Socket Options

You can set and get several options on sockets through setsockopt() and getsockopt(). For example, you can change the send or receive buffer space. For more information, see the setsockopt(3C) and getsockopt(3C) man pages.

The general forms of the calls are in the following formats:

setsockopt(s, level, optname, optval, optlen);

getsockopt(s, level, optname, optval, optlen);

The operating system can adjust the values appropriately at any time.

The arguments of setsockopt and getsockopt calls are in the following list:

s

Socket on which the option is to be applied

level

Specifies the protocol level, such as socket level, indicated by the symbolic constant SOL_SOCKET in sys/socket.h

optname

Symbolic constant defined in sys/socket.h that specifies the option

optval

Points to the value of the option

optlen

Points to the length of the value of the option

For getsockopt, optlen is a value-result argument. This argument is initially set to the size of the storage area pointed to by optval. On return, the argument's value is set to the length of storage used.

When a program needs to determine an existing socket's type, the program should invoke inetd by using the SO_TYPE socket option and the getsockopt call:

#include <sys/types.h>
#include <sys/socket.h>
 
int type, size;
 
size = sizeof (int);
if (getsockopt(s, SOL_SOCKET, SO_TYPE, (char *) &type, &size) < 0) {
    ...
}

After getsockopt, type is set to the value of the socket type, as defined in sys/socket.h. For a datagram socket, type would be SOCK_DGRAM.