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Communications Programming Concepts

Reading Internet Datagrams Example Program

/*
 * This program creates a datagram socket, binds a name to it, and
 * then reads from the socket.
 */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
main()
{
   int sock, length;
   struct sockaddr_in name;
   char buf[1024];
   /* Create a socket from which to read. */
   sock = socket(AF_INET, SOCK_DGRAM, 0);
   if (sock < 0) {
      perror("opening datagram socket");
      exit(1);
   }
   /* Create name with wildcards. */
   name.sin_family = AF_INET;
   name.sin_addr.s_addr = INADDR_ANY;
   name.sin_port = 0;
   if (bind(sock, (struct sockaddr *)&name, sizeof(name))) {
      perror("binding datagram socket");
      exit(1);
   } 


  /* Find assigned port value and print it out. */
   length = sizeof(name);
   if (getsockname(sock, (struct sockaddr *)&name, &length)) {
      perror("getting socket name");
      exit(1);
   }

   printf("Socket has port #%d\n", ntohs(name.sin_port));
   /* Read from the socket. */
   if (read(sock, buf, 1024) < 0)
      perror("receiving datagram packet");
   printf("-->%s\n", buf);
   close(sock);
}


/*
 * recvfrom() can also be used in place of the read. recvfrom()
 * provides an extra field for setting flags.
 */

More explanation is available in Socket Data Transfer.

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