Producer Consumer stuck in deadlock with consumer - c

Question

I'm trying to implement the producer consumer with conditional variables so I can learn about synchronization. I'm using the github to guide me and solved some seg faults but now it seems that my consumer never gets executed or is stuck in deadlock. I'm not sure what could be the error. I have included printf statements in the producer to execute everytime it runs and it completes producing any string in messages.txt less than 5. The consumer though does not and is stuck in a deadlock.
#define max 5
int par = 0;
// Data structure for queue
struct queue
{
char *items; // array to store queue elements
int maxsize; // maximum capacity of the queue
int front; // front points to front element in the queue (if any)
int rear; // rear points to last element in the queue
int size; // current capacity of the queue
pthread_mutex_t mutex; // needed to add/remove data from the buffer
pthread_cond_t can_produce; // signaled when items are removed
pthread_cond_t can_consume; // signaled when items are added
};
// Utility function to initialize queue
struct queue* newQueue(int size)
{
struct queue *pt = NULL;
pt = (struct queue*)malloc(sizeof(struct queue));
pt->items = (char*)malloc(size * sizeof(char));
pt->maxsize = size;
pt->front = 0;
pt->rear = -1;
pt->size = 0;
pthread_mutex_init(&pt->mutex, NULL);
pthread_cond_init(&pt->can_produce, NULL);
pthread_cond_init(&pt->can_consume, NULL);
return pt;
}
// Utility function to return the size of the queue
int size(struct queue *pt)
{
return pt->size;
}
// Utility function to check if the queue is empty or not
int isEmpty(struct queue *pt)
{
return !size(pt);
}
// Utility function to return front element in queue
char front(struct queue *pt)
{
if (isEmpty(pt))
{
//printf("UnderFlow\nProgram Terminated\n");
}
return pt->items[pt->front];
}
// Utility function to add an element x in the queue
void enqueue(struct queue *pt, char x)
{
if (size(pt) == pt->maxsize)
{
//printf("OverFlow\nProgram Terminated\n");
}
//printf("Inserting %c\t", x);
pt->rear = (pt->rear + 1) % pt->maxsize; // circular queue
pt->items[pt->rear] = x;
pt->size++;
//printf("front = %c, rear = %c\n", pt->front, pt->rear);
}
// Utility function to remove element from the queue
void dequeue(struct queue *pt)
{
if (isEmpty(pt)) // front == rear
{
//printf("UnderFlow\nProgram Terminated\n");
}
//printf("Removing %c\t", front(pt));
pt->front = (pt->front + 1) % pt->maxsize; // circular queue
pt->size--;
//printf("front = %d, rear = %c\n", pt->front, pt->rear);
}
void consumer_f(void *arg)
{
struct queue *pt = (struct queue*)arg;
while (par==0 && !isEmpty(pt))
{
pthread_mutex_lock(&pt->mutex);
if (pt->size == 0)
{ // empty
// wait for new items to be appended to the buffer
pthread_cond_wait(&pt->can_consume, &pt->mutex);
}
printf("%c", pt->front);
dequeue(pt);
pthread_cond_signal(&pt->can_produce);
pthread_mutex_unlock(&pt->mutex);
}
}
void producer_f(void *arg)
{
struct queue *pt = (struct queue*)arg;
char tmp;
FILE *fp;
fp = fopen("messages.txt", "r");
if (fp == NULL)
{
//fprintf(stderr, "error opening messages.txt");
return -1;
}
while ((tmp = fgetc(fp)) != EOF)
{
pthread_mutex_lock(&pt->mutex);
if (pt->size == max)
pthread_cond_wait(&pt->can_produce, &pt->mutex);
enqueue(pt, tmp);
printf("sent");
pthread_cond_signal(&pt->can_consume);
pthread_mutex_unlock(&pt->mutex);
}
par = 1; //denotes EOF for consumer */
}
int main()
{
printf("nop");
struct queue *pt = newQueue(5);
pthread_t producer;
pthread_t consumer;
printf("got here");
if (pthread_create(&producer, NULL, &producer_f, pt))
{
//fprintf(stderr, "Error creating producer thread\n");
return -1;
}
if (pthread_create(&consumer, NULL, &consumer_f, pt))
{
//fprintf(stderr, "Error creating consumer thread\n");
return -1;
}
if (pthread_join(producer_f, NULL))
{
//fprintf(stderr, "Error joining proucer thread\n");
return -1;
}
if (pthread_join(consumer_f, NULL))
{
//fprintf(stderr, "Error joinging consumer thread\n");
return -1;
}
return 0;
}

Answer 1

The consumer thread is not going to deadlock state but it is exiting without consuming as EOS is reached before consumer started consuming.
As you know, threads can get scheduled in a random way by OS (I mean, at least you can assume that they get scheduled in a random way). With this assumption, the producer might have started and read all bytes and enabled eos flag i.e. par=1. If consumer thread starts after par=1, it is not consuming at all.
To handle this case, you need to update the consumer_f() function.
while (1) //Run the loop always
{
pthread_mutex_lock(&pt->mutex);
if ((pt->size == 0) && (par == 0)) //Make sure that EOS is not already reached.
{ // empty
// wait for new items to be appended to the buffer
pthread_cond_wait(&pt->can_consume, &pt->mutex);
}
if(par && isEmpty(pt))
{
//If Array is empty and eos is reached, unlock and exit loop
pthread_mutex_lock(&pt->mutex);
break;
}
//Other code
}
Similarly, you need to enable eos flag inside mutex in producer_f().
while ((tmp = fgetc(fp)) != EOF)
{
//Your code
}
pthread_mutex_lock(&pt->mutex);
par = 1;
pthread_cond_signal(&pt->can_consume); //To make sure that consumer thread wakesup
pthread_mutex_unlock(&pt->mutex);
PS: pt->size == 0 can be replaced with isEmpty(pt) for more readability.