
int setsockopt(int s, int level, int optname, const void *optval, size_t optlen);
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 value of 0 may be supplied via optval. If the size of the option value is greater than optlen, the object pointed to by optval will be silently truncated.
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 /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 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 comes into play when a connection has had no traffic on it for two hours. At that point either side of the connection that has SO_KEEPALIVE set will send a special packet guaranteed to elict a response from a properly implemented TCP/IP product. If a response is not received, (after several short-interval timeouts), the connection is declared down and processes using the socket are notified by sending 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.
SO_BROADCAST requests permission to send broadcast datagrams on the socket.
SO_OOBINLINE works with protocols that support out-of-band data to request 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.
SO_SNDBUF and SO_RCVBUF 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.
SO_TYPE and SO_ERROR are 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.