//-------------------------------------------------------------------
//	switcher.cpp
//
//	This program implements a crude version of user-mode thread-
//	scheduling (as distinguished from the kernel-mode scheduling 
//	of threads which is directly supported by Linux).  We create
//	a queue of read-to-run threads, and we use signals generated
//	by an interval timer to trigger our "round-robin" scheduling
//	among four distinct threads (which all exist within a common 
//	process address-space).  What is unusual here is that we use
//	a common stack for all of our threads, for simplicity.  This
//	is possible because our threads always resume execution from 
//	their initial entry-point (i.e., they "start over"), instead 
//	of resuming from the point where they were interrupted.  But
//	this is purely a demo, to illustrate the concept of threads. 
//
//	programmer: ALLAN CRUSE
//	written on: 27 SEP 2004
//-------------------------------------------------------------------

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

int	done = 0;	// global variable

int thread1( void )
{
	printf( "Hello from thread1 \n" );
	while ( !done );
	printf( "\nThread1 is quitting\n" );
	exit(0);
}
	
int thread2( void )
{
	printf( "Hello from thread2 \n" );
	while ( !done );
	printf( "\nThread2 is quitting\n" );
	exit(0);
}

int thread3( void )
{
	printf( "Hello from thread3 \n" );
	while ( !done );
	printf( "\nThread3 is quitting\n" );
	exit(0);
}

int thread4( void )
{
	printf( "Hello from thread4 \n" );
	while ( !done );
	printf( "\nThread4 is quitting\n" );
	exit(0);
}





typedef int (*THREAD)(void);

THREAD 	ready[] = { thread1, thread2, thread3, thread4 }; 
const int thread_count = sizeof( ready )/sizeof( THREAD );

int 	next_thread = 0;	// array-index for 'ready' list

void upon_signal( int signum )
{
	unsigned long 	*tos;	// this is used for addressing the stack-frame

	asm(" movl %%ebp, %0 " : "=m" (tos) );	// initializes frame-pointer

	// This was useful during development -- to display the stack elements
	// for (int i = 0; i < 22; i++) printf( "tos[%d]=%08X \n", i, tos[i] );

	tos[17] = (unsigned long)ready[ next_thread ];	// modify return-point

	next_thread = ( 1 + next_thread ) % thread_count;  // "circular" array

	if ( signum == SIGINT )	done = 1; 	// to terminate on <CONTROL>-C
}	



int main( int argc, char **argv )
{
	signal( SIGINT, upon_signal );
	
	struct itimerval  it;
	it.it_value.tv_sec = 1;
	it.it_value.tv_usec = 0;
	it.it_interval.tv_sec = 0;
	it.it_interval.tv_usec = 200000;
	setitimer( ITIMER_REAL, &it, NULL );
	signal( SIGALRM, upon_signal );
	
	while ( !done );

	printf( "\nQuitting\n" );
}