Socket Programming in C

What are Sockets?

 Sockets provide a standard connection protocol which enables the communication of data over a network.

 Sockets supports both connection oriented (Transmission Control Protocol) and connectionless (User Datagram Protocol)
 Sockets can be easily configured to act like a client (The one which initiates the connection), or can act as a server (The one which accepts the connection).

The Socket Structures:
 The first structure is struct sockaddr that holds socket information.

         struct sockaddr
         {
                    unsigned short   sa_family;      /* address family */ 
                    char                    sa_data[14];  /* 14 bytes of protocol address */ 
          };

Attribute	Values	Description
sa_family	AF_INET AF_UNIX AF_NS AF_IMPLINK	This represents an address family. In most of the Internet based applications we use AF_INET.
sa_data	Protocol Specific Address	The content of the 14 bytes of protocol specific address are interpreted according to the type of address. For the Internet family we will use port number IP address which is represented by sockaddr_in structure defined below.

But, there's another structure (struct sockaddr_in) that help you to reference to the socket's elements.

         struct sockaddr_in
         {
                    short int                     sin_family; /* Address family */
                    unsigned short int     sin_port; /* Port */
                    struct in_addr            sin_addr; /* Internet Address */
                    unsigned char           sin_zero[8]; /* Same size as struct sockaddr */
         };

Attribute	Values	Description
sa_family	AF_INET AF_UNIX AF_NS AF_IMPLINK	This represents an address family. In most of the Internet based applications we use AF_INET.
sin_port	Service Port	A 16 bit port number in Network Byte Order.
sin_addr	IP Address	A 32 bit IP address in Network Byte Order.
sin_zero	Not Used	You just set this value to NULL as this is not being used.

struct in_addr
{
         unsigned long    s_addr;
};

Attribute	Values	Description
s_addr	service port	A 32 bit IP address in Network Byte Order.

There is one more important structure. This structure is used to keep information related to host.

struct hostent
 {
         char *                    h_name; /* Official name of host. */       
         char **                   h_aliases; /* Alias list. */
         int                          h_addrtype; /* Host address type. */       
         int                          h_length; /* Length of address. */
         char **                   h_addr_list; /* List of addresses from name server. */
         #define h_addr      h_addr_list[0] /* Address, for backward compatibility. */
 };

Attribute	Values	Description
h_name	ti.com etc	This is official name of the host. For example tutorialspoint.com, google.com etc.
h_aliases	TI	This will hold a list of host name aliases.
h_addrtype	AF_INET	This contains the address family and in case of Internet based application it will always be AF_INET
h_length	4	This will hold the length of IP address which is 4 for Internet Address.
h_addr_list	in_addr	For the Internet addresses the array of pointers h_addr_list[0], h_addr_list[1] and so on are points to structure in_addr.

Following structure is used to keep information related to service and associated ports.

struct servent
{
         char *s_name;
         char **s_aliases;
         int s_port;
         char *s_proto;
};

Attribute	Values	Description
s_name	http	This is official name of the service. For example SMTP, FTP POP3 etc.
s_aliases	ALIAS	This will hold list of service aliases. Most of the time this will be set to NULL.
s_port	80	This will have associated port number. For example for HTTP this will be 80.
s_proto	TCP UDP	This will be set to the protocol used. Internet services are provided using either TCP or UDP.