GETSOCKOPT(2) Linux Programmer's Manual GETSOCKOPT(2)
getsockopt, setsockopt - get and set options on sockets
int getsockopt(int s, int level, int optname, void *optval, socklen_t
int setsockopt(int s, int level, int optname, const void *optval,
Getsockopt and setsockopt manipulate the options associated with a
socket. 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 socket 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 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 sup-
plied 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; con-
sult 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 disabled.
For a description of the available socket options see socket(7) and the
appropriate protocol man pages.
On success, zero is returned. On error, -1 is returned, and errno is
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
SVr4, 4.4BSD (these system calls first appeared in 4.2BSD). SVr4 docu-
ments 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
ioctl(2), socket(2), getprotoent(3), protocols(5), socket(7), unix(7),
Linux Man Page 1999-05-24 GETSOCKOPT(2)