Reverse byte order using union, How?

[jbp2004]

Hello !

Now, I am able to get reverse byte order using the following function:

void GetReverseHexa(int n) {
	int a, b, c, d, e;
	a = n & 0x000000FF;
	b = n & 0x0000FF00;
	c = n & 0x00FF0000;
	d = n & 0xFF000000;

	a <<= 24;
	b <<= 8;
	c >>= 8;
	d >>=24;

	e = a|b|c|d;

	printf("\nReverse byte order: %2x\n\n",e);
}

parameter n is the given number (user input). Now, the task it to come up with the same output but using union. Anyone who has an idea?

[Kheun]

union Word
{
	DWORD dword;
	struct
	{
		BYTE lo_byte_short0;
		BYTE hi_byte_short0;
		BYTE lo_byte_short1;
		BYTE hi_byte_short1;
	} bytes;
};

Word Reverse(Word& w)
{
	Word result;
	result.bytes.lo_byte_short0 = w.bytes.hi_byte_short1;
	result.bytes.hi_byte_short0 = w.bytes.lo_byte_short1;
	result.bytes.lo_byte_short1 = w.bytes.hi_byte_short0;
	result.bytes.hi_byte_short1 = w.bytes.lo_byte_short0;

	return result;
}

int main()
{
	Word a;
	a.dword = 0x12345678;
	
	printf("\nReverse byte order: %2x\n\n", Reverse(a).dword);


	return 0;
}

[jbp2004]

Ok, I got the answer I want. Now, I want to share the code here

void GetReverseByteOrder(int n) {
   union {
      int nGiven;
      unsigned char val[4];
   } HEXA;

   HEXA.nGiven = n; /* n is input of user */
   for (int i=0; i<4; i++)
      printf("%02x",HEXA.val[i]);
}

What does this piece of code do?
-- user types in a number, say 123456
-- 123456 is stored to n and passed to HEXA.nGiven
-- we know that 123456 is 1E240 in base 16
-- now, we wanted to reverse 1E240 making it 40E20100
-- 40E20100 is generated by the code above

How come? Can u imagine how? :-)

That's it! A generous man from codeguru shared the above code. I just did a little modification. I forgot his username here in this forum but anyway, many many thanks to him!

[jbp2004]

Kheun,

Thank you for your reply.
Anyway, I found a shorter code though I do not know if the one I found is good to use. What do you think?

Anyway, how does your code differs to the code I got?

[Kheun]

Well, both are doing almost the same thing except that your code only displays in reverse order without the actual reversing of the int.