#include <sys/socket.h>
int getsockopt(int sockfd, int level, int optname,
void *restrict optval, socklen_t *restrict optlen);
int setsockopt(int sockfd, int level, int optname,
const void *optval, socklen_t optlen);
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.
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.