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];
};

ss_family :

Type : sa_family_t

Value : AF_INET or AF_INET6.

Purpose : To distinguish address family it contains.

5.5 KiB Raw Blame History

Network Programming in C

struct(s) used are :-

5.5 KiB

Raw Blame History