Weird pointer dereferencing phenomenon

[asjt3]

Hey guys,
I've got a decode function which decodes from a buffer. It reads the buffer to a var, _int64 myValue. Since the buffer stores a pointed double value, here's how I extract the original encoded val after the decoding itself is finished:

void* valuePointer = &myValue;
double finalVal = *(double*)valuePointer;

When i try something like: printf("final val = %f",finalVal), I get 0.0000 (which is wrong)

However when I try printf("myValue = %i, finalValue = %i",myValue,finalValue); i get 1083399168 and 1239.0 (which is right.)

It seems that finalValue only prints out correctly when it is preceded in a printf statement by myValue, as shown.

I really need to extract finalValue alone, does anyone know what is going on here and how to solve it?

thanks,

doct

[Duthomhas]

Assuming that you don't have endian or fp-format issues, you can do a simple cast:

#include <iostream>
using namespace std;

int main()
  {
  // IEEE double precision value == -2 (little-endian format)
  unsigned char s[ 8 ] = { 0x00, 0x00,  0x00, 0x00,  0x00, 0x00,  0x00, 0xC0 };
  double d = *(double*)( s );
  cout << "-2 == " << d << endl;
  return 0;
  }

Also, please be careful how you cast pointers around. For example, you say:
printf("... finalValue = %i", ..., finalValue );
The types don't match, and could easily smash your stack.

Hope this helps.

[asjt3]

For some reason, it only prints correctly when in tandem with myval, and it doesn't even store the correct value.

Right sorry, I meant to write %f haha.

Thannks for the reply, I'm not sure if this helps me though. Let me try to explain more clearly what I'm doing:

encoding:

double value = 4556.00;
_int64* myValue = (_int64*)&value;
unsigned char* pLoc = (unsigned char*)myValue;
unsigned char* buf = (unsigned char*)buffer; //buffer has been passed to 
//this function as a void pointer

//in a for loop:
   buf[i] = pLoc[i];

//therefor you have buf = {40,B1,CC,00,00,00,00,00} (hexadecimal) 

decoding:

_int64 myval = 0;

//then in decode, buffer is passed again, and through some byte-masking 
//you get this:

myval = 40B1CC0000000000; // (hexadecimal)

//so then of course i try:

void* vp = (void*)&myval;
value = *(double*)vp;

printf("myval = %i, value = %f\n",myval,value);
//from the above printf, i get some random int for myval and 4556.0 for value
//but then if i do this just below:
printf("val = %f\n",value);
//i get value = 0.000 

what's goin on here??