From: "Graham Warren" <Moosehead AT tesco DOT net>
Newsgroups: comp.os.msdos.djgpp
Subject: Puzzling pointer behaviour
Date: Sat, 13 Oct 2001 10:22:21 +0100
Organization: Tesco ISP
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To: djgpp AT delorie DOT com
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Hello,
Thankyou to all who have helped me in the past.

I've come across something slightly strange about byte pointers. Can anyone
explain this. This small program demonstrates my query. When a byte pointer
is made to point to an long int pointer, it actually points to the 8 least
significant bits of the long int. Also, when this byte pointer is
incremented it points 'left' of its original position. Why is this?


/* POINTER.C */
#include <stdio.h>

int main (void)
{
int j;
unsigned long int dword1 = 0;                       /* set a 32-bit int to
zero */
unsigned long int *ptr_dword1 = &dword1; /* make a pointer to it */
unsigned char *ptr_byte1;

ptr_byte1 = (unsigned char *) ptr_dword1; /* make ptr_byte1 point to the
32-bit int */
*ptr_byte1 = 255;                                           /* set
*ptr_byte1 to 255 */

/* optional lines */
ptr_byte1 += 2;
*ptr_byte1 = 255;

for (j = 0; j < 32; j++) {                                     /* display
the 32 bits of memory */
  printf ("%i", (*ptr_dword1 & 0x80000000) ? 1 : 0);
  *ptr_dword1 = *ptr_dword1 << 1;
  }
printf ("\n");

return 0;
}
/* end */






Thankyou,
Graham Warren