CODEWITHSUNDEEP
csocketsendianness
user3542317
user3542317

Reputation: 313

inet_netof() and returned host-byte order

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>

int main(int argc, char **argv) {
  unsigned long addr;
  unsigned long net_id;
  struct sockaddr_in address;

  addr = inet_addr("192.168.50.25");
  // prints 1932A8C0 in host byte-order which in this case is little-endian
  printf("IP-Address in Host-Byte Order: %08X\n", (unsigned int) addr);


  // zero the struct out
  memset(&address, 0, sizeof address);

  // copy address into the struct   
  memcpy(&address.sin_addr.s_addr, &addr, sizeof addr); 

  // inet_netof returns the network ID in host byte order
  net_id = inet_netof(address.sin_addr);

  // prints 00C0A832 Why not 0032A8C0?
  printf("Network ID in Host-Byte-Order: %08X\n", (unsigned int) net_id);
  return 0;
}

I have a little-endian machine so the host byte-order is little endian. The IP-Address 192.168.50.25 is 0x1932A8C0 in host-byte order. This is obvious to me.

Now, the inet_netof function returns the networkID of the address 192.168.50.25 in host-byte-order. The output of that is 0x00C0A832. This is confusing me. This does not really look like little-endianness to me. If I would convert that into dotted decimal it would be: 0.192.168.50.

I would have assumed the network-ID would look like this in little-endian: 0.50.168.192 or 0x0032A8C0 hexadecimal instead inet_netof() returns 0x00C0A832.

Why does inet_netof() return 0x00C0A832 instead of 0x0032A8C0?

Upvotes: 1

Views: 1161

Answers (2)

alk
alk

Reputation: 70931

  1. inet_addr() returns network byte order.
  2. The network number you receive is three bytes.
  3. inet_netof() returns host byte order, that is Little-endian. As your machine is Little-endian the bytes returned by inet_netof() are in revers order, compared to the output of what inet_addr() returned.

For reference please see man 3 inet.

Upvotes: 1

Karoly Horvath
Karoly Horvath

Reputation: 96266

0x00C0A832 is, as you said, 0.192.168.50.

It's in host byte order, and the correct value.

The phrasing in the documentation is a bit vague("returns the network number part of the Internet Address in"), but if I check an implementation, there's a right shift, so everything seems to be fine.

Upvotes: 1

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