Unix Socket - Structures


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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:

Attribute Values Description
sa_family

AF_INET

AF_UNIX

AF_NS

AF_IMPLINK

It 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.

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:

Attribute Values Description
sa_family

AF_INET

AF_UNIX

AF_NS

AF_IMPLINK

It 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.

in addr

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

struct in_addr {
   unsigned long s_addr;
};

Here is the description of the member fields:

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

hostent

This structure is used to keep information related to host.

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

Here is the description of the member fields:

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

NOTE : h_addr is defined as h_addr_list[0] to keep backward compatibility.

servent

This particular 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;
};

Here is the description of the member fields:

Attribute Values Description
s_name http This is the official name of the service. For example, SMTP, FTP POP3, etc.
s_aliases ALIAS It holds the list of service aliases. Most of the time this will be set to NULL.
s_port 80 It will have associated port number. For example, for HTTP, this will be 80.
s_proto

TCP

UDP

It is set to the protocol used. Internet services are provided using either TCP or UDP.

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., '\0') by using memset() for bzero() functions, otherwise it may get unexpected junk values in your structure.



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