tcp_client.c

/* Run it on a Linux System.
   Put it inside: "~/Sockets/TCPClient" directory.*/

/*
 Here is depicted the workflow for the TCP client:
 
 +---------------+
 |               |
 |   socket()    |  Create a socket through a call to socket()
 |               |
 +---------------+
         |
         |
         v
 +---------------+
 |               |
 |   connect()   |  Connect to a remote address through a call to connect()
 |               |
 +---------------+      
         |
         |
         v
 +---------------+
 |               |
 |    recv()     |  Starts receiving data
 |               |
 +---------------+
       
*/ 

//We want to include some stuff from stdio and stdlib
#include <stdio.h>
#include <stdlib.h>

// The following are for definition of socket functions and API
#include <sys/types.h>
#include <sys/socket.h>

//... for the structures we need to store address infos
#include <netinet/in.h>

int main () {
    
  //Definition of a socket:
  int network_socket;
  network_socket = socket(AF_INET, SOCK_STREAM, 0); /* call to the socket() function
                                                       AF_INET is the socket Domain
                                                       SOCKET_STREAM is the type of the socket (eg: TCP or UDP... In this case it is TCP)
                                                       0 means we want to use the defaut protocol (TCP) 
                                                       then we have create the socket.*/
  
  /* Now we want to create the other end of the communication, to which we want to Connect. 
     Then we want to call the connect() function. 
     Before to call the connect functionwe need to specify the address we want to connect to; this is why we need netinet.in.h 
     who contains a Structure in which we can define IP and Port numbers. */
  // Definition of an address Structure for the socket:
  struct sockaddr_in server_address;
  //sin_family and  and sin_port are some of the fields that constitute the struct
  server_address.sin_family = AF_INET; // type of the address
  server_address.sin_port = htons(9002); // we could pass an integer, but better to use the conversion function htons(). 
                                         // 9002 is a port numer that is not used by the OS. 
  server_address.sin_addr.s_addr = INADDR_ANY; //here we specify the actual address we want to connect to.
                                               // INADDR_ANY == 0.0.0.0
  
  /* then we have defined the address we want to connect to. So now it make sense to kalling the connect() function.
     the first param we pass to connect() is the socket network_socket.
     then we need to cast the server address struct to a slightly different struck 
     then we pass the size of the address. 
     NB: connect() returns an integer that lets us know if the connection was successful or not.*/
  int connection_status = connect (network_socket, (struct sockaddr *) &server_address, sizeof(server_address)); 
  
  // then now we test if the connection was successful or not
  // if connection_status is zero then it was ok; if -1 something went wrong.
  if (connection_status == -1) {
    printf ("There was an error making the connection to the remote socket.\n\n");
  }
  
  // Now we can start sending or receiving data from the server.
  // Let's see recv()  function.
  // the first param we pass to recv() is the socket
  // second we pass a string we want to use to send back data from the server. We pass the address of this string &server_response.
  // the third param is the size of the server response (of the string)
  // is an options parameter.
  char server_response[256];
  recv (network_socket, &server_response, sizeof (serve_response), 0);
  
  // then we just print the data we got back from the server
  printf ("The server sent the data: %s\n", server_response);
   
  // now we can close the socket by useing the close funcion
  // this function takes the socket as a parameter
  close(network_socket);
  
  return 0;
}