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GETSOCKOPT(2) Linux Programmer's Manual GETSOCKOPT(2)
NAME
getsockopt, setsockopt - get and set options on sockets
SYNOPSIS
#include <sys/socket.h>
int getsockopt(int sockfd, int level, int optname,
void *restrict optval, socklen_t *restrict optlen);
int setsockopt(int sockfd, int level, int optname,
const void *optval, socklen_t optlen);
DESCRIPTION
getsockopt() and setsockopt() manipulate options for the socket referred to by the file descriptor sockfd. 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 sockets API
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 sup‐
plied. 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 arguments 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 argument, initially containing the size of the buffer pointed to by optval, and modi‐
fied 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 defi‐
nitions 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 argument for optval. For setsockopt(), the argument should be nonzero to enable a boolean option, or zero if the option
is to be disabled.
For a description of the available socket options see socket(7) and the appropriate protocol man pages.
RETURN VALUE
On success, zero is returned for the standard options. On error, -1 is returned, and errno is set to indicate the error.
Netfilter allows the programmer to define custom socket options with associated handlers; for such options, the return value on success is the value returned by
the handler.
ERRORS
EBADF The argument sockfd is not a valid file 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 optlen invalid in setsockopt(). In some cases this error can also occur for an invalid value in optval (e.g., for the IP_ADD_MEMBERSHIP option described
in ip(7)).
ENOPROTOOPT
The option is unknown at the level indicated.
ENOTSOCK
The file descriptor sockfd does not refer to a socket.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these system calls first appeared in 4.2BSD).
NOTES
For background on the socklen_t type, see accept(2).
BUGS
Several of the socket options should be handled at lower levels of the system.
SEE ALSO
ioctl(2), socket(2), getprotoent(3), protocols(5), ip(7), packet(7), socket(7), tcp(7), udp(7), unix(7)
Linux 2021-03-22 GETSOCKOPT(2)