C: malloc arrays- unexpected output

kpachopoulos · 03-24-2006, 09:02 AM

Hi,
i've written this:

Code:

#include <stdio.h>
#include <stdlib.h>

#define CHUNKS 10

int *allocate_int_array(int chunks_num)
{
	return (int *)malloc(chunks_num*sizeof(int));
}



main()
{
	int *exp_array=allocate_int_array(CHUNKS);

	
	
	int i;
	
	for (i=0; i<10; i++)
	{		
		*exp_array=i;
		exp_array+=sizeof(int *);
	}
	
	int j;
	for (j=10;j>0;j--)
	{
		printf("%i \n",*exp_array);
		exp_array-=sizeof(int *);
	}
	
	free(*exp_array);
}

When i execute it i get:

Code:

Why don't i get this? What is this 663604? (looks like an address)

Code:

bc8o8 · 03-24-2006, 10:13 AM

The lines

Code:

===snip===
   exp_array+=sizeof(int *);
===snip===
   exp_array-=sizeof(int *);
===snip===

are incrementing your array location by sizeof(int *) which (on my system) is 4. So you are skipping valid locations and reading/writing other areas in memory. What you want to do instead is actually just increment the pointer -- pointers are smart enough to know their size, so it will increment appropriately.

For example the following should do what you want

Code:

for (i=0; i<10; i++)
{		
	*exp_array=i;
	exp_array++;
}

int j;
for (j=10;j>0;j--)
{
	printf("%i \n",*exp_array);
	exp_array--;
}

although, instead of moving your exp_array pointer around, a better approach would be to just use exp_array[ i ] = i; and either printf("%i \n",exp_array[ i ]); or printf("%i \n",exp_array+i); (which are identical)

addy86 · 03-24-2006, 04:49 PM

Quote:

Originally Posted by bc8o8

Code:

int j;
for (j=10;j>0;j--)
{
	printf("%i \n",*exp_array);
	exp_array--;
}

This will still not work; instead:

Code:

{
	exp_array--;
	printf("%i \n",*exp_array);
}

freegianghu · 03-24-2006, 09:30 PM

Quote:

Originally Posted by nocturna_gr

Hi,
i've written this:

Code:

#include <stdio.h>
#include <stdlib.h>

#define CHUNKS 10

int *allocate_int_array(int chunks_num)
{
	return (int *)malloc(chunks_num*sizeof(int));
}



main()
{
	int *exp_array=allocate_int_array(CHUNKS);

	
	
	int i;
	
	for (i=0; i<10; i++)
	{		
		*exp_array=i;
		exp_array+=sizeof(int *); // exp_array++; 
	}
	
	int j;
	for (j=10;j>0;j--)
	{
		printf("%i \n",*exp_array);
		exp_array-=sizeof(int *); // exp_array--;
	}
	
	free(*exp_array); // free(exp_array);
}

................

Cheers,
GH.