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.
- 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.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these system calls first appeared 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 system.
ioctl(2),
socket(2),
getprotoent(3),
protocols(5),
ip(7),
packet(7),
socket(7),
tcp(7),
udp(7),
unix(7)