Thread: help with pointers

Hi,
I need help with this program . I am doing something wrong with pointer addition.

Code:

/* program allocates 16 MB of memory on heap and writes to every 2MB interval*/

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

#define INTERVAL  (2UL * 1024 *1024) /* 2MB interval*/

int main(int argc, char *argv[])
{

unsigned long p = malloc(atoi(argv[1]));
assert(p);

int i = 0;

unsigned long interval  =  INTERVAL;

int number_of_chunks  = (atoi(argv[1])/INTERVAL);

for(i = 0; i < number_of_chunks;  i++){
	p[i *INTERVAL] = (char) 1; /* write 1 at start location of each chunk */
	}

free(p);
return 0;
}

Getting a seg fault after i = 3 and gdb says each iternation is incrementing the pointer 10MB instead of 2MB.

It shouldn't even compile since p is not a pointer, so it can't be dereferenced. If your compiler allows it anyway, get a better compiler, or at least turn on warnings.

Originally Posted by christop

It shouldn't even compile since p is not a pointer, so it can't be dereferenced. If your compiler allows it anyway, get a better compiler, or at least turn on warnings.

Yes u are right. I think it was an overseen error as a result of an unchecked keystroke in vim. But the actual code had a pointer declared.
Thank you oogabooga. I think that will work. I thought all the memory addresses in RAM need to be unsigned long. But yeah unsigned char should work.

Originally Posted by livin

Yes u are right. I think it was an overseen error as a result of an unchecked keystroke in vim. But the actual code had a pointer declared.
Thank you oogabooga. I think that will work. I thought all the memory addresses in RAM need to be unsigned long. But yeah unsigned char should work.

A pointer is whatever size an address needs to be on the system, which might be smaller or larger than an unsigned long. A variable declared as a "char *" is a pointer (address) that refers to a char or to an array of char.