c - Understanding htonl() and ntohl()

Question

I am trying to use unix sockets to test sending some udp packets to localhost.

It is my understanding that when setting ip address and port in order to send packets, I would fill my sockaddr_inwith values converted to network-byte order. I am on OSX and I'm astonished that this

printf("ntohl: %d
", ntohl(4711));
printf("htonl: %d
", htonl(4711));
printf("plain: %d
", 4711);

Prints

ntohl: 1729232896
htonl: 1729232896
plain: 4711

So neither function actually returns the plain value. I would have expected to see either the results differ, as x86 is little-endian (afaik), or be identical and the same as the actual number 4711. Clearly I do not understand what htonl and ntohl and their variants do. What am I missing?

The relevant code is this:

int main(int argc, char *argv[])
{
   if (argc != 4)
   {
      fprintf(stderr, "%s
", HELP);
      exit(-1);
   }

   in_addr_t rec_addr = inet_addr(argv[1]); // first arg is '127.0.0.1'
   in_port_t rec_port = atoi(argv[2]);      // second arg is port number
   printf("Address is %s
Port is %d
", argv[1], rec_port);
   char* inpath = argv[3];

   char* file_buf;
   unsigned long file_size = readFile(inpath, &file_buf); // I am trying to send a file
   if (file_size > 0)
   {
      struct sockaddr_in dest;
      dest.sin_family      = AF_INET;
      dest.sin_addr.s_addr = rec_addr; // here I would use htons
      dest.sin_port        = rec_port;
      printf("ntohs: %d
", ntohl(4711));
      printf("htons: %d
", htonl(4711));
      printf("plain: %d
", 4711);
      int socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
      if (socket_fd != -1)
      {
         int error;
         error = sendto(socket_fd, file_buf, file_size + 1, 0, (struct sockaddr*)&dest, sizeof(dest));
         if (error == -1)
            fprintf(stderr, "%s
", strerror(errno));
         else printf("Sent %d bytes.
", error);
      }
   }

   free(file_buf);
   return 0;
}

Answer 1

As others have mentioned, both htons and ntohs reverse the byte order on a little-endian machine, and are no-ops on big-endian machines.

What wasn't mentioned is that these functions take a 16-bit value and return a 16-bit value. If you want to convert 32-bit values, you want to use htonl and ntohl instead.

The names of these functions come from the traditional sizes of certain datatypes. The s stands for short while the l stands for long. A short is typically 16-bit while on older systems long was 32-bit.

In your code, you don't need to call htonl on rec_addr, because that value was returned by inet_addr, and that function returns the address in network byte order.

You do however need to call htons on rec_port.