Network Programming in C

Quick reference to struct(s) and apis' used for networking applicaton in C language.

Let's first discuss about the important struct and then how we use apis' to establish networking functionality.

struct(s) used are :-

• struct addrinfo -

    struct addrinfo {
        int             ai_flags;       // AI_PASSIVE, AI_CANONNAME, etc.
        int             ai_family;      // AF_INET, AF_INET6, AF_UNSPEC
        int             ai_socktype;    // SOCK_STREAM, SOCK_DRAM
        int             ai_protocol;    // use 0 for "any"
        size_t          ai_addrlen;     // size of ai_addr in bytes
        struct scokaddr *addr;          // struct sockaddr_in or _in6
        char            *ai_canonname;  // full canonical hostname
        struct addrinfo *ai_next;       // linked list, next node
    };
  

  1. ai_flags :

     Type : int

     Value : AI_PASSIVE, AI_CANONNAME.

     Purpose : Help determine the processing path for getaddrinfo().

     • For ex : AI_PASSIVE suggest the getaddrinfo() to get the address of the host machine the program is running on.

  2. ai_family :

     Type : int

     Value : AF_INET (IPv4), AF_INET6 (IPv6) or AF_UNSPEC.

     Purpose : Specify which type of IP address (in terms of representation) does it hold.

  3. ai_socktype :

     Type : int

     Value : SOCK_STREAM, SOCK_DRAM.

     Purpose : Specify what kind of socket we are interested in using.

  4. ai_protocol :

     Type : int

     Value : use 0 for "any".

     Purpose : What kind of protocol we are intending to use.

  5. ai_addrlen :

     Type : size_t

     Value : size of ai_addr in bytes.

     Purpose : What is the size of struct sockaddr pointer holds.

  6. addr :

     Type : struct sockaddr *

     Value : address in form of struct sockaddr.

     Purpose : Holding the actual address.

  7. ai_cannonname :

     Type : char *

     I don't know what's it's used for.

  8. ai_next :

     Type : struct addrinfo *

     Value : address of next struct addrinfo.

     Purpose : Intending to have a linked list.

---

• struct sockaddr -

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

  1. sa_family :

     Type : unsigned short

     Value : AF_XXX

     Purpose : Specify the address family type.

  2. sa_data :

     Type : char

     Value : IP address in for raw byte data.

     Purpose : Buffer for storing the actual address in Network Byte Order.

Important

Using this structure and hard coding every thing on it is very tedious. Use struct sockaddr_in for IPv4 or struct sockaddr_in6 for IPv6.

---

• struct sockaddr_in -

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

  1. sin_family :

     Type : short int

     Value : AF_INET.

     Purpose : Specifying the IP address belongs to IPv4.

  2. sin_port :

     Type : unsigned short int

     Value : Port number.

     Purpose : Stores the port number which will be used by the created socket.

  3. sin_addr :

     Type : struct in_addr

     Value : Internet address

     Purpose : Act as a buffer to store Internet address in raw byte.

  4. sin_zero :

     Type : unsigned char

     Value : All element set to zero.

     Purpose : Used for padding to match the size of struct sockaddr.

• struct in_addr -

      struct in_addr{
          uint32_t s_addr;
      };
  

  1. s_addr :

     Type : uint32_t

     Value : IP address in raw byte in Network Byte Order.

     Purpose : Act as buffer to store IP address.

---

• struct sockaddr_in6 -

      struct sockaddr_in6{
          u_int16_t      sin6_family;  // address family, AF_INET6
          u_int16_t      sin6_port;    // port number, Network Byte Order
          u_int32_t      sin6_flowinfo;// IPv6 flow information
          struct in6_addr sin6_addr;   // IPv6 address
          u_int32_t      sin6_scope_id;// Scope ID
      };
  

sin6_family, sin6_port and sin6_addr are similar to the corresponding fields of struct sockaddr_in6.

• struct in6_addr -

      struct in6_addr{
          unsigned char s6_addr[16];  //IPv6 address
      };
  

  1. s6_addr :

     Type : unsigned char array

     Value : IPv6 address in raw byte.

     Purpose : Act as a buffer to store IPv6 address.

---

• struct sockaddr_storage -

Important

Because sockaddr_in6 is larger in size than sockaddr_in and we may not know which to use when, we can use sockaddr_storage for storing any of the two and type-cast to their respective type.

```
    struct sockaddr_storage{
        sa_family_t     ss_family;  // address family

        /*
        * all this is padding, implementation specific, ignore it.
        */

        char        __ss_pad1[_SS_PAD1SIZE];
        int64_t     __ss_align;
        char        __ss_pad2[_SS_PAD2SIZE];
    };
```

1. ss_family :

   Type : sa_family_t

   Value : AF_INET or AF_INET6.

   Purpose : To distinguish address family it contains.

---

Functions' used are :-

Functions for conversion between network and host byte order :

1. uint32_t htonl( uint32_t hostlong )

   • Convert hostlong in host byte order to network byte order.

2. uint16_t htons( uint16_t hostshort )

   • Convert hostshort in host byte order to network byte order.

3. uint32_t ntohl( uint32_t netlong )

   • Convert netlong in network byte order to host byte order.

4. uint16_t ntohs( uint16_t netshort )

   • Convert netshort in network byte order to host byte order.

Functions for conversion between network representation to presentation representation :

Since the IP address are presentable in form of (for eg.) 194.10.11.5 for IPv4, but the actual representation in network is in raw byte form. So, for easy conversion between the two, apis are provided for such conversion.

1. const char *inet_ntop(int af, const void* src, char* dst, socklen_t size)

   Parameters :

   • af :

     Type : int

     Value : AF_INET, AF_INET6.

     Purpose : Used to specify type of address we are converting (IPv4 or IPv6).

   • src :

     Type : const void *

     Value : Buffer containing raw byte address in form.

   • dst :

     Type : char *

     Value : Buffer for storing the IP address in presentable form.

   • size :

     Type : socklen_t

     Value : INET_ADDRSTRLEN or INET6_ADDRSTRLEN.

     Purpose : Maximum size of buffer to store the IP address.

   Return type :

   • Type : const char *

   • Purpose : Pointer to dst if no error occured, otherwise returns null pointer and set global error variable errno.

2. int inet_pton(int af, const char *src, void *dst)

   Parameter :

   • af :

     Type : int

     Value : AF_INET, AF_INET6.

     Purpose : Used to specify type of address we are converting (IPv4 or IPv6).

   • src :

     Type : const char *

     Value : String to convert address in presentable form to network compatible form.

   • dst :

     Type : void *

     Value : empty buffer.

     Purpose : To store address in network compatible form.

---

System Calls :-

Below are the system calls that can be used to access network functionality.

• getaddrinfo() :

  1. Function prototye

  int getaddrinfo(const char *node,    // e.g. "www.example.com" or IP
                  const char *service, // e.g. "http" or port number
                  const struct addrinfo *hints,
                  struct addrinfo **res);