//-------------------------------------------------------------------
//	racecure.cpp
//
//	This demo revises our 'racedemo.cpp' program; it eliminates 
//	the 'race condition' by using signals to synchronize access
//	to the stdout stream (in the manner suggested by W. Richard 
//	Stevens in "Advanced Programming in the UNIX Environment"). 
//	The 'sigsuspend()' function is used here to enable tasks to 
//	avoid 'busy-waiting' in an especially efficient fashion (by
//	not being sheduled for CPU time until conditions exist that
//	allow them to make forward progress).	
//
//	programmer: ALLAN CRUSE
//	written on: 10 OCT 2004
//-------------------------------------------------------------------

#include <stdio.h>	// for perror(), setbuf(), putc() 
#include <unistd.h>	// for fork() 
#include <stdlib.h>	// for exit() 
#include <signal.h>	// for signal(), sigsuspend(), kill(), etc

#define DELAY_COUNT 20000000

sigset_t		newmask, oldmask, zeromask;	// signal-sets
volatile sig_atomic_t 	sigflag = 0;	// modified by signal-handler  

void upon_signal( int signum ) { sigflag = 1; }


void INITIALIZE_TELL_WAIT( void )
{
	sigemptyset( &zeromask );

	sigemptyset( &newmask );
	sigaddset( &newmask, SIGUSR1 );
	sigaddset( &newmask, SIGUSR2 );
	sigprocmask( SIG_BLOCK, &newmask, &oldmask );

	signal( SIGUSR1, upon_signal );
	signal( SIGUSR2, upon_signal );
}


void WAIT_PARENT( void )
{
	while ( sigflag == 0 ) sigsuspend( &zeromask );
	sigflag = 0;
	sigprocmask( SIG_SETMASK, &oldmask, NULL );
}

void TELL_PARENT( int pid )
{
	kill( pid, SIGUSR2 );	// tells parent we're done
}


void WAIT_CHILD( void )
{
	while ( sigflag == 0 ) sigsuspend( &zeromask );
	sigflag = 0;
	sigprocmask( SIG_SETMASK, &oldmask, NULL );
}

void TELL_CHILD( int pid )
{
	kill( pid, SIGUSR1 );	// tells child we're done
}



//---------------------------------------------------------------------
// Below is the original 'racedemo' code -- with synchronization added
//---------------------------------------------------------------------

void one_character_at_a_time( char *msg )
{
	setbuf( stdout, NULL );		// set to 'unbuffered'
	while ( int c = *msg++ ) 
		{
		putc( c, stdout );
		int	i = DELAY_COUNT;
		while( --i );
		}
}	


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

	int	pid = fork();
	if ( pid < 0 ) { perror( "fork" ); exit(1); }
	else if ( pid == 0 )
		{
		WAIT_PARENT();		 // lets parent go first
		one_character_at_a_time( "output from child process \n" );
		TELL_PARENT( getppid() );  // ok parent can exit
		}
	else	{
		one_character_at_a_time( "output from parent process \n" );
		TELL_CHILD( pid );	// tells child to go now
		WAIT_CHILD();		// but lets child finish
		}
	exit(0);
}