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09.24.2008 at 11:50AM PDT, ID: 23759904
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7.5

pthreads condition variable problem

Asked by Novice_ in C Programming Language, Linux, Linux Programming

Tags:

I am trying simulate multiple requests that are handled by threads . When a request is created it is pushed in a queue . The main on inserting value into queue signals the condition wait variable for threads to pick up data from queue and execute. The thread in turn prints that data .

Now the code i have written works fine 50% of the time but many time the threads are not picking up task even after signaling the condition varaible and at time two thread are picking up same data from queue .


Desired output
INserting 1
Socket serviced 1
INserting 2
Socket serviced 2
INserting 3
Socket serviced 3
INserting 4
Socket serviced 4
..........

Output that is wrong:
INserting 1
INserting 2
INserting 3
INserting 4
INserting 5

in this case the program exits post insertion and no thread executes inspite of cond var being signaled each time
 or

INserting 1
Socket serviced 1
INserting 2
Socket serviced 2
INserting 3
Socket serviced 3
Socket serviced 3


I am not able to identify what is wrongStart Free Trial 
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#include <sys/types.h>
#include <string.h>
#include <unistd.h>
#include <stdio.h>
#include<pthread.h>
#include<stdlib.h>
 
typedef struct queue_struct
{
	int head,tail;
	int IsEmpty;
	int data[30];
		
}queue;
 
#define Queue_Size 30
pthread_mutex_t mutex =  PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condPool = PTHREAD_COND_INITIALIZER;
queue q;
 
void queue_init(queue *q)
{
	q->head=-1;
	q->tail=-1;
	q->IsEmpty=1;	
}
 
void queue_insert(queue *q,int element)
{
	if(q->head==(q->tail+1)%Queue_Size)
	printf("Queue size exceeded\n");
	else
	{
		q->tail=(q->tail+1)%Queue_Size;
		q->data[q->tail]=element;
		if (q->head == -1 )
		{
			 q->head = 0;
			 q->IsEmpty=0;
		}
	}		
}
 
int queue_fetch(queue *q)
{
	int temp=-1;
	
	if((q->head==q->tail) && (q->tail==-1))
	{
		printf("Queue empty\n");
	}
	else
	{
		temp= q->data[q->head];
	
		if(q->tail==q->head)
			{
				q->head=q->tail=-1;
				q->IsEmpty=1;
			}
		else
		q->head=(q->head+1)%Queue_Size;
 
	}
	return temp;
}
void EnterPool()
{
     int iData=0;
	while(1)
	{
		pthread_mutex_lock(&mutex);
		
		while(q.IsEmpty==1)
			pthread_cond_wait(&condPool,&mutex);
		iData=queue_fetch(&q);
		pthread_mutex_unlock(&mutex);
			
		printf("Socket serviced %d \n",iData);	
	}	
}
 
int main(int argc, char* argv[])
{
	    pthread_t WorkerPool[5];
	     int i;
   	//Initialize pool queue	
		queue_init(&q);
		
		//Create thread pool
		for(i=0;i<5;i++)
			pthread_create(&WorkerPool[i],NULL,(void *) EnterPool,NULL);
int x=0;
    while(1)
    {
        x++;
        if(x>25)
        exit(0);
        
        pthread_mutex_lock(&mutex);
        printf("INserting %d \n",x);
		queue_insert(&q,x);	
		pthread_cond_signal(&condPool);
		pthread_mutex_unlock(&mutex)	;				
    }   
}
[+][-]09.24.2008 at 03:11PM PDT, ID: 22564538

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About this solution

Zones: C Programming Language, Linux, Linux Programming
Tags: C
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Solution Provided By: sweetfa2
Participating Experts: 3
Solution Grade: B
 
 
[+][-]09.24.2008 at 03:14PM PDT, ID: 22564563

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[+][-]09.24.2008 at 03:18PM PDT, ID: 22564598

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[+][-]09.24.2008 at 08:55PM PDT, ID: 22566148

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[+][-]10.04.2008 at 07:10AM PDT, ID: 22640906

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