IPV4 TCP server client program with Poll system call
====================================================

.. tab-set::

    .. tab-item:: IPv4 AF_INET TCP

 	* In this program, you are going to learn

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                * How to create a Socket ?

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                * How to bind a socket ?

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                * How to listen a socket ?

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                * How to connect a socket ?

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                * How to accept a socket ?

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                * How to send a data ?

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                * How to recv a data ?

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                * How to use socket APIs ?

                        * `socket <https://man7.org/linux/man-pages/man2/socket.2.html>`_

                        * `bind <https://man7.org/linux/man-pages/man2/bind.2.html>`_

                        * `listen <https://man7.org/linux/man-pages/man2/listen.2.html>`_

			* `connect <https://man7.org/linux/man-pages/man2/connect.2.html>`_

			* `accept <https://man7.org/linux/man-pages/man2/accept.2.html>`_

			* `send <https://man7.org/linux/man-pages/man2/send.2.html>`_

			* `recv <https://man7.org/linux/man-pages/man2/recv.2.html>`_

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        	Topics in this section,
			
     			* :ref:`IPV4 AF_INET TCP SOCKET FAQs <poll_ipv4_af_inet_tcp_socket>`
                        * :ref:`Step 1: Sequence Diagram for SERVER.c <poll_ipv4_af_inet_tcp_socket_server_sequence_diagram>`
                        * :ref:`Step 2: Program for Server.c <poll_ipv4_af_inet_tcp_socket_server_code>`
                        * :ref:`Step 3: Compile and Execute Server.c <poll_ipv4_af_inet_tcp_socket_server_side_compile_and_execute>`
                        * :ref:`Step 4:Sequence Diagram for CLIENT.c <poll_ipv4_af_inet_tcp_socket_client_sequence_diagram>`
                        * :ref:`Step 5: Program for Client.c <poll_ipv4_af_inet_tcp_socket_client_code>`
                        * :ref:`Step 6: Compile and Execute Client.c <poll_ipv4_af_inet_tcp_socket_client_side_compile_and_execute>`
			* :ref:`Summary <poll_ipv4_af_inet_tcp_Summary>`

.. _poll_ipv4_af_inet_tcp_socket:

.. tab-set::

        .. tab-item:: IPV4 AF_INET TCP SOCKET : FAQs

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                        Let us answer few basic questions in this socket

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			What does the AF_INET parameter represent in the socket() function call?

                        .. dropdown:: See Answer

		     		Address Family for IPv4

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                        Why is SOCK_STREAM used as the second parameter in the socket() function call?

                        .. dropdown:: See Answer

		     		It indicates that the socket will use a reliable, connection-oriented communication (TCP).

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                        What is the default protocol for a TCP socket created with socket(AF_INET, SOCK_STREAM, ?? )?

                        .. dropdown:: See Answer

		     		IPPROTO_TCP

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			What is the primary characteristic of TCP communication?

                        .. dropdown:: See Answer

				Connection-oriented and reliable

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			What does a return value of -1 indicate in socket functions?

                        .. dropdown:: See Answer

		     		In many socket programming APIs, a return value of -1 usually indicates an error.
				Functions often set an error code that can be retrieved using perror (or an equivalent mechanism) to determine the specific nature of the error.

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			Why might bind() or listen() fail in socket programming?

                        .. dropdown:: See Answer

				bind() might fail if the specified address is already in use, or if the process lacks the necessary permissions.
				listen() might fail if the socket is not bound, or the operating system limit for pending connections is reached.

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			How should you handle errors when using accept() in socket programming?

                        .. dropdown:: See Answer

				Check the return value and handle errors appropriately

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			Why is it important to check the return value of send() and recv() in socket programming?

                        .. dropdown:: See Answer

				It detects issues such as network errors or closed connections.

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			What is a common practice for handling timeouts in socket programming?

                        .. dropdown:: See Answer

				Use non-blocking sockets with functions like poll() or epoll().

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			Can you use a TCP socket (SOCK_STREAM) for sending and receiving data concurrently between a client and server?

                        .. dropdown:: See Answer

		     		Yes, TCP sockets support bidirectional communication.

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			What is the purpose of the poll system call?

                        .. dropdown:: See Answer
				
		     		To block and wait for activity on one or more file descriptors.

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			How does poll differ from poll in terms of usability?

                        .. dropdown:: See Answer
	
		     		poll is more efficient than poll for monitoring multiple file descriptors.

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                    	What types of file descriptors can be monitored using poll?

                        .. dropdown:: See Answer

				sockets, files, timerfd, socketpair, message_queue, Namedpipes and shared_memory.

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			How does poll handle a set of file descriptors with different states (e.g., reading, writing, exception)?	

                        .. dropdown:: See Answer
			
		     		It uses different structures for each state in the pollfd array.

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			How do you handle errors when using the poll system call?

                        .. dropdown:: See Answer

                               Check the return value for -1 to detect errors, Use perror to print error messages.

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			How does poll handle a set of file descriptors with different states (e.g., reading, writing, exception)?

                        .. dropdown:: See Answer

		     	       Array of pollfd Structures:
			       	   Before calling poll, you need to create an array of pollfd structures, 
				   where each structure represents a file descriptor and its associated events.

			       	   ``struct pollfd fds[NUM_FDS];``

				   NUM_FDS is the number of file descriptors you want to monitor.

			       Initialize pollfd Structures:
				   For each file descriptor you want to monitor, initialize the corresponding pollfd structure with the following information:

				   `fd:` The file descriptor to monitor.
				   `events:` The events of interest (e.g., POLLIN for readability, POLLOUT for writability).
                                   `revents:` Initially set to zero. After the poll call, this field is updated to indicate the events that occurred.
				   
				   ``fds[0].fd = fd1;``

				   ``fds[0].events = POLLIN;``

				   ``fds[0].revents = 0;``

				   ``fds[1].fd = fd2;``

                                   ``fds[1].events = POLLIN;``

                                   ``fds[1].revents = 0;``


			       Call poll:
 				  After initializing the pollfd array, call the poll function, providing the array, the number of file descriptors, and a timeout

				   ``int ready_fds = poll(fds, NUM_FDS, timeout_ms);``

				   `ready_fds` will contain the number of file descriptors that are ready.


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                        How does poll Checking Ready File Descriptors?

                        .. dropdown:: See Answer

				After the poll call, loop through the pollfd array and check the revents field for each file descriptor to determine which events occurred.

				.. code-block:: c

			     		for (int i = 0; i < NUM_FDS; ++i) {
			     			if (fds[i].revents & POLLIN) {
			     				// File descriptor i is ready for reading
			     			}

			     			if (fds[i].revents & POLLOUT) {
			     				// File descriptor i is ready for writing
			     			}
			     			// Check other events if needed (e.g., POLLERR, POLLHUP)
			     		}

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			What does it mean if poll returns 0?

                        .. dropdown:: See Answer

				No file descriptors are ready within the specified timeout.


.. _poll_ipv4_af_inet_tcp_socket_server_sequence_diagram:

.. tab-set::

    .. tab-item:: Step 1: Sequence Diagram for SERVER.c

	.. plantuml::

           @startuml
	   !theme spacelab
           start
	   :socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
           :bind(tcp_server_fd, (struct sockaddr*)&tcp_addr, sizeof(tcp_addr));
           :listen(tcp_server_fd, MAX_CLIENTS);
	   :tcp_client_fd = accept(tcp_server_fd, (struct sockaddr*) &tcp_addr, &tcp_addr_len);
	   :fds[0].fd = tcp_client_fd;
	   :fds[0].events = POLLIN;
           while (while(1)) is (yes)
           :poll(fds, MAX_CLIENTS + 1, -1);
	   if (fds[0].revents & POLLIN) then (yes)
	      :recv(tcp_client_fd, buffer, sizeof(buffer) - 1, 0);
	      :send(tcp_client_fd, buffer, strlen(buffer), 0);
	   else (no)
	   endif
           endwhile (CTRL+c)
	   :(void)close(tcp_client_fd);
           stop
           @enduml


.. _poll_ipv4_af_inet_tcp_socket_server_code:

.. tab-set::

    .. tab-item:: Step 2: Program for Server.c

        * There are many functions used in socket. We can classify those functions based on functionalities.

                * Create Socket

                * Bind Socket

                * Listen Socket

		* Accept Socket

		* Poll

		* Recv data_packet

		* Send data_packet

		* Close socket

	* ``socket()`` is used to create a new socket. For example,

	.. code-block:: c

		tcp_server_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

	* ``bind()`` is used to associate the socket with a specific address and port. For example,

	.. code-block:: c

		ret = bind(tcp_server_fd, (struct sockaddr*)&tcp_addr, sizeof(tcp_addr));

	* ``listen()`` is used to set up a socket to accept incoming connections. For example,

	.. code-block:: c

		ret = listen(tcp_server_fd, MAX_CLIENTS);		

	* ``poll()`` is used for monitoring multiple file descriptors to see if I/O is possible on any of them.

	.. code-block:: c

		ret = poll(fds, MAX_CLIENTS + 1, -1);		

	* ``accept()`` is used in network programming on the server side to accept a connection request from a client. For example,

	.. code-block:: c

      		tcp_client_fd = accept(tcp_server_fd, (struct sockaddr*) &tcp_addr, &tcp_addr_len);

	* ``recv``  is used in network programming to receive data from a connected socket. For example,

	.. code-block:: c
	
		len = recv(tcp_client_id, buffer, sizeof(buffer) - 1, 0);

	* ``send`` is used in network programming to send data over a connected socket. For example,

	.. code-block:: c

		ret = send(tcp_client_fd, buffer, strlen(buffer), 0);

	* ``close`` is used to close the socket To free up system resources associated with the socket. For example,

	.. code-block:: c

     		(void)close(tcp_client_fd);

	* See the full program below,

	.. literalinclude:: IPV4_AF_INET_TCP/server/server.c
                :language: c
		:emphasize-lines: 73, 74, 89, 90, 136, 137, 138, 145, 146, 147, 155, 156, 166, 167, 168, 185, 197, 198

.. _poll_ipv4_af_inet_tcp_socket_server_side_compile_and_execute:

.. tab-set::

    .. tab-item:: Step 3: Compile and Execute Server.c

        .. code-block:: c
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		$ gcc -o server server.c

		$ sudo ./server 8080 

		Port: 8080
		Server is listening...
		Connection Accept
		Received: HI
		Sentbuffer = HELLO
		Received: HI
		Sentbuffer = HELLO
		Received: HI
		Sentbuffer = HELLO
		Received: HI
		Sentbuffer = HELLO
		Received: HI
		Sentbuffer = HELLO
		Received: HI
		Sentbuffer = HELLO
		Received: HI
		Sentbuffer = HELLO
		Received: HI
		Sentbuffer = HELLO
		Received: HI
		^CCaught sigINT!


.. _poll_ipv4_af_inet_tcp_socket_client_sequence_diagram:

.. tab-set::

    .. tab-item:: Step 4: Sequence Diagram for CLIENT.c

	.. plantuml::

   	   @startuml
	   !theme spacelab
           start
           :socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
	   :connect(client_fd, (struct sockaddr*)&tcp_addr, tcp_addr_len);
	   while (while(1)) is (yes)
	   :send(client_fd, buffer, strlen(buffer), 0);
	   :fds[0].fd = client_fd;
           :fds[0].events = POLLIN;
	   :poll(fds, 2, -1);
           if (fds[0].revents & POLLIN) then (yes)
	      :recv(client_fd, buffer, sizeof(buffer) - 1, 0); 
           else (no)
           endif
           endwhile (CTRL+c)
	   :(void)close(client_fd);
           stop
           @enduml

.. _poll_ipv4_af_inet_tcp_socket_client_code:

.. tab-set::

    .. tab-item:: Step 5: Program for Client.c

        * There are many functions used in socket. We can classify those functions based on functionalities.

                * Create Socket

                * Connect Socket

		* Poll

                * Recv data_packet

                * Send data_packet

		* Close socket

        * ``socket`` is used to create a new socket. For example,

        .. code-block:: c

		client_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

	* ``connect`` is used in network programming to establish a connection from a client to a server. For example,

	.. code-block:: c

		cli_connect = connect(client_fd, (struct sockaddr*)&tcp_addr, tcp_addr_len);

	* ``poll()`` is used for monitoring multiple file descriptors to see if I/O is possible on any of them.

        .. code-block:: c

		ret = poll(fds, 2, -1);       

	* ``send`` is used in network programming to send data over a connected socket. For example,

        .. code-block:: c

     		ret = send(client_fd, buffer, strlen(buffer), 0);

	* ``recv`` is used in network programming to receive data from a connected socket. For example,

        .. code-block:: c

     		len = recv(client_fd, buffer, sizeof(buffer) - 1, 0);

	* ``close`` is used to close the socket To free up system resources associated with the socket. For example,

        .. code-block:: c

		(void)close(client_fd);

	* See the full program below,

        .. literalinclude:: IPV4_AF_INET_TCP/client/client.c
                :language: c
		:emphasize-lines: 79, 80, 103, 104, 155, 156, 157, 164, 165, 166, 181, 193

.. _poll_ipv4_af_inet_tcp_socket_client_side_compile_and_execute:

.. tab-set::

    .. tab-item:: Step 6: Compile and Execute Client.c

        .. code-block:: c
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                :emphasize-lines: 1, 3

	     	$ gcc -o client client.c

		$ sudo ./client 8080 127.0.0.1
		
		Port: 8080
		IP Address: 127.0.0.1
		connected
		sentbuffer = HI
		Received: HELLO
		sentbuffer = HI
		Received: HELLO
		sentbuffer = HI
		Received: HELLO
		sentbuffer = HI
		Received: HELLO
		sentbuffer = HI
		Received: HELLO
		sentbuffer = HI
		Received: HELLO
		sentbuffer = HI
		Received: HELLO
		sentbuffer = HI
		Received: HELLO
		sentbuffer = HI
		Received: HELLO
		^CCaught sigINT!


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                $ sudo ./server 8080 127.0.0.1

		$ sudo ./client 8080 127.0.0.1

		program to run with elevated privileges, listen on port 8080, and bind to the loopback address 127.0.0.1.

		<port_number> <ip_address> decided by the user based on the connection.

.. tab-set::

        .. tab-item:: Enhanced Socket Flexibility with ``AF_INET`` and ``PF_INET`` Domains

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                        **Default Domain:**

                                By default, the socket is configured to work in the ``AF_INET`` domain, handling all types of network data.

                        **Additional Domain Support:**

                                We expand the socket's capabilities to also function in the ``PF_INET`` domain, allowing it to operate similarly to ``AF_INET``.

                        **Socket Creation:**

                                We set up a network connection point known as a socket using ``socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)``.

                        **Working Scenario:**
                                
                                Despite the change in domain to ``PF_INET``, the socket continues to operate the same way, handling general network data.

.. _poll_ipv4_af_inet_tcp_Summary:

.. tab-set::

    .. tab-item:: Summary

        ============== =========================================================================================
        Socket API     Learning
        ============== =========================================================================================
        socket         Create a new socket
        bind	       Associate the socket with a specific address and port
        listen	       Set up a socket to accept incoming connections.
	connect        Establish a connection from a client to a server.
	accept	       Server side to accept a connection request from a client.
	poll	       Monitor multiple file descriptors (usually sockets) for read, write, or error conditions.
	recv           Receive data from a connected socket.
	send	       Send data over a connected socket.
	============== =========================================================================================

.. card:: See Also

	* previous topic

                * :doc:`../../../sockets/ipv4_af_inet_tcp/select/select`
	
	* current topic 
	
                * :doc:`../../../sockets/ipv4_af_inet_tcp/poll/poll`

        * Next topic

                * :doc:`../../../sockets/ipv4_af_inet_tcp/epoll/epoll`

	* Other sockets
		
		* :doc:`../../../sockets/ipv4_af_inet_udp`
                * :doc:`../../../sockets/af_unix_tcp`
                * :doc:`../../../sockets/af_unix_udp`
                * :doc:`../../../sockets/ipv4_raw_af_inet_icmp`
                * :doc:`../../../sockets/ipv4_raw_af_inet_raw`
                * :doc:`../../../sockets/ipv4_raw_af_inet_tcp`
                * :doc:`../../../sockets/ipv4_raw_af_inet_udp`
                * :doc:`../../../sockets/ipv6_af_inet6_tcp`
                * :doc:`../../../sockets/ipv6_af_inet6_udp`
                * :doc:`../../../sockets/ipv6_raw_af_inet6_icmp`
                * :doc:`../../../sockets/ipv6_raw_af_inet6_tcp`
                * :doc:`../../../sockets/ipv6_raw_af_inet6_udp`
                * :doc:`../../../sockets/ipv6_raw_af_inet6_raw`
		* :doc:`../../../sockets/raw_af_packet_raw_htons_ETH_P_ALL`
                * :doc:`../../../sockets/raw_af_packet_tcp_htons_ETH_P_ALL`
                * :doc:`../../../sockets/raw_af_packet_udp_htons_ETH_P_ALL`
	
	* Other IPCs

                * :doc:`../../../Message_queues/Message_queues`
                * :doc:`../../../NamedPipes/NamedPipes`
                * :doc:`../../../Netlink/Netlink`
                * :doc:`../../../Shared_Memory/Shared_Memory`
                * :doc:`../../../Shared_Memory_2_FDS/Shared_Memory_2_FDS`
                * :doc:`../../../SocketPair/SocketPair`
                * :doc:`../../../Timerfd/Timerfd`