//-------------------------------------------------------------------
//	brkdemo.cpp
//
//	This program demonstrates the effects of directly invoking 
//	the Linux kernel's 'brk' system-call as distinguished from
//	the 'brk()' library-function as documented on 'man' pages. 
//
//	programmer: ALLAN CRUSE
//	date begun: 14 APR 2005
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), perror() 
#include <fcntl.h>	// for open() 
#include <stdlib.h>	// for exit() 
#include <unistd.h>	// for read(), write(), close() 

#define __NR_brk 45		// from <asm/unistd.h>
#define BUFFERSZ (16<<10)	// 16-kilobytes (=0x4000)


// global variables (used for easy assembler access)
unsigned int sys_call_id = __NR_brk;
unsigned int bufferlength = BUFFERSZ;
unsigned int original_brk, modified_brk;


int main( int argc, char **argv )
{
	// ask kernel for the current 'brk' value
	asm("	movl	sys_call_id, %eax	");
	asm("	xorl	%ebx, %ebx		");
	asm("	int	$0x80			");
	asm("	movl	%eax, original_brk	");
	printf( "original brk-value: %p \n", original_brk );

	// ask kernel to modify the 'brk' value
	asm("	movl	sys_call_id, %eax	");
	asm("	movl	original_brk, %ebx	");
	asm("	addl	bufferlength, %ebx	");
	asm("	int	$0x80			");
	
	// ask kernel for the current 'brk' value
	asm("	movl	sys_call_id, %eax	");
	asm("	xorl	%ebx, %ebx		");
	asm("	int	$0x80			");
	asm("	movl	%eax, modified_brk	");
	printf( "modified brk-value: %p \n", modified_brk );
}