help with data type sizes Post: 302488055

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Top Forums Programming help with data type sizes Post 302488055 by Corona688 on Friday 14th of January 2011 05:38:01 PM

01-14-2011

Registered User

Where the variable is stored has nothing to do with how it does pointer math. I can't see your code from here, but I'd venture they were all on the stack, with nothing in the heap. What type it's stored in does matter. Math on pointers is done in multiples of the base size, math on integers is just plain math.

So &a-&b is arithmetic on pointers themselves, and happens in multiples of the base size.

But if you convert them to integers first -- integers, not pointers to integers -- the compiler is no longer aware of any base size and does just plain arithmetic.

Try this:

Code:

#include <stdio.h>

int main(void)
{
        int a;
        int *b=&a;
        int *c=b+1;
        int d=b;
        int e=c;

        printf("diff between pointers is: %d\n", (unsigned long)(c-b));
        printf("diff between ints is: %d\n", e-d);
}

This makes it more obvious what's going on.

When you do math on a pointer, it puts it in terms of it's base size. An integer pointer plus one, actually gets sizeof(int) added to it, in this case 4. For plain integers, it makes no adjustment.

Last edited by Corona688; 01-14-2011 at 06:44 PM..

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ultoa_r

ltostr(3C)																ltostr(3C)

NAME

       ltostr(),  ultostr(),  ltoa(), ultoa() - convert long integers to strings

SYNOPSIS

   Obsolescent Interfaces
DESCRIPTION

	      Convert a signed long integer to the corresponding string
			     representation in the specified base.  The argument base must be between 2 and 36, inclusive.

	      Convert an unsigned long integer to the corresponding string
			     representation in the specified base.  The argument base must be between 2 and 36, inclusive.

	      Convert a signed long integer to the corresponding base 10
			     string representation, returning a pointer to the result.

	      Convert an unsigned long integer to the corresponding base 10
			     string representation, returning a pointer to the result.

       These functions are smaller and faster than using for simple conversions (see printf(3S)).

   Obsolescent Interfaces
       convert long integers to strings.

ERRORS

       If the value of base is not between 2 and 36, and return NULL and set the external variable to ERANGE.

WARNINGS

       The return values for and point to data whose content is overwritten by subsequent calls to these functions by the same thread.

       and  are  obsolescent interface supported only for compatibility with existing DCE applications. New multi-threaded applications should use
       and

AUTHOR

       and were developed by HP.

SEE ALSO

       strtol(3C), printf(3S), thread_safety(5).

																	ltostr(3C)

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