getsockopt(), setsockopt() - get and set options on sockets
#include <sys/types.h>
#include <sys/socket.h>
int getsockopt (int s, int level, int optname, void *optval,
int *optlen)
int setsockopt (int s, int level, int optname,
const void *optval, int optlen)
The getsockopt(2) and setsockopt(2) functions manipulate the options associated with a socket. Options are always present at the uppermost socket level and they may also exist at other protocol levels.
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(2) .
The parameters optval and optlen are used to access option values for setsockopt(2). For getsockopt(2) they identify a buffer in which the value for the requested option(s) are to be returned. For getsockopt(2), 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 NULL.
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 4 of the manual.
Most socket-level options utilize an int parameter for optval. For setsockopt(2), 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(2) and set with setsockopt(2).
| Option | Default | Description |
|---|---|---|
| SO_ACCEPTCONN | 0 (off) | socket is listen()ing |
| SO_BROADCAST | 0 (off) | enables permission to transmit broadcast messages |
| SO_DEBUG | 0 (off) | enables recording of debugging information |
| SO_DONTROUTE | 0 (off) | enables routing bypass for outgoing messages |
| SO_ERROR | N/A | get and clear error on the socket (get only) |
| SO_EXCLUSIVEADDRUSE | 0 (off) | enables a socket to be bound for exclusive access |
| SO_KEEPALIVE | 0 (off) | enables keep connections alive |
| SO_LINGER | 0 (off) | linger on close if data present |
| SO_OOBINLINE | 0 (off) | enables reception of out-of-band data in band |
| SO_RCVBUF | ? | set buffer size for input |
| SO_REUSEADDR | 0 (off) | enables local address reuse |
| SO_SNDBUF | ? | set buffer size for output |
| SO_TYPE | N/A | get the type of the socket (get only) |
| TCP_NODELAY | 0 (off) | disables the Nagle algorithm |
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(2) or read(2) calls without the MSG_OOB flag. Some protocols always behave as if this option is set. 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.
SO_DEBUG enables debugging in the underlying protocol modules.
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_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 and processes using the socket are notified through a SIGPIPE signal when attempting to send data.
SO_LINGER controls the action taken when unsent messages are queued on 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(2) 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(2) call when SO_LINGER is requested). If SO_LINGER is disabled and a close(2) is issued, the system will process the close in a manner that allows the process to continue as quickly as possible.
SO_REUSEADDR indicates that the rules used in validating addresses supplied in a bind(2) call should allow reuse of local addresses.
The TCP_NODELAY option disables the Nagle algorithm for minimizing network congestion. Briefly, the algorithm requires that TCP segments not be sent until all outstanding data is acknowledged, or there's a maximum segment size packet to send. The stack is allowed to condense data into multibyte segments. Most applications are not affected by the Nagle algorithm and therefore should not set TCP_NODELAY. Some applications are affected:
A mouse- or pointer-using application such as an X Server is an application that should disable the Nagle algorithm; it needs to track and respond to the mouse movements.
Both getsockopt(2) and setsockopt(2) return 0 if they succeed. If they fail, they returns -1 and set errno to indicate the error.
The call succeeds unless:
ioctl(2)
socket(2)