[aclug-L] Re: Nice code and negative memory addresses

[Tom Hull <thull@xxxxxxxxxxx>]

Lars von dem Ast wrote:
> 
> Here's some nice code (taken from a C text):
> 
> /* pmath.c */
> 
> #define STRSIZ 20
> 
> typedef struct {
>   char name[STRSIZ];
>   double diameter;
>   int moons;
>   double orbit_time, rotation_time;
> } planet_t;
> 
> int main(void)
> {
>   planet_t pl[2] = {{"Earth", 12713.5, 1, 1.0, 24.0},
>                     {"Jupiter", 142800.0, 4, 11.9, 9.925}};
>   int nm[5] = {4, 8, 10, 16, 22};
>   planet_t *p;
>   int *np;
> 
>   p = pl + 1;
>   np = nm + 1;
> 
>   printf("sizeof (planet_t) = %d          sizeof (int) = %d\n",
>          sizeof (planet_t), sizeof (int));
>   printf("pl = %d                nm = %d\n", pl, nm);
>   printf(" p = %d (pl + %d)      ", p, (int)p - (int)pl);
>   printf("np = %d (nm + %d)\n", np, (int)np - (int)nm);
>   printf(" p - pl = %d\n", p - pl);
> 
> }
> 
> Okay, here are the results:
> 
> sizeof (planet_t) = 48          sizeof (int) = 4
> pl = -1073743632                nm = -1073743664
>  p = -1073743584 (pl + 48)      np = -1073743660 (nm + 4)
>  p - pl = 1
> 
> The pl, p, nm, and np variables should have memory addresses in them, but why
> are they negative numbers?

Because you told printf() to print the pointers as signed integers. printf
doesn't know better -- it just takes a sizeof(int) chunk of data off the
argument stack and does what you tell it to.

Instead of %d (signed integer), you could use %u (unsigned integer), or %x
(unsigned hexadecimal integer), or %p (some format appropriate for pointers,
usually hexadecimal, except that it will work even if sizeof(int) != 
sizeof(void*).

Also, your (int) casts are bogus. The proper cast of pointers to do byte address
calculations is to use (size_t). size_t is usually unsigned int or unsigned 
long,
depending on the compiler and the machine architecture. It would also work to
cast both pointers to (char*), since sizeof(char) == 1.

> Lb
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 *  Tom Hull * thull at kscable.com * http://www.tomhull.com/
 */
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