getsockopt, setsockopt - get and set options on sockets
int getsockopt(int s, int level, int optname, void *opt
val, socklen_t *optlen);
int setsockopt(int s, int level, int optname, const void
*optval, socklen_t optlen);
Getsockopt and setsockopt manipulate the options associ
ated with a socket. Options may exist at multiple proto
col levels; they are always present at the uppermost
When manipulating socket options the level at which the
option resides and the name of the option must be speci
fied. 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 proto
col 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 parameters 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
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 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 parameter for
optval. For setsockopt, the parameter should be non-zero
to enable a boolean option, or zero if the option is to be
For a description of the available socket options see
On success, zero is returned. On error, -1 is returned,
and errno is set appropriately.
EBADF The argument s is not a valid descriptor.
The argument s is a file, not a socket.
The option is unknown at the level indicated.
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.
SVr4, 4.4BSD (these system calls first appeared in
4.2BSD). SVr4 documents additional ENOMEM and ENOSR error
codes, but does not document the SO_SNDLOWAT, SO_RCVLOWAT,
SO_SNDTIMEO, SO_RCVTIMEO options
The fifth argument of getsockopt and setsockopt is in
reality an int [*] (and this is what BSD 4.* and libc4 and
libc5 have). Some POSIX confusion resulted in the present
socklen_t. The draft standard has not been adopted yet,
but glibc2 already follows it and also has socklen_t [*].
See also accept(2).
Several of the socket options should be handled at lower
levels of the system.
ioctl(2), socket(2), getprotoent(3), protocols(5),
socket(7), unix(7), tcp(7)