Re: htons() and ntohs()

[Thomas Maeder]

alaudo <alaudo@gmail.com> writes:

> I've got a strange problem with changing my unsigned short values from
> host to the network byte order and vice versa.
> I have a client and a server, which communicate via their own binary
> protocol. I need to send some numerical values of unsigned short type,
> so in the client I do the following:

Please next time post a minimal program that your audience can
copy&paste&compile&run to (attempt to) see what you are seeing.


> typedef unsigned short order_id_t;
> ...
> order_id_t  orderNum; // variable declaration
> ...
> /* I generate a binary message of MSG_LENGTH length and copy the id to
> it */
> void *msg;  //binary message
> msg = (void *) malloc(MSG_LENGTH);

1. variables are best initialized in their definition.
2. why do you cast from void * to void *?


> orderNum = htons(orderNum);
> memcpy(msg+OFFSET,&orderNum,sizeof(order_id_t));
>
> /* Now I face a strange problem */
> fprintf(stdout,"Your ID is:%d",ntohs(orderNum)); // here the correct
> number is written to the stdout

%d indicates that the following argument is of type int (or a type
promoted to int). In fact, it has an unsigned type (uint16_t on my
machine).


> fprintf(stdout,"Your ID is:%d",ntohs( (order_id_t) *(msg+OFFSET))); //
> here t I get strange values, as if I had only the higher byte
> // e.g., if orderNum is < 255, then I get 0, 256 gives me 256 correct,
> 257...260 all give 256 etc.

The expression *(msg+OFFSET) dereferences a pointer to void.

And depending on the actual value of OFFSET, msg+OFFSET may not be
suitable aligned for type order_id_t.


When I compile

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>

int main()
{
  size_t const MSG_LENGTH = 100;
  size_t const OFFSET = 0;

  typedef unsigned short order_id_t;
  order_id_t  orderNum = 17;

  void *msg = malloc(MSG_LENGTH);

  orderNum = htons(orderNum);
  memcpy(msg+OFFSET,&orderNum,sizeof(order_id_t));

  fprintf(stdout,"Your ID is:%u",ntohs(orderNum));

  fprintf(stdout,"Your ID is:%u",ntohs( (order_id_t) *(msg+OFFSET)));
}

as C++ using gcc 4.1.2, I get:

test.cpp: In function 'int main()':
test.cpp:17: error: pointer of type 'void *' used in arithmetic
test.cpp:22: error: pointer of type 'void *' used in arithmetic
test.cpp:22: error: 'void*' is not a pointer-to-object type


When I compile it as C, I get:

test.cpp: In function 'main':
test.cpp:22: warning: dereferencing 'void *' pointer
test.cpp:22: error: void value not ignored as it ought to be