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

#define PARENT_FLAG1 "PFLAG1"
#define PARENT_FLAG2 "PFLAG2"
#define CHILD_FLAG1 "CFLAG1"
#define CHILD_FLAG2 "CFLAG2"

void wait_flag(char *pname)
{
	FILE *pfile = 0x0;
	while(!pfile) pfile = fopen(pname,"r");
	fclose(pfile);
}

void send_flag(char *pname)
{
	FILE *pfile = fopen(pname,"w");
	if(pfile) fclose(pfile);
}

void hide_flag(char *pname)
{
	remove(pname); /* actually deletes the flag file! */
}

int main(int argc, char *argv[])
{
	pid_t child;
	if((child=fork())==0)
	{
		/** child stuff - wait for parent */
		printf("[Child-%d] Waiting for parent... ",getpid());
		wait_flag(PARENT_FLAG1); /* wait for parent signal */
		printf("YES!\n");
		/** child stuff - send ack to parent */
		printf("[Child-%d] Sending acknowledgement to parent.\n",getpid());
		send_flag(CHILD_FLAG1);
		/** child stuff - wait for parent */
		printf("[Child-%d] Waiting for parent... ",getpid());
		wait_flag(PARENT_FLAG2); /* wait for parent signal */
		printf("YES!\n");
		/** child stuff - send ack to parent */
		printf("[Child-%d] Sending acknowledgement to parent.\n",getpid());
		send_flag(CHILD_FLAG2);
	}
	else
	{
		/** parent stuff - send 'command' to child */
		printf("[Parent-%d] Sending 'command' to child.\n",getpid());
		send_flag(PARENT_FLAG1);
		/** parent stuff - wait for child */
		printf("[Parent-%d] Waiting for child response... ",getpid());
		wait_flag(CHILD_FLAG1); /* wait for child signal */
		printf("YES!\n");
		/** parent stuff - cleanup! */
		hide_flag(PARENT_FLAG1);
		hide_flag(CHILD_FLAG1); /** should be done by child? */
		/** parent stuff - send 'command' to child */
		printf("[Parent-%d] Sending 'command' to child.\n",getpid());
		send_flag(PARENT_FLAG2);
		/** parent stuff - wait for child */
		printf("[Parent-%d] Waiting for child response... ",getpid());
		wait_flag(CHILD_FLAG2); /* wait for child signal */
		printf("YES!\n");
		/** parent stuff - cleanup! */
		hide_flag(PARENT_FLAG2);
		hide_flag(CHILD_FLAG2); /** should be done by child? */
	}
	printf("[%d] Done\n",getpid());
	return 0;
}