Hi friends,
you have been explained the initial part of perl programming by Ameya.Here i am going to explain an important feature of perl i.e. network programming, eg how to make programs for client-server relationship.
socket: end points of a two way communication path.

To make an application for client-server relationship, you have to write one program for server & one for client.

server program

• First you have to acquire a port which is not yet being used by any other program by getservbyport function.
  syntax:
  (name,aliases,portnum,protoname)=getservbyport(searchportnumber,searchproto);
  this searches for the port searchportnumber in the /etc/services file & if search is succesful it returns the values denoted on L.H.S. which is a four element list otherwise empty list is returned.
  eg.
  $port=2000; while(getservbyport($port,"tcp")){port++;}
  
• Get the number of the protocol you want to use by using getprotobyname function.
  syntax:
  (name,aliases,number)=getprotobyname(searchname)
  It searches for the protocol by name searchname in the /etc/protocols file & if search is succesful it returns the values denoted on L.H.S. which is a three element list otherwise empty list is returned.
  eg
  ($d1,$d2,$prototype)=getprotobyname("tcp");
  
• Get the name of host using gethostbyname function & giving the name of your machine(string) as its argument & if you don't know the name of your machine then you can give UNIX command hostname as argument(i.e. 'hostname' , single quotes are given to execute the UNIX command hostname).
  syntax:
  (name,altnames,addrtype,len,addrs)=gethostbyname(hname);
  It is similar to above two commands except it searches for machine name in /etc/hosts file.
  ($d1,$d2,$d3,$d4,$rawserver)=gethostbyname('hostname');
  
• you have to create a socket which will act as server socket. use socketfunction for this purpose.
  syntax:
  socket(socketname,domain,type,format);
  eg
  socket(SSOCKET,2,1,$prototype);
  
  • socketname is the file variable associated with new socket under creation.
  • domain is protocol family to use eg. PF_UNIX,PF_INET,PF_IMPLINK & PF_NS.(listed in /usr/include/sys/socket.h file)
  • type is the type of socket to be created eg SOCK_STREAM, SOCK_DGRAM etc.
  • format is the no. of protocol to be used with the socket.
    
    socket func. returns non-zero value if successful otherwise zero is returned.
    
    • After creating a socket you have to bind this socket to particular network address which is in the form of four byte packed address,packed by pack function & consists of following elements:
      syntax:
      bind(socketname,address);
      socketname is the name of socket which is to be binded with this network address.
      address consists of:
      
      1. address type: which is always AF_INET but in call to pack you should provide the corresponding integer value of AF_INET for your system which can be found from /usr/include/netdb.h
      2. second is the number of port to use when connecting for which we used getservbyport.
      3. third is the 4-byte representation of the Internet address of the machine to which the socket is to be bound
      
      all these three are packed by pack function as:
      eg. $address=pack("Sna4x8",2,$portnum,$inetaddress);
      bind(SSOCKET,$address);
      pack function packs the comma separated list starting from second argument according to the pack format characters specified in the first argument of pack.
      
    • Now, this server should wait for some client to request service.So, listen func. is used for this purpose.It can wait for maximum 5 clients,which wait in a queue.
      syntax:
      listen(socketname,number);
      socketname is same as described before.
      eg
      listen(SSOCKET,1);
      number is the maximum number of the clients that can be queued up to connect to this server.
      returns non-zero value if successful else zero
      
    • Now,this waiting or listening server should accept the connection from client when it requests.
      syntax:
      accept(clientsocket,socketname);
      clientsocket represents the newly created connection.
      eg
      $clientaddr=accept(SOCKET,SSOCKET);
      
    • do other processings like print etc.
      
    
    
    

    client program

    The processes that connect to server are called clients.
    main steps involved

    • Get the number of protocol used by using getprotobyname.
    • Get the hostname of client & server by providing their machine names as input using gethostbyname.
    • Create socket for client(or for current program) using socket func.
    • bind this socket to network address of client machine(i.e. bind socket with current program) using bind.
    • Connect to the server process by calling connect function
      syntax:
      connect(clientsocket,address);
      clientsocket is the client socket created using socket & address is the internal representation of the internet address to which we want to connect.
      eg
      connect(SOCKET,$serveraddr);
      
    • do other processings like print etc.
    
    select function
    syntax
    select(outputfile);
    default output file is standard output device i.e. screen but if you don't want the output to be printed on screen but redirect to a file then use select & pass outputfile as argument. Then the further calls to print ,write etc will print to outputfile.