#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>


int semID=0;
struct sembuf sopList[3];

int mutex_init(){
	/* free state */
	sopList[0].sem_num=0;
	sopList[0].sem_op=0;
	sopList[0].sem_flg=SEM_UNDO|IPC_NOWAIT;

	/* lock it */
	sopList[1].sem_num=0;
	sopList[1].sem_op=1;
	sopList[1].sem_flg=SEM_UNDO|IPC_NOWAIT; // Non blocking

	/* unlock it */
	sopList[2].sem_num=0;
	sopList[2].sem_op=-1;
	sopList[2].sem_flg=SEM_UNDO|IPC_NOWAIT;

	if((semID=semget(IPC_PRIVATE,1,IPC_CREAT|0666))<0){
		perror("mutex_init :: ");
		return 1;
	}

	return 0;
}
/* NOTE: I am not queuing the sem ops, because i dont want my processes blocked 
 * hence causing a potential race, but this condition has been handled
 * */
int mutex_lock(){
	if(semop(semID,&sopList[0],1)!=-1){
#ifdef DEBUG
		printf("mutex_lock :: process %i found it unlocked\n",getpid());
#endif
		if(semop(semID,&sopList[1],1)!=-1){ // perform the lock
#ifdef DEBUG
			printf("mutex_lock :: process %i locked it\n",getpid());
#endif
			return 0;
		}
	}
#ifdef DEBUG
	printf("mutex_lock %i :: return 1\n",getpid());
#endif
	return 1;
}

/* RETURN value: 0 if unlocked, else 1 */
int mutex_unlock(){
	return(semop(semID,&sopList[2],1)); // perform the unlock
}

int main(void){
	pid_t pid;
	int stat;
	mutex_init();
	double slept=0;	

	if((pid=fork())<0)return 1;
		if(pid==0)printf("Child %i created\n",getpid());
	

	for(slept=0;slept<1;slept+=0.1){
			if(!(stat=mutex_lock()))break;
			sleep(0.1);
	}
	if(stat){
		printf("process %i, tried locking for 1 second but failed \n",getpid());
	}else{
		printf("process %i, locked it, and is sleeping\n",getpid());
		sleep(2);
		for(slept=0;slept<1;slept+=0.1){
			if(!(stat=mutex_unlock()))break;
			sleep(0.1);
		}	
		if(stat){
			printf("process %i, tried to UNlock for 1 second but failed \n",getpid());
		}else{
			printf("process %i, UNlocked it,\n",getpid());
		}
	}
	if(pid>0){
		printf("Parent is done and is waiting for child to die \n");
		wait(&stat);
	}
	printf("process %i, has finished \n",getpid());

	return 0;
}