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Socket, also known as "Socket" (socket), is typically used by applications to send requests over a network or respond to network requests, enabling communication between hosts or processes on a single computer. In this chapter, we will introduce how to use Socket services in the Perl language. * * * ### Creating a Server * Use the **socket** function to create a socket service. * Use the **bind** function to bind a port. * Use the **listen** function to listen on the port. * Use the **accept** function to accept client requests. ### Creating a Client * Use the **socket** function to create a socket service. * Use the **connect** function to connect to the socket server. The following diagram illustrates the communication flow between the client and server: !(#) * * * ## Server Socket Functions ### socket Function In Perl, we use the socket() function to create a socket. The syntax is as follows: socket( SOCKET, DOMAIN, TYPE, PROTOCOL ); Parameter analysis: * **DOMAIN** specifies the protocol family for the created socket. For example: * `AF_INET` represents IPv4 network protocol * `AF_INET6` represents IPv6 * `AF_UNIX` represents local sockets (using a file) * **TYPE** specifies the socket type, which can be SOCK_STREAM for connection-oriented or SOCK_DGRAM for connectionless communication. * **PROTOCOL** should be **(getprotobyname('tcp'))**. It specifies the actual transport protocol to be used. Therefore, the socket function call is as follows: use Socket # Defines PF_INET and SOCK_STREAM socket(SOCKET,PF_INET,SOCK_STREAM,(getprotobyname('tcp'))); ### bind() Function Use bind() to assign an address to a socket: bind( SOCKET, ADDRESS ); SOCKET is a socket descriptor. ADDRESS is the socket address (TCP/IP) containing three elements: * Address family (TCP/IP uses AF_INET, which might be 2 on your system) * Port number (e.g., 21) * Network address (e.g., 10.12.12.168) After creating a socket with socket(), only the protocol is assigned, not an address. Before accepting connections from other hosts, you must call bind() to assign an address to the socket. A simple example is as follows: use Socket # Defines PF_INET and SOCK_STREAM $port = 12345; # Port to listen on $server_ip_address = "10.12.12.168"; bind( SOCKET, pack_sockaddr_in($port, inet_aton($server_ip_address))) or die "Cannot bind port! n"; **or die** executes if binding the address fails. By setting the setsockopt() option SO_REUSEADDR, the port can be reused immediately. The **pack_sockaddr_in()** function converts the address to binary format. ### listen() Function After a socket is bound to an address, the listen() function starts listening for possible connection requests. However, this can only be used when reliable data streams are guaranteed: listen( SOCKET, QUEUESIZE ); SOCKET: A socket descriptor. QUEUESIZE: An integer that determines the size of the listening queue. When a connection request arrives, it enters this queue; when a connection request is accepted by accept(), it is removed from the queue; when the queue is full, new connection requests return an error. Once a connection is accepted, 0 is returned for success, and -1 for an error. ### accept() Function The accept() function accepts a requested socket connection. If successful, it returns the compressed network address; otherwise, it returns FALSE: accept( NEW_SOCKET, SOCKET ); NEW_SOCKET: A socket descriptor. SOCKET: A socket descriptor. accept() is typically used in an infinite loop: while(1) { accept( NEW_SOCKET, SOCKT ); .......} The above example can listen for client requests in real-time. * * * ## Client Functions ### connect() Function The connect() system call sets up a connection for a socket, with parameters including the file descriptor and host address. connect( SOCKET, ADDRESS ); The following example creates a connection to a server socket: $port = 21; # FTP port $server_ip_address = "10.12.12.168"; connect( SOCKET, pack_sockaddr_in($port, inet_aton($server_ip_address))) or die "Cannot bind port! n"; * * * ## Complete Example Next, let's understand the application of all socket functions through a complete example: Server server.pl code: ## Example #!/usr/bin/perl -w use strict; use Socket; my$port = shift || 7890; my$proto = getprotobyname('tcp'); my$server = "localhost"; socket(SOCKET, PF_INET, SOCK_STREAM, $proto)or die"Cannot open socket $!n"; setsockopt(SOCKET, SOL_SOCKET, SO_REUSEADDR, 1)or die"Cannot set SO_REUSEADDR $!n"; bind(SOCKET, pack_sockaddr_in($port, inet_aton($server)))or die"Cannot bind port $port! n"; listen(SOCKET, 5)or die"listen: $!"; print"Server started: $portn"; my$client_addr; while($client_addr = accept(NEW_SOCKET, SOCKET)){my$name = gethostbyaddr($client_addr, AF_INET); print NEW_SOCKET"I am a message from the server"; print"Connection received from $namen"; close NEW_SOCKET; } Open a terminal and execute the following code: $ perl server.pl Server started: 7890 Client client.pl code: ## Example #!/usr/bin/perl -w use strict; use Socket; my$host = shift || 'localhost'; my$port = shift || 7890; my$server = "localhost"; socket(SOCKET,PF_INET,SOCK_STREAM,(getprotobyname('tcp')))or die"Cannot create socket $!n"; connect(SOCKET, pack_sockaddr_in($port, inet_aton($server)))or die"Cannot connect: port $port! n"; my$line; while($line = ){print"$linen"; }close SOCKET or die"close: $!"; Open another terminal and execute the following code: $ perl client.pl I am a message from the server