htonl printing garbage value

Question

The variable 'value' is uint32_t

    value = htonl(value);

    printf("after htonl  is %ld\n\n",value);

    This prints -201261056

    value = htons(value);

    printf("after htons  is %ld\n\n",value);

    This prints  62465

Please suggest what could be the reason?

Answer 1

Host Order is the order which your machine understands the data (assuming your machine is little endian) correctly. Network Order is Big Endian, which cannot be understood by your system properly. This is the reason for your so called garbage values.

So, basically, there is nothing with the code. : )

Google "Endianness" to get all the details about Big Endian and Little Endian.

Providing some more info, In Big endian, first byte or lowest address will have the most significant byte and in little endian, at the same place, the least significant byte will be present. So, when you use htonl, your first byte will now contain the most significant byte, but your system will think it is as the least significant byte.

Considering the wikipedia example of decimal 1000 (hex 3E8) in big endian will be 03 E8 and in little endian will be E8 03. Now, if you pass 03 E8 to a little machine, it will consider to be decimal 59395.

Answer 2

I guess your input is 500, isn't it?

500 is 2* 8+2 *7+2* 6+2 *5+2* 4+2 *2 or 0x00 0x00 0x01 0xF4 in little endian order.

TCP/IP uses big endian. So after the htonl, the sequence is 0xF4 0x01 0x00 0x00 .

If you print it as signed integer, since the first digit is 1, it is negative then. Negative numbers are regarded as complement, The value is -(2* 27 + 2 *25+2* 24+2 *23+2* 22+2 *21+2* 20+2 *19+2* 18+2 *17+2**16) == -201261056

Answer 3

htonl() and htons() are functions which is used to convert the data from host's endianess to networks endiness.

Network uses big-endian. so if your system is X86, then it is little-endian.

Host to Network byte order(long data) is htonl(). ie converts 32bit value to network byte order.

Host to Network byte order(short data) is htons(). ie converts 16bit value to network byte order.

sample program to show how htonl() works as well as effect of using 32bit value in htons() function.

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

int main()
{
   long data = 0x12345678;
   printf("\n After htonl():0x%x  , 0x%x\n", htonl(data), htons(data));
   return 0;
}

It will print After htonl():0x78563412 , 0x7856 on X86_64.

Reference:

http://en.wikipedia.org/wiki/Endianess

http://msdn.microsoft.com/en-us/library/windows/desktop/ms738557%28v=vs.85%29.aspx

http://msdn.microsoft.com/en-us/library/windows/desktop/ms738556%28v=vs.85%29.aspx

Answer 4

@halfelf>I jut want to put my findings.I just tried the below program with the same 
value 500.I guess you have mistakenely mentioned output of LE as BE and vice versa
Actual output i got is 0xf4 0x01 0x00 0x00 as little Endian format.My Machine is 
LE
#include <stdio.h>
#include <netinet/in.h>
/* function to show bytes in memory, from location start to start+n*/
void show_mem_rep(char *start, int n)
{
    int i;
     for (i = 0; i < n; i++)
      printf(" %.2x-%p", start[i],start+i);
     printf("\n");
}

/*Main function to call above function for 0x01234567*/
int main()
{
  int i = 500;//0x01234567;
   int y=htonl(i); --->(1)
   printf("i--%d , y---%d,ntohl(y):%d\n",i,y,ntohl(ntohl(y)));
   printf("_------LITTLE ENDIAN-------\n");
   show_mem_rep((char *)&i, sizeof(i));
   printf("-----BIG ENDIAN-----\n");/* i just used int y=htonl(i)(-1) for reversing 
   500 ,so that
   i can print as if am using a BE machine. */

   show_mem_rep((char *)&y, sizeof(i));

   getchar();
   return 0;
  }
output is 
i--500 , y----201261056,ntohl(y):-201261056
_------LITTLE ENDIAN-------
 f4-0xbfda8f9c 01-0xbfda8f9d 00-0xbfda8f9e 00-0xbfda8f9f
 -----BIG ENDIAN-----
  00-0xbfda8f98 00-0xbfda8f99 01-0xbfda8f9a f4-0xbfda8f9b