GETSOCKOPT(2)

NAME
getsockopt, setsockopt - get and set options on sockets

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int
getsockopt(int s, int level, int optname, void *optval, socklen_t *optlen);
int
setsockopt(int s, int level, int optname, const void *optval, socklen_t optlen);

DESCRIPTION
Getsockopt() and setsockopt() manipulate the 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(3).

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 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(), 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). SO_SNDTIMEO and SO_RCVTIMEO use a struct timeval parameter, defined in <sys/time.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 recording of debugging information
SO_REUSEADDR | enables local address reuse
SO_REUSEPORT | enables duplicate address and port bindings
SO_KEEPALIVE | enables keep connections alive
SO_DONTROUTE | enables routing bypass for outgoing messages
SO_LINGER | linger on close if data present
SO_BROADCAST | enables permission to transmit broadcast messages
SO_OOBINLINE | enables reception of out-of-band data in band
SO_SNDBUF | set buffer size for output
SO_RCVBUF | set buffer size for input
SO_SNDLOWAT | set minimum count for output
SO_RCVLOWAT | set minimum count for input
SO_SNDTIMEO | set timeout value for output
SO_RCVTIMEO | set timeout value 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(2) call should allow reuse of local addresses. SO_REUSEPORT allows completely duplicate bindings by multiple processes if they all set SO_REUSEPORT before binding the port. This option permits multiple instances of a program to each receive UDP/IP multicast or broadcast datagrams destined for the bound port. 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 via a SIGPIPE signal when attempting to send data. 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 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 seconds in the setsockopt() 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.

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 maximum on these values, which is accessible through the sysctl(3) MIB variable “kern.ipc.maxsockbuf”.

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(2) 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 (non-zero) 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 the 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 from that which was returned.

SO_SNDTIMEO is an option to set a timeout value for output operations. It accepts a struct timeval parameter with the number of seconds and microseconds used to limit waits for output operations to complete. If a send operation has blocked for this much time, it returns with a partial count or with the error EWOULDBLOCK if no data were sent. In the current implementation, this timer is restarted each time additional data are delivered to the protocol, implying that the limit applies to output portions ranging in size from the low water mark to the high water mark for output. SO_RCVTIMEO is an option to set a timeout value for input operations. It accepts a struct timeval parameter with the number of seconds and microseconds used to limit waits for input operations to complete. In the current implementation, this timer is restarted each time additional data are received by the protocol, and thus the limit is in effect an inactivity timer. If a receive operation has been blocked for this much time without receiving additional data, it returns with a short count or with the error EWOULDBLOCK if no data were received.

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.

IMPLEMENTATION NOTES

In the non-threaded library getsockopt() is implemented as the getsockopt syscall.

In the threaded library, the getsockopt syscall is assembled to _thread_sys_getsockopt() and getsockopt() is implemented as a function which locks s for read and write, then calls _thread_sys_getsockopt(). Before returning, getsockopt() unlocks s.

In the non-threaded library setsockopt() is implemented as the setsockopt syscall.

In the threaded library, the setsockopt syscall is assembled to _thread_sys_setsockopt() and setsockopt() is implemented as a function which locks s for read and write, then calls _thread_sys_setsockopt(). Before returning, setsockopt() unlocks s.

RETURN VALUES
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.

[EFAULT]
    The address pointed to by optval is not in a valid part of the process address space. For getsockopt(), this error may also be returned if optlen is not in a valid part of the process address space.

SEE ALSO
ioctl(2), socket(2), getprotoent(3), sysctl(3), protocols(5), sysctl(8)

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

HISTORY
The getsockopt() system call appeared in 4.2BSD.