Question

I have to make a program which has a shared variable (counter with initial value = 35) and 5 threads. I have to make the program so that each thread accesses the value of the counter and decrements it by 1. This should continue until counter = 0.

This is what I have for the code so far, but the problem is that only one thread is decrementing the value of counter to 0.

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <pthread.h>

#define NTHREADS 5
void *thread_function1(void *);
void *thread_function2(void *);
void *thread_function3(void *);
void *thread_function4(void *);
void *thread_function5(void *);
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
short int  counter = 35;


main()
{
   pthread_t thread_id[NTHREADS];
   pthread_t thread1, thread2, thread3, thread4, thread5;
int status, status2, status3, status4, status5;

status = pthread_create(&thread1, NULL, thread_function1, NULL);
if(status!=0){
    fprintf(stderr,"thread 1 failed\n");    
}

status2 = pthread_create(&thread2, NULL, thread_function2, NULL);
if(status2!=0){
    fprintf(stderr,"thread 2 failed\n");    
}

status3 = pthread_create(&thread3, NULL, thread_function3, NULL);
if(status3!=0){
    fprintf(stderr,"thread 3 failed\n");    
}

status4 = pthread_create(&thread4, NULL, thread_function4, NULL);
if(status4!=0){
    printf("thread 4 failed\n");    
}

status5 = pthread_create(&thread5, NULL, thread_function5, NULL);
if(status5!=0){
    fprintf(stderr,"thread 5 failed\n");    
}

//pthread_join(thread1, NULL);
//int x = counter;

printf("created all the threads \n");

printf("joining thread 1");
    pthread_join(thread1, NULL);
printf("joining thread 2");
    pthread_join(thread2, NULL);
printf("joining thread 3");
    pthread_join(thread3, NULL);
printf("joining thread 4");
    pthread_join(thread4, NULL);
printf("joining thread 5");
    pthread_join(thread5, NULL);                          

   printf("Final counter value: %d\n", counter);
}

void *thread_function1(void *dummyPtr)
{
   printf("Thread number %ld\n", pthread_self());   

while(counter>0){
   srand(time(NULL));

   int r = rand()%3;
  printf(" %d\n", r);
   pthread_mutex_lock( &mutex1 );
  printf("entered the mutex");
   counter--;
   printf(" %d\n", counter);
   sleep(r);
   pthread_mutex_unlock( &mutex1 );
   printf("mutex unlocked");
   pthread_yield();
}

}

void *thread_function2(void *dummyPtr)
{
   printf("Thread number %ld\n", pthread_self());   

while(counter>0){
   srand(time(NULL));

   int r = rand()%3;
  printf(" %d\n", r);
   pthread_mutex_lock( &mutex1 );
  printf("entered the mutex");
   counter--;
   printf(" %d\n", counter);
   sleep(r);
   pthread_mutex_unlock( &mutex1 );
   pthread_yield();
}

}

void *thread_function3(void *dummyPtr)
{
   printf("Thread number %ld\n", pthread_self());   

while(counter>0){
   srand(time(NULL));
   int r = rand()%3;
  printf(" %d\n", r);
   pthread_mutex_lock( &mutex1 );
  printf("entered the mutex");
   counter--;
   printf(" %d\n", counter);
   sleep(r);
   pthread_mutex_unlock( &mutex1 );
   pthread_yield();
}

}

void *thread_function4(void *dummyPtr)
{
   printf("Thread number %ld\n", pthread_self());   

while(counter>0){
   srand(time(NULL));

   int r = rand()%3;
  printf(" %d\n", r);
   pthread_mutex_lock( &mutex1 );
  printf("entered the mutex");
   counter--;
   printf(" %d\n", counter);
   sleep(r);
   pthread_mutex_unlock( &mutex1 );
   pthread_yield();
}

}

void *thread_function5(void *dummyPtr)
{
   printf("Thread number %ld\n", pthread_self());   

while(counter>0){
   srand(time(NULL));

   int r = rand()%3;
   printf(" %d\n", r);
   pthread_mutex_lock( &mutex1 );
   printf("entered the mutex");
   counter--;
   printf(" %d\n", counter);
   sleep(r);
   pthread_mutex_unlock( &mutex1 );
   pthread_yield();
}

}

Can anyone please help ?
Thanks

Answer 1

int r = rand()%3;
/* ... */
sleep(rand);

r is a random number between 0 and 2, but you sleep rand seconds, which is the address of a function, which will be implicitly casted to an unsigned int – so the thread will sleep for a very very long time. Use sleep(r); instead.

Also, note that you read counter while not holding the mutex (in while(counter > 0)), which may lead to the program working incorrectly depending on architecture, caching and compiler optimizations. You should lock the mutex, read the value of counter to a local variable, then unlock the mutex and check whether the value is positive.