A few of the common socket options are:

SO_REUSEADDR:
This is to report whether the same port can be reused again that is socket can be bind to the same port or not. This is a Boolean option.

SO_KEEPALIVE:
This is to report whether the keep-alive message is flowing between client and server that connections are kept active with the periodic transmission of the message. This is a Boolean option.

SO_SENDBUF:
This reports the send buffer size information.

SO_RECVBUF:
This reports the received buffer size information.

SO_TYPE:
This reports the type of socket.

SO_DONTROUTE:
This reports whether the message can pass through the router.

These options can be manipulated by two system call API:

• getsockopt()
• setsockopt()

getsockopt():

int getsockopt (int sock_fd, int level, int options, void* optval, 
                socklen_t *len);

• With this API, the current value of the options for the given socket can be fetched.
• The first parameter is the socket descriptor.
• The level argument specifies the protocol at which the options resides. To retrieve the socket level option, specify the argument as SOL_SOCKET, for TCP and UDP,level can be set to IPPROTO_TCP and IPPROTO_UDP respectively.
• The option argument specifies the option for which the current value to be fetched.
• The fourth argument is the pointer to variable which holds the current value for the option.
• The length field, initially is the size of buffer pointed by optval and later modified to indicate actual size of optval on return.
• On success it returns 0 and -1 on failure.

setsockopt():

int setsockopt (int sock_fd, int level, int options, const void * 
                 optval, socklen_t optlen)

• With this API, the current value of the options for the given socket can be set to the new value.
• The first parameter is the socket descriptor.
• The level argument specifies the protocol at which the options reside. To retrieve the socket level option, specify the argument as SOL_SOCKET, for TCP and UDP, the level can be set to IPPROTO_TCP and IPPROTO_UDP respectively.
• The option argument specifies the option for which the current value to be set.
• The fourth argument is the pointer to a variable that holds the new value for the option.
• The length field is the actual size of optval to set.
• On success it returns 0 and -1 on failure.

Sample Program:

    int optval;
    unsigned int len = sizeof(optval);
    int sock_fd = socket (AF_INET, SOCK_STREAM, 0);
    if (sock_fd == -1)
    {
        printf ("\n Socket creation failed %d %s \n", errno, 
                strerror(errno));
        return -1; 
    }
    /* Getting the value of Keepalive */
    int ret = getsockopt (sock_fd, SOL_SOCKET, SO_KEEPALIVE, &optval, 
                          &len);
    if (ret == 1)
    {
        printf("\n Fetching value for keepalive option failed\n");
        return -1;
    }

    printf ("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));
    optval = 1;

    /* Setting the value of Keepalive */
    ret = setsockopt (sock_fd, SOL_SOCKET, SO_KEEPALIVE, &optval, len);
    if (ret ==1)
    {
        printf("\n Fetching value for keepalive option failed\n");
        return -1;
    }
    printf ("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));

Relevant Posts:

• Sockets Introduction
• Socket APIs
• TCP Socket Program
• UDP Socket Program
• Multi-client server support- select() API
• Host and Network Byte ordering
• Little-endian and big-endian

Categories: Operating system (OS)