Thread: Program Crashing on Pointer Assignment

Code:

#include <stdlib.h>
#include <math.h>

int div32(long dividend, long divisor, long *quotient, long *rmdr)
{
	*rmdr = dividend;
	*quotient = 0;
	int shifts = 0;
	
	while ((divisor & 0x40) != 0x40)
	{
		divisor = divisor << 1;
		shifts++;
	}
	int i = 0;
	for (i; i <= shifts; i++)
	{
		if (*rmdr-divisor >= 0)
		{
			*rmdr -= divisor;
			*quotient++;
		}
		
		divisor = divisor >> 1;
		if ( i!=shifts)
			*quotient = *quotient << 1;
	}
	printf("&#37;l / %l = %l", dividend, divisor, *quotient);
	
	return 0;
}

void main (int argc, char *argv[])
{
	div32(50, 25, 0, 0);
}

crashes on line 6. even if line 6 is *rmdr=25

if line 6 is commented out, it crashes on line 7


WTF!??!?!

I usually use global variables if I can, and hence avoid stuff like this and the problems
it can bring, so I am not used to all this parameter passing malarky


However it appears you pass 0 to the routine in the place of *quotient.
So you are probably then trying to put something into the address 0000
when you do *quotient = 0.
When you try to do this the computer thinks WTF! he's trying to overwrite
the operating system and hence crashes in disgust, or something like that

You could try changing the line to

Code:

printf("\n%d",(int *)*quotient);

or something like that, to see what value you were trying to overwrite.

Kindly post well indented code using code tags.

Anyway, you are passing null pointers and then trying to dereference them, which is obviously a problem since they do not point to any object.

Originally Posted by esbo

I usually use global variables if I can, and hence avoid stuff like this and the problems
it can bring, so I am not used to all this parameter passing malarky

And don't listen to esbo's global variables mumbo-jumo. Global variables are bad practice.

Originally Posted by foxman

Second, it's totally normal it's crashing. You are passing 0 as argument value for your pointer. You can't acces memory at position 0. Note that your function call is all but semantically good (at least, for the last two arguments).

Technically, a null pointer does not have to point to position 0. But the rest is right. You can't dereference a null pointer.

Originally Posted by vsanandan

Pnevma,
You can follow what foxman says. I would suggest one more way to avoid the crash. In div32(..) function definition, you can add the following line as 6th line:
quotient = (long*) malloc(sizeof(long));
Then 7th line can be:
*quotient = 0;
If you don't need the value of quotient outside the function div32(...), you can free within the function -
free(quotient);

Yes, that would avoid the crash, but if you don't actually want the quotient outside the function, you probably would not want to allocate, assign and free inside the function, but rather use a local variable for quotient calculation, and pass in the pointer as NULL, then check if the pointer is NOT NULL, then set the quotient from the local variable.

Allocating memory could easily take as much time as the entire rest of the function.

[Of course, the function won't know if the data is needed or not, but passing in NULL as a pointer can indicate it's not needed to the function].

--
Mats