pthreads/arrays Problem

[Jedel]

Hey guys, I have a bit of a head scratcher as far as this program goes right now.

The program is a simulation of a movie rental using pthreads. It starts by reading in a file, which sets all of the variables (worker threads, client threads, how many movies and of what, etc). I have the movies that are found in the init file put into a char * array, and printing them right after I store them yields the correct movie title found at the correct index, code snippet below from my init() function

int nMovies = 0;
...
while ( fgets ( line, sizeof line, file ) != NULL ) 
		{
                    ...(if movie found)
				movies[nMovies] = line;
				cMovies[nMovies] = line;
				printf("Title:%s",cMovies[nMovies]); //Prints out Correct Title
				nMovies++;
				...
		}

movies[] are the movies found on the server
cMovies[] are the movies that the client will try to rent (which is the same array with some extra dummie titles to simulate clients trying to rent an invalid movie).

This part works fine, However, this is where the weird part comes in: after reading the init file, I call to create the client threads:

void startClients(int clients ){
	clients = nClients;
	numClients[nClients]; //Global variable of type pthread_t
	int rc;
   	long t;

   	for(t=0; t<nClients; t++){

    	  	printf("In main: creating thread %ld\n", t);
    		rc = pthread_create(&numClients[t], NULL, clientThread, (void *)t);
      		
		if (rc){
        		printf("ERROR; return code from pthread_create() is %d\n", rc);
         	exit(-1);
      		}

		
   	}
}

which calls this function after threads are created:

void *clientThread(void *threadid){
	int tid;
   	tid = (int)threadid;
        //Number of steps (actions of clients) to simulate
	int steps = nSteps;
	int i;
	printf("ThreadID:%d\n",tid);
	int rv;
	
	for(i = 0; i < steps; i++){
		coin = 0;
		//wait until the lock is not taken
		while(true){
			rv = pthread_mutex_trylock( &clientMutex );
			if(rv == 0){
				printf("Working???");
				break;
			}

		};
		printf("Working???");
		if (coin == 0){
			randomCheckout(tid);
                        pthread_exit(NULL);
			break;
			
			
		}else{
			//randomReturn();
		}
		
		sleep(150);
	}
	pthread_mutex_unlock( &clientMutex );
	printf("\nInside of checkoutVideo \n");
	pthread_exit(NULL);

}

This is the first problem: When I run the program, it creates all of the threads perfectly fine, but only the first thread to get the lock will get into the bolded section (even though coin ALWAYS equals 0 right now), and the whole program will only go through 1 step. Am I not releasing the mutex lock correctly or in the wrong place? I tried moving it to after randomCheckout(tid); in the bolded section (instead of break), and ended up with segmentation faults after the first thread went through a step.

without pthread_exit(NULL); in the bolded section (just break), it gets about half way through the # of client threads and seg faults.

The next problem comes next, in the randomcheckout() method:

void randomCheckout(int tid){
	
        //Number of entries in the client's movie array
	int j = (sizeof cMovies)/4;	
	printf("\n%d\n", j);
        
        //Randomly pick a number from the array        
	int selection = rand() % j;
	printf("%d\n", selection); 


	int a;
	for(a = 0; a < j; a++){
		printf("\n%s",cMovies[a]);

	}

	printf("%s\n", cMovies[selection]);
	checkoutVideo(numClients[tid], cMovies[selection]);
}

In the bolded section (which is just debugging code cycling through the array), the output is just the number of entries in the array, but all the same movie title (the last one in the init file/stored into the array)...I'm not accessing the array anywhere in between populating it and here, so why are all of the values changing to the last entry??

Sorry for the bombardment of questions, I've been spending hours upon hours trying to fix these bugs and I just want to pull my hair out

If you need anything other parts of my code/more description of what's going on, just let me know.

Thanks for any input!!

Also for any Mods: I have an old account that I haven't accessed in a while and I guess it was deactivated to posting due to inactivity? Is there anyway to reactivate it?

[C_ntua]

void startClients(int clients ){
	clients = nClients;
	numClients[nClients]; //Global variable of type pthread_t
	int rc;
   	long t;

   	for(t=0; t<nClients; t++){

    	  	printf("In main: creating thread %ld\n", t);
    		rc = pthread_create(&numClients[t], NULL, clientThread, (void *)t);
      		
		if (rc){
        		printf("ERROR; return code from pthread_create() is %d\n", rc);
         	exit(-1);
      		}

		
   	}
}

Don't have time atm to look on the rest, but the red lines make no sense.
The first assigns the argument to something. Maybe you wanted

nClinets = clients;

The second does absolutely nothing!

[ZuK]

                   ...(if movie found)
				movies[nMovies] = line;
				cMovies[nMovies] = line;
				printf("Title:%s",cMovies[nMovies]); //Prints out Correct Title
				nMovies++;

This will set each movies and cMovies to the same value ( line ).
You have to copy the string
e.g.

                   ...(if movie found)
				cMmovies[nMovies] = malloc( strlen(line) +1);
                                strcpy(cMovies[nMovies], line);
				printf("Title:%s",cMovies[nMovies]); //Prints out Correct Title
				nMovies++;

and don't forget to free the strings

Kurt

[Jedel]

Thanks for the input guys!

yeah, my code is a bit messy from trying a whole bunch of things, so there probably are a few statements that don't do much

Anyway, now I'm stuck getting my pthreads to join...My code for the clients:

void startClients(int clients ){
	clients = nClients;
	numClients[nClients];
	int rc;
   	long t;

	pthread_attr_init(&attr);
  	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
   	for(t=0; t<nClients; t++){

    	  	//printf("In main: creating thread %ld\n", t);
    		rc = pthread_create(&numClients[t], &attr, clientThread, (void *)t);
      		
		if (rc){
        		printf("ERROR; return code from pthread_create() is %d\n", rc);
         	exit(-1);
      		}
	
   	}
}

void waitForClients( int clients){
	int rc;
   	int t;
	printf("HERE??\n");
	for(t=0; t< clients; t++) {
		printf("\nTHIS IS HERE");
      		rc = pthread_join(numClients[t], &attr);
      		if (rc) {
	 		printf("ERROR; return code from pthread_join() is %d\n", rc);
	 	exit(-1);
	 	}

		//pthread_exit(NULL);
      	printf("Main: completed join with thread %ld having a status of %ld\n",t,(long)status);
      	}
}

//Called when client threads are created
void *clientThread(void *threadid){
	int tid;
   	tid = (int)threadid;
	int steps = nSteps;
	int i;
	printf("ThreadID:%d\n",tid);
	int rv;
	int count = 0;
	

	for(i = 0; i < steps; i++){
		coin = rand() % 1;
		//wait until the lock is not taken
		while(true){
			rv = pthread_mutex_trylock( &c.clientMutex );
			if(rv == 0){
				count++;
				break;
			}
		}
		
		if (count < SIMULMOVES) {
			pthread_cond_wait(&c.empty, &c.clientMutex);
		     // pthread_cond_signal(&c.condVar);
		      printf("inc_count(): thread , count =   Threshold reached.\n");
		}

		if (coin == 0){
			//printf("HERE");
			randomCheckout(tid);
			//pthread_exit(NULL);
			pthread_mutex_unlock( &c.clientMutex );
			count = 0;
			//pthread_cond_signal(&c.empty);
			//break;
			
			
		}else{
			printf("SHOULD RETURN HERE");
			randomReturn(tid);
			count = 0;
			//pthread_cond_signal(&c.empty);
		}
		
		sleep(2000);
		
	}
	
	//pthread_mutex_unlock( &c.clientMutex );
	//printf("\nInside of checkoutVideo \n");
	pthread_exit(NULL);

}

my main so far is just:

int main(){
	init();
	startClients(nClients);
	waitForClients(nClients);
	

	return 0;
}

so startClients() is called, which creates the threads, then clientThread() is run, then waitForClients() is called to wait for all of the threads to finish.

What happens is that all of the threads will go through clientThread() once trying to rent a movie, then after that it goes to waitForClients()..The bolded printf statement is the last thing that outputs before it segfaults on me.

the thing that I can't figure out is that it is segfaulting even before the call to pthread_join (the statement printf("\nTHIS IS HERE"); never outputs, even if I get rid of the for loop...) which just downright makes no sense to me.

I have a feeling it has something to do with the way I'm trying to synchronize my threads (the bolded section of code is trying to use a condition variable to see if the lock is taken as I read cond'n variable and mutex's need to be used together, but I'm not quite sure of its implementation).

thanks!