[Tfug] Need help with a C++ algorithm

[Stephen Hooper]

On 3/15/07, Brian Murphy <murphy+tfug at email.arizona.edu> wrote:
> Quoting Jude Nelson <judecn at gmail.com>:
> > Is there a way to determine the size of an allocated block of memory if only
> > a pointer to it is known?
>
>
> No, there is not.
>

Hate to disagree with you there dude, but yes there is:

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

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

char *start,*seek;
char *start2;
int i;

	if(argc < 2)
		return EXIT_FAILURE;

	start = malloc(strtol(argv[1],NULL,16) * sizeof(char));
	seek = start;
	seek -= 4;
	for(;seek != start; seek++)
		printf("%X %02X\n",seek,*seek & 0xFF);

	start2 = malloc(1 * sizeof(char));
	printf("next alloc starts at %X (%X difference)\n",start2,start2 - start);
	
	free(start); // How do you think this magic works...
	free(start2);

	return EXIT_SUCCESS;
}

It is horribly system specific, but quite fun.  For a 64 bit machine
you would obviously want to change the "seek -=", and be sure to read
things on Linux.

Run this like:

allocsize 0x3F7F

You may be surprised by the results, but an understanding of the
processor will tell you why it allocates the low order bytes as it
does.