getsockopt, setsockopt - get and set options on sockets
Standard C library (libc, -lc)
#include <sys/socket.h>
int getsockopt(int sockfd, int level, int optname,
void optval[restrict *.optlen],
socklen_t *restrict optlen);
int setsockopt(int sockfd, int level, int optname,
const void optval[.optlen],
socklen_t optlen);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 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
getsockopt(), 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 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 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.
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.
The argument sockfd is not a valid file descriptor.
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.
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.1-2008.
POSIX.1-2001, SVr4, 4.4BSD (first appeared in 4.2BSD).
Several of the socket options should be handled at lower levels of the system.