Why this uint16_t variable declaration doesn't work?

Question

I have to send a package from a udp client to udp server (i made) but the problem is that this declaration doesn't work

#define RRQ 1
.
.
.

uint16_t op = htons(RRQ);

i tryed also

uint16_t op = htons(1);

and

int k = 1;
uint16_t op = htons(k);

but this doesn't put 1 in op but 256

I tryed to send this package to the server, i receive the package and i get the same result server side (i find op == 256 and not op == 1 like it should) Thanks for the reading!

Impossible to know since we have no idea how `htons` looks like. Please provide a [mre] — klutt, Sep 23 '19 at 18:33
@Barmar Looks like it's (thankfully) [part of the POSIX standard](https://pubs.opengroup.org/onlinepubs/9699919799/functions/htonl.html). — tadman, Sep 23 '19 at 18:40
@Barmar My bad there. But still, this questions lacks a mre nonetheless. — klutt, Sep 23 '19 at 18:42
I don't understand what doesn't work. Sounds like the function is flipping the bytes as expected from it on a little-endian machine. — Eugene Sh., Sep 23 '19 at 18:42
It sounds like the server just isn't using `ntohs()` as it should. — Barmar, Sep 23 '19 at 18:50

Support Ukraine · Answer 1 · 2019-09-23T19:20:02.883

Why this uint16_t variable declaration doesn't work?

Well, it does...

You are probably on a little endian machine so the result is correct.

On a little endian machine the memory is like:

uint16_t v1 = 1;     --> Memory: 0x01 0x00
uint16_t v256 = 256; --> Memory: 0x00 0x01

In network order it's like:

uint16_t v1 = 1;     --> Memory: 0x00 0x01
uint16_t v256 = 256; --> Memory: 0x01 0x00

So k have the value 1 which in memory will be 0x01 0x00

Then op will be assigned k converted to network order so in memory it will be 0x00 0x01.

When you print that as a uint16_t on the host, you will see the result 256.

This little program illustrates what is going on:

#include <stdio.h>
#include <inttypes.h>
#include <arpa/inet.h>

int main()
{
  uint16_t k = 1;
  unsigned char* p = (unsigned char*)&k;
  printf("k=%hd\n", k);
  for (size_t i = 0; i<sizeof k; ++i)
  {
      printf("%02X\n", *p++);
  }
  uint16_t op = htons(k);
  unsigned char* pop = (unsigned char*)&op;
  printf("op=%hd\n", op);
  for (size_t i = 0; i<sizeof op; ++i)
  {
      printf("%02X\n", *pop++);
  }
  return 0;
}

Output:

k=1
01
00
op=256
00
01

Why this uint16_t variable declaration doesn't work?

1 Answers1