Some Client Code

• We need to connect to a server.
• To do this, we will need the server name.
• gethostbyname
  • #include <netdb.h>
  • takes a cstring, the name of the host we want to look up
  • returns a hostent struct pointer.

  • 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 */
    }
    

    • h_name is the official name
    • h_aliases is a list of aliases or cnames
    • h_addrtype is either AF_INET or AF_INET6
    • h_length is the size, in bytes of the address
    • h_addr_list is a list of all applicable addresses
  • Or returns NULL on failure
  • We are really interested in the h_addr_list[0] field.
  • A code snippet

    struct  hostent *server_info;
    char * hostname = "cslab103.cs.edinboro.edu";
    
    if (NULL == (server_info = gethostbyname(hostname))){
           sprintf(buf, "Name lookup failed for %s",hostname);
           perror("Name lookup failed ");
           exit(-1);
    }
    	

• connect
  • #include <sys/types.h>
  • #include <sys/socket.h>
  • The first parameter is the socket file descriptor
  • The second is a struct sockaddr pointer
  • The last is the length of the sockaddr field
  • The sockaddr pointer
    • This changes based on the type of socket.
    • In our case we want a sockaddr_in

      struct sockaddr_in {
                 short    sin_family
                 u_short  sin_port;      
      	   struct   in_addr sin_addr;
      }
      

    • The sin_family is always AF_INET
    • The sin_port is the port you want to connect to.
    • The sin_addr is what was returned from gethostbyname
    • sin_addr can also be INADDR_ANY, for on the server.
  • htons is a utility routine which will convert a number from the host representation to the network representation (host to network short).
  • The return value is 0, or -1 for error.

    struct  sockaddr_in  where;
    ...
          where.sin_addr.s_addr = *((u_long *) server_info->h_addr_list[0]);
          where.sin_port = htons(5600);
          where.sin_family = AF_INET;
    
          if (-1 == connect(s,&where,sizeof(struct sockaddr_in))){
             perror("Connection failed");
             exit(-1);
          }
    	

• inet_ntoa will convert a network address in network byte order to a string.
• My code