Socket Programming

• For those of you who have not had uses, this might be new.
• I will use the following sources:
  • UNIX System Programming for SVR4, Curry, July 1996, O'Reilly
  • The unix man pages.
  • The net.
  • Stalling's appendix C.
• Most things are locked down, so you will need to run your server on
  • cslab100
  • With ports 5000 - 5999
• My code is in C because I am lazy. There is no reason it can't be in c++
  • And aside from the printf stuff it should all be the same anyway.
• This code is OLD.
  • There is a slight migration in features and what was right and wrong at the time.
  • But there it is (note the date on the link)
• There are some common system calls that both servers and clients use.
• And there are some that are not.
• There are some routines that are strictly related to getting information needed to configure the communication.
• In any case, most of the work involves configuring the connection.
• The socket call creates a communication endpoint.
  • This is much like a file handle.
  • We will eventually read/write information from this entity.
  • A socket is an integer

    int mySocket;
    	  

  • You will need to include <sys/socket.h>
  • A socket is created with a call to socket which has three parameters.
    • the domain
      • One of the following
        • AF_INET
        • AF_INET6
        • Or others including
        • AF_UNIX, PF_LOCAL
        • AF_IPX - novel
        • AF_APPLETALK
    • the type
      • SOCK_STREAM
        • This forms a duplex connection oriented connection.
        • This is essentially a tcp connection.
        • Message boundaries are not preserved.
        • This is essentially a TCP connection.
        • read, write, send, recv
        • OOB data is available.
      • SOCK_SEQPACKET
        • Same as SOCK_STREAM but message boundaries are preserved.
      • SOCK_DGRAM
        • No in-order, no circuit, no guarantee
        • send, recvfrom
      • And others
    • The protocol
      • A 0 can be used if only a single protocol supports the socket type requested.
      • getprotoent() can be used otherwise.
    • Return Value
      • -1 indicates error
      • otherwise the socket handle.
      • See the man pages for error codes.
  • getprotoent
    • Takes a c style string.
    • Returns a protoent structure or NULL

    •               struct protoent {
                            char    *p_name;        /* official protocol name */
                            char    **p_aliases;    /* alias list */
                            int     p_proto;        /* protocol number */
                    }
      	     

• perror(string)
  • from <stdio.h>
  • Called when a system call fails
  • This will print out an associated message.
  • perror("Socket not created ");

    Socket not created : Protocol not supported
    	 

• sprintf (and printf too)
  • from <stdio.h>
  • sprintf prints to a buffer, which is the first argument
  • The next is a format string
  • The last is a list of variables to be printed.
  • printf is the same, but without the buffer
  • %s is a string, %d is an int, %f is a float, %c is a char.
  • see man sprintf for more formats
  • sprintf(buf, "The error code is %d for %s", intVar, stringVar);
  • printf("%s is %d, x=%f, y=%f\n", srtingVar, intVar, floatVar, floatVar);
• A code snippet

     char *  protoName = "udp";
     int     s;
     struct  protoent *  myproto;
  
  ...
     myproto = getprotobyname(protoName);
     if (NULL == myproto) {
         sprintf(buf, "Could not get protocol %s",protoName);
         perror(buf);
         exit(-1);
     }
     if ((s =  socket(AF_INET, SOCK_STREAM, myproto->p_proto)) == -1){
           perror("Socket not created ");
           exit(-1);
        }
       

• Eventually we will want to get data into and out of a socket.
  • This is done with read and write
  • Needs <unistd.h>
  • write
    • Takes the socket
    • A buffer to read from
    • The size, in bytes, of the buffer
    • Returns
      • -1 in the face of an error
      • The number of bytes written in other cases.

    • write(s,buf,strlen(buf));
      	     

  • read
    • The same parameters and operation.

    • rlen = read(s,buf,200);
      buf[rlen] = NULL;