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

int flag_SIGUSR1 = 0;
int flag_SIGUSR2 = 0;

void signal_handler(int signum)
{
	switch(signum)
	{
	case SIGUSR1:
		flag_SIGUSR1 = 1;
		flag_SIGUSR2 = 0;
		break;
	case SIGUSR2:
		flag_SIGUSR1 = 0;
		flag_SIGUSR2 = 1;
		break;
	/** other signals are ignored? */
	}
}

int main(int argc, char *argv[])
{
	pid_t child;
	if(signal(SIGUSR1,signal_handler)==SIG_ERR)
	{
		printf("Cannot set handler for SIGUSR1\n");
		exit(1);
	}
	if(signal(SIGUSR2,signal_handler)==SIG_ERR)
	{
		printf("Cannot set handler for SIGUSR2\n");
		exit(1);
	}
	if((child=fork())==0)
	{
		/** child stuff - wait for parent */
		printf("[Child-%d] Waiting for parent... ",getpid());
		while(!flag_SIGUSR1); /* wait for parent signal */
		printf("YES! (flag1=%d)(flag2=%d)\n",flag_SIGUSR1,flag_SIGUSR2);
		/** child stuff - send ack to parent */
		printf("[Child-%d] Sending acknowledgement to parent.\n",getpid());
		kill(getppid(),SIGUSR1);
		/** child stuff - wait for parent */
		printf("[Child-%d] Waiting for parent... ",getpid());
		while(!flag_SIGUSR2); /* wait for parent signal */
		printf("YES! (flag1=%d)(flag2=%d)\n",flag_SIGUSR1,flag_SIGUSR2);
		/** child stuff - send ack to parent */
		printf("[Child-%d] Sending acknowledgement to parent.\n",getpid());
		kill(getppid(),SIGUSR2);
	}
	else
	{
		/** parent stuff - send 'command' to child */
		printf("[Parent-%d] Sending 'command' to child.\n",getpid());
		kill(child,SIGUSR1);
		/** parent stuff - wait for child */
		printf("[Parent-%d] Waiting for child response... ",getpid());
		while(!flag_SIGUSR1); /* wait for child signal */
		printf("YES! (flag1=%d)(flag2=%d)\n",flag_SIGUSR1,flag_SIGUSR2);
		/** parent stuff - send 'command' to child */
		printf("[Parent-%d] Sending 'command' to child.\n",getpid());
		kill(child,SIGUSR2);
		/** parent stuff - wait for child */
		printf("[Parent-%d] Waiting for child response... ",getpid());
		while(!flag_SIGUSR2); /* wait for child signal */
		printf("YES! (flag1=%d)(flag2=%d)\n",flag_SIGUSR1,flag_SIGUSR2);
	}
	printf("[%d] Done\n",getpid());
	return 0;
}