GETSOCKOPT(2) Linux Programmer's Manual GETSOCKOPT(2)
getsockopt, setsockopt - get and set options on sockets
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);
getsockopt() and setsockopt() manipulate options for the socket re‐
ferred 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 pro‐
tocol 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 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 getsock‐
opt(), optlen is a value-result argument, 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 ap‐
propriate 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; con‐
sult the appropriate entries in section 4 of the manual.
Most socket-level options utilize an int argument for optval. For set‐
sockopt(), 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.
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.
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
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)).
The option is unknown at the level indicated.
The file descriptor sockfd does not refer to a socket.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these system calls first ap‐
peared in 4.2BSD).
For background on the socklen_t type, see accept(2).
Several of the socket options should be handled at lower levels of the
ioctl(2), socket(2), getprotoent(3), protocols(5), ip(7), packet(7),
socket(7), tcp(7), udp(7), unix(7)
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Linux 2021-03-22 GETSOCKOPT(2)