#include <sys/types.h> #include <sys/socket.h>
int getsockopt(int s, int level, int optname, void *optval, socklen_t *optlen);
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 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.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
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(2) and setsockopt(2) 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.