GETSOCKOPT(2)

GETSOCKOPT(2) | System Calls Manual | GETSOCKOPT(2)

# NAME

`getsockopt`, `setsockopt`, `getsockopt2` —
get and set options on sockets

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <sys/socket.h>`

int
`getsockopt`(int s, int level, int optname, void * restrict optval, socklen_t * restrict optlen);

int
`setsockopt`(int s, int level, int optname, const void *optval, socklen_t optlen);

int
`getsockopt2`(int s, int level, int optname, void * restrict optval, socklen_t * restrict optlen);

# DESCRIPTION

`getsockopt`(), `setsockopt`() and `getsockopt2`() manipulate the options associated with a socket.

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. Options may exist at multiple protocol levels. 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`() and `getsockopt2`() they identify a buffer in which the value for the requested option(s) are to be returned. For `getsockopt2`() they are also used to provide an extra argument to select which value to return. For `getsockopt`() and `getsockopt2`(), 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 the size of the requested option value to be stored in optval is greater than the size indicated in optlen then only optlen bytes will be stored in optval and the result will be silently truncated. 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, including: faith(4), icmp6(4), ip(4), ip6(4), ipsec(4), multicast(4), pim(4), route(4), tcp(4), and unix(4).

Most socket-level options use 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`() or `getsockopt2`() 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_TIMESTAMP` | enables reception of a timestamp with datagrams
`SO_ACCEPTFILTER` | set accept filter on listening socket
`SO_RERROR` | enables receive size error reporting
`SO_NOSIGPIPE` | controls generation of `SIGPIPE` for the socket
`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_RERROR` indicates that receive buffer overflows should be handled as errors. Historically receive buffer overflows have been ignored and programs could not tell if they missed messages or messages had been truncated because of overflows. Since programs historically do not expect to get receive overflow errors, this behavior is not the default, but the default can be changed by setting the `SO_RERROR` flag using sysctl(8) and `kern.sooptions`.

`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, measured in seconds, termed the linger interval, is specified 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 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) or poll(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 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 `EAGAIN` 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 `EAGAIN` if no data were received.

If the `SO_TIMESTAMP` option is enabled on a `SOCK_DGRAM` socket, the recvmsg(2) call will return a timestamp corresponding to when the datagram was received. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by a struct timeval. The cmsghdr fields have the following values:

    cmsg_len = sizeof(struct timeval)
    cmsg_level = SOL_SOCKET
    cmsg_type = SCM_TIMESTAMP

`SO_ACCEPTFILTER` places an accept_filter(9) on the socket, which will filter incoming connections on a listening socket before being presented for accept(2). The `setsockopt`() system call will fail if the socket already has a filter set, and listen(2) must be called on the socket before trying to install a filter. The optval argument should point to a struct accept_filter_arg that will select and configure the accept_filter(9), defined as follows:

    struct  accept_filter_arg {
            char    af_name[16];
            char    af_arg[256-16];
    };

The af_name argument should be filled with the name of the accept filter that the application wishes to place on the listening socket. The optional argument af_arg can be passed to the accept filter specified by af_name to provide additional configuration options at attach time. Passing in an optval of `NULL` will remove the filter.

Finally, `SO_TYPE` and `SO_ERROR` are options used only with `getsockopt`() or `getsockopt2`(). `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.

# 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.
[`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.
[`EINVAL`]
  The socket s was not suitable for installing an accept_filter(9).
[`ENOPROTOOPT`]
  The option is unknown at the level indicated.
[`ENOTSOCK`]
  The argument s is a file, not a socket.

# SEE ALSO

ioctl(2), poll(2), select(2), socket(2), getprotoent(3), faith(4), icmp6(4), ip(4), ip6(4), ipsec(4), multicast(4), pim(4), route(4), tcp(4), unix(4), protocols(5), accept_filter(9)

# HISTORY

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

The `getsockopt2`() system call appeared in NetBSD 9.0.

# BUGS

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

August 7, 2021 | NetBSD 10.0