--- name: "getsockopt" section: "2" platform: "macos" package: "xnu" version: "12377.121.6" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/getsockopt" --- # getsockopt(2) get and set options on sockets ## NAME **getsockopt**, **setsockopt** – get and set options on sockets ## SYNOPSIS **#include \** *int*\ **getsockopt**(*int socket*, *int level*, *int option\_name*, *void \*restrict option\_value*, *socklen\_t \*restrict option\_len*); *int*\ **setsockopt**(*int socket*, *int level*, *int option\_name*, *const void \*option\_value*, *socklen\_t option\_len*); ## 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)](/getprotoent/3.md). The parameters *option\_value* and *option\_len* 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**(), *option\_len* is a value-result parameter, initially containing the size of the buffer pointed to by *option\_value*, and modified on return to indicate the actual size of the value returned. If no option value is to be supplied or returned, *option\_value* may be NULL. *option\_name* 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 *option\_value*. 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\_NOSIGPIPE | do not generate SIGPIPE, instead return EPIPE | | SO\_NREAD | number of bytes to be read (get only) | | SO\_NWRITE | number of bytes written not yet sent by the protocol (get only) | | SO\_LINGER\_SEC | linger on close if data present with timeout in seconds | SO\_DEBUG enables debugging in the underlying protocol modules. SO\_REUSEADDR indicates that the rules used in validating addresses supplied in a [bind(2)](/bind/2.md) 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)](/close/2.md) 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\_LINGER\_SEC is the same option as SO\_LINGER except the linger time is in seconds for SO\_LINGER\_SEC. 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. 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\_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)](/select/2.md) 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 2048. 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 than that 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. The struct timeval parameter must represent a positive time interval; otherwise, **setsockopt**() returns with the error EDOM. SO\_NOSIGPIPE is an option that prevents SIGPIPE from being raised when a write fails on a socket to which there is no reader; instead, the write to the socket returns with the error EPIPE when there is no reader. Finally, SO\_TYPE, SO\_ERROR, SO\_NREAD, and SO\_NWRITE 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. SO\_NREAD returns the amount of data in the input buffer that is available to be received. For datagram oriented sockets, SO\_NREAD returns the size of the first packet -- this differs from the **ioctl**() command FIONREAD that returns the total amount of data available. SO\_NWRITE returns the amount of data in the output buffer not yet sent by the protocol. ## RETURN VALUES The **getsockopt()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ## ERRORS The **getsockopt**() and **setsockopt**() system calls will succeed unless: - \[EBADF\] The argument *socket* is not a valid file descriptor. - \[EFAULT\] The address pointed to by *option\_value* is not in a valid part of the process address space. For **getsockopt**(), this error may also be returned if *option\_len* is not in a valid part of the process address space. - \[EINVAL\] The option is invalid at the level indicated. - \[ENOBUFS\] Insufficient system resources available for the call to complete. - \[ENOMEM\] Insufficient memory available for the system call to complete. - \[ENOPROTOOPT\] The option is unknown at the level indicated. - \[ENOTSOCK\] The argument *socket* is not a socket (e.g., a plain file). The **setsockopt**() system call will succeed unless: - \[EDOM\] The argument *option\_value* is out of bounds. - \[EISCONN\] *socket* is already connected and a specified option cannot be set while this is the case. - \[EINVAL\] The socket has been shut down. ## LEGACY SYNOPSIS **#include \** **#include \** The include file ⟨sys/types.h⟩ is necessary. ## SEE ALSO [socket(2)](/socket/2.md), [bind(2)](/bind/2.md), [ioctl(2)](/ioctl/2.md), [getprotoent(3)](/getprotoent/3.md), [protocols(5)](/protocols/5.md) ## BUGS Several of the socket options should be handled at lower levels of the system. ## HISTORY The **getsockopt**() system call appeared in 4.2BSD.