Example: A connectionless client

/**************************************************************************/
/* This sample program provides a code for a connectionless client.       */
/**************************************************************************/

/**************************************************************************/
/* Header files needed for this sample program                            */
/**************************************************************************/
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

/**************************************************************************/
/* Constants used by this program                                         */
/**************************************************************************/
#define SERVER_PORT     3555
#define BUFFER_LENGTH    100
#define FALSE              0
#define SERVER_NAME     "ServerHostName"

/* Pass in 1 parameter which is either the */
/* address or host name of the server, or  */
/* set the server name in the #define      */
/* SERVER_NAME                             */
void main(int argc, char *argv[])
{
   /***********************************************************************/
   /* Variable and structure definitions.                                 */
   /***********************************************************************/
   int    sd, rc;
   char   server[NETDB_MAX_HOST_NAME_LENGTH];
   char   buffer[BUFFER_LENGTH];
   struct hostent *hostp;
   struct sockaddr_in serveraddr;
   int    serveraddrlen = sizeof(serveraddr);

   /***********************************************************************/
   /* A do/while(FALSE) loop is used to make error cleanup easier.  The   */
   /* close() of the socket descriptor is only done once at the very end  */
   /* of the program.                                                     */
   /***********************************************************************/
   do
   {
      /********************************************************************/
      /* The socket() function returns a socket descriptor representing   */
      /* an endpoint.  The statement also identifies that the INET        */
      /* (Internet Protocol) address family with the UDP transport        */
      /* (SOCK_STREAM) will be used for this socket.                      */
      /********************************************************************/
      sd = socket(AF_INET, SOCK_DGRAM, 0);
      if (sd < 0)
      {
         perror("socket() failed");
         break;
      }

      /********************************************************************/
      /* If an argument was passed in, use this as the server, otherwise  */
      /* use the #define that is located at the top of this program.      */
      /********************************************************************/
      if (argc > 1)
         strcpy(server, argv[1]);
      else
         strcpy(server, SERVER_NAME);

      memset(&serveraddr, 0, sizeof(serveraddr));
         serveraddr.sin_family      = AF_INET;
         serveraddr.sin_port        = htons(SERVER_PORT);
         serveraddr.sin_addr.s_addr = inet_addr(server);
         if (serveraddr.sin_addr.s_addr == (unsigned long)INADDR_NONE)
      {
         /*****************************************************************/
         /* The server string that was passed into the inet_addr()        */
         /* function was not a dotted decimal IP address.  It must        */
         /* therefore be the hostname of the server.  Use the             */
         /* gethostbyname() function to retrieve the IP address of the    */
         /* server.                                                       */
         /*****************************************************************/
         hostp = gethostbyname(server);
         if (hostp == (struct hostent *)NULL)
         {
            printf("Host not found --> ");
            printf("h_errno = %d\n", h_errno);
            break;
         }

         memcpy(&serveraddr.sin_addr,
                hostp->h_addr,
                sizeof(serveraddr.sin_addr));
      }

      /********************************************************************/
      /* Initialize the data block that is going to be sent to the server */
      /********************************************************************/
      memset(buffer, 0, sizeof(buffer));
      strcpy(buffer, "A CLIENT REQUEST");

      /********************************************************************/
      /* Use the sendto() function to send the data to the server.        */
      /********************************************************************/
      rc = sendto(sd, buffer, sizeof(buffer), 0,
                  (struct sockaddr *)&serveraddr,
                  sizeof(serveraddr));
      if (rc < 0)
      {
         perror("sendto() failed");
         break;
      }

      /********************************************************************/
      /* Use the recvfrom() function to receive the data back from the    */
      /* server.                                                          */
      /********************************************************************/
      rc = recvfrom(sd, buffer, sizeof(buffer), 0,
                    (struct sockaddr *)&serveraddr,
                    & serveraddrlen);
      if (rc < 0)
      {
         perror("recvfrom() failed");
         break;
      }

      printf("client received the following: <%s>\n", buffer);
      printf("from port %d, from address %s\n",
             ntohs(serveraddr.sin_port),
             inet_ntoa(serveraddr.sin_addr));

      /********************************************************************/
      /* Program complete                                                 */
      /********************************************************************/

   } while (FALSE);

   /***********************************************************************/
   /* Close down any open socket descriptors                              */
   /***********************************************************************/
   if (sd != -1)
      close(sd);
}