CODEWITHSUNDEEP
c++multithreading
Callat
Callat

Reputation: 3044

How to make a thread contain a loop using mutexes to control flow?

I am working on a homework assignment and I've hit a wall. The assignment is to use 2 threads with controlling mutex to reduce a number x over a loop of 5 iterations.

The first thread does x=x-5
The second thread does x = x/5

The proper result should reduce x to 5 from 19530 over 5 iterations alternating between thread1 and thread2 at each iteration.

I have the following result as of now:

Thread1: x = 19525
Thread1: x = 19520
Thread1: x = 19515
Thread1: x = 19510
Thread1: x = 19505


From above it is clear that my second thread is not only not doing it's job but it's not doing anything at all.

Below is my code, it is written in C++ but the style is using the tools we learned in class which is how one would do it in C but it should work the same either way. 

#include<iostream>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
using namespace std;

void *first(void *);
void *second(void *);

pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

int x = 19530; // global var that is being manipulated
int i = 1; // counter for the loops

int main() {
    int t1, t2;
    pthread_t thread1, thread2;

    if((t1 = pthread_create( &thread1, NULL, first, NULL))) {
        printf("Thread creation failed: %d\n", t2);
    }

    if((t2 = pthread_create( &thread2, NULL, second, NULL))) {
        printf("Thread creation failed: %d\n", t2);
    }

    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);
    return(0);
}

void *first(void *){ // function for thread1
  for(; i <=5; i++){
  pthread_mutex_lock(&mymutex);
  x = x-5;
  cout << "Thread1: x = " << x << endl;
  pthread_mutex_unlock(&mymutex);
  }
}

void *second(void *){ // function for thread2
  for(; i<=5; i++){
  pthread_mutex_lock(&mymutex);
  x = x/5;
  cout << "Thread2: x = " << x << endl;
  pthread_mutex_unlock(&mymutex);
  }
}

Note I am brand new to the concept threads and mutex. And I'd prefer to stick to the way I have learned in class which I believe is called "the C way".

Upvotes: 0

Views: 469

Answers (1)

Callat
Callat

Reputation: 3044

Thanks to rclgdr who commented above I was able to answer my own question the following code works as intended:

#include<iostream>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
using namespace std;

void *first(void *);
void *second(void *);

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER; // thread1 mutex
pthread_mutex_t mutex2 = PTHREAD_MUTEX_INITIALIZER; // thread2 mutex

int x = 19530; // global var that is being manipulated

int main() {
    cout << "x = " << x << endl << endl;
    int t1, t2;
    pthread_t thread1, thread2;
    pthread_mutex_lock(&mutex2);
    if((t1 = pthread_create( &thread1, NULL, first, NULL))) {
        printf("Thread creation failed: %d\n", t2);
    }

    if((t2 = pthread_create( &thread2, NULL, second, NULL))) {
        printf("Thread creation failed: %d\n", t2);
    }

    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);
    return(0);
  }

void *first(void *){ // function for thread1
  for(int i = 1; i <=5; i++){
  pthread_mutex_lock(&mutex1);
  x = x-5;
  cout << "Iteration " << i <<endl;
  cout << "Thread1: x = " << x << endl;
  pthread_mutex_unlock(&mutex2);
  }
}

void *second(void *){ // function for thread2
  for(int i = 1; i<=5; i++){
  pthread_mutex_lock(&mutex2);
  x = x/5;
  cout << "Thread2: x = " << x << endl << endl;
  pthread_mutex_unlock(&mutex1);
  }
}

Upvotes: 1

Related Questions