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 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 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 disabled.
For a description of the available socket options see socket(7) and the
appropriate protocol man pages.
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, also used by glibc. See also