//----------------------------------------------------------------
//	signal6.cpp
//
//	This program will attempt to dereference a null pointer,
//	but beforehand it installs a signal-handler for SIGSEGV. 
//	Now our signal-handler will 'return' instead of 'exit',
//	but not until it has adjusted the saved image of the EIP
//	register, located at tos[15] on the stack, in order that
// 	the 2-byte instruction 'mov (%eax), %eax' which attempts
//	to read an invalid memory-address will now be 'skipped'.     
//
//	  compile-and-link using: $ g++ signal6.cpp -o signal6
//
//	programmer: ALLAN CRUSE
//	written on: 20 MAR 2006
//----------------------------------------------------------------

#include <stdio.h>	// for printf()
#include <signal.h>	// for signal()
#include <stdlib.h>	// for exit()

void upon_signal( int signo )
{
	printf( "\nCaught signal number %d \n\n", signo );

	int	*tos = &signo;
	for (int k = 1; k <= 20; k++)
		{
		int	i = 20 - k;	
		printf( "%08X: %08X =tos[%d] \n", &tos[i], tos[i], i );
		}
	printf( "\n" );

	tos[15] += 2;	// exit( signo );
}

int main( int argc, char *argv[] )
{
	signal( SIGSEGV, upon_signal );

	int	*ptr = NULL;

	asm(" movl $0xAAAAAAAA, %eax ");
	asm(" movl $0xBBBBBBBB, %ebx ");
	asm(" movl $0xCCCCCCCC, %ecx ");
	asm(" movl $0xDDDDDDDD, %edx ");
	asm(" movl $0xEEEEEEEE, %esi ");
	asm(" movl $0xFFFFFFFF, %edi ");
	asm(" nop ");

	int	x = *ptr;

	printf( "\nx = %08X \n\n", x );
}