Unix Socket - Structures

Various structures are used in Unix Socket Programming to hold information about the address and port, and other information. Most socket functions require a pointer to a socket address structure as an argument. Structures defined in this chapter are related to Internet Protocol Family.

sockaddr

The first structure is sockaddr that holds the socket information −

struct sockaddr {
   unsigned short   sa_family;
   char             sa_data[14];
};

This is a generic socket address structure, which will be passed in most of the socket function calls. The following table provides a description of the member fields −

sockaddr in

The second structure that helps you to reference to the socket s elements is as follows −

struct sockaddr_in {
   short int            sin_family;
   unsigned short int   sin_port;
   struct in_addr       sin_addr;
   unsigned char        sin_zero[8];
};

Here is the description of the member fields −

in addr

This structure is used only in the above structure as a structure field and holds 32 bit netid/hostid.

struct in_addr {
   unsigned long s_addr;
};

Here is the description of the member fields −

hostent

This structure is used to keep information related to host.

struct hostent {
   char *h_name; 
   char **h_apases; 
   int h_addrtype;  
   int h_length;    
   char **h_addr_pst
	
#define h_addr  h_addr_pst[0]
};

Here is the description of the member fields −

NOTE − h_addr is defined as h_addr_pst[0] to keep backward compatibipty.

servent

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

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

Here is the description of the member fields −

Tips on Socket Structures

Socket address structures are an integral part of every network program. We allocate them, fill them in, and pass pointers to them to various socket functions. Sometimes we pass a pointer to one of these structures to a socket function and it fills in the contents.

We always pass these structures by reference (i.e., we pass a pointer to the structure, not the structure itself), and we always pass the size of the structure as another argument.

When a socket function fills in a structure, the length is also passed by reference, so that its value can be updated by the function. We call these value-result arguments.

Always, set the structure variables to NULL (i.e., ) by using memset() for bzero() functions, otherwise it may get unexpected junk values in your structure.