Pi calculator with mutex Synchronization

Question

Finishing up an assignment here. Got the code to work and calculate pie except for random values where I receive the following errors:

1. ./piesync 10 3 pi computed with 10 terms in 3 threads is 3.14183961892940200045 * Error in `./piesync': free(): invalid next size (fast): 0x0000000001ca3010 *

2. ./piesync 100 5 * Error in `./piesync': double free or corruption (out): 0x0000000000ee5040 *

I know it might be something with the array or the mutex but cant figure out what.

Code:

//Pini Vaknine
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

//global variables
int N, T;
double gpie = 3.0;
pthread_mutex_t mutex;

//pie function
void* pie_runner(void* arg)
{
    long j = (long)arg;    
    long lower = (N/T)*(j-1)+1;
    long upper = ((N/T)*(j));
    double myPartialSum = 0;
    //printf("lower=%lu upper=%lu\n",lower , upper);

    for(long i = lower; i <= upper; i++)
    {        
        if(i % 2 == 0){
            myPartialSum -= 4.0/((2*i)*(2*i+1)*(2*i+2)); 
            //printf("vsum %lu = %f\n", j, vsum[j]);
                  }
        else{
            myPartialSum += 4.0/((2*i)*(2*i+1)*(2*i+2));
            //printf("vsum %lu = %f\n", j, vsum[j]);
                  }


    }
    pthread_mutex_lock (&mutex);
    gpie = gpie + myPartialSum;
    pthread_mutex_unlock (&mutex);    

    pthread_exit(0);
    //return NULL;
}

int main(int argc, char **argv)
{

    if(argc != 3) {
        printf("Error: Must send it 2 parameters, you sent %d\n", argc-1);
        exit(1);
    }
    N = atoi(argv[1]);
    T = atoi(argv[2]); 


    if(N <= T) {
        printf("Error: Number of terms must be greater then number of threads.\n");
        exit(1);    
    }



    //launch threads
    pthread_attr_t attr;
    pthread_t *tids = (pthread_t *) calloc(T, sizeof(pthread_t));
    if(tids == NULL) {
        fprintf(stderr, "Memory allocation problem\n");
        exit(1);
    }


        pthread_mutex_init(&mutex, NULL);
        pthread_attr_init(&attr);
        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

    for(long i = 1; i<=T; i++)
    {

        int r = pthread_create(&tids[i], &attr, pie_runner, (void*)i);
        if(r<0) {
            printf("ERROR: pthread_create() returned %d\n", r);
            exit(2);
        }
    }

    //wait for threads...
    for(int k = 1; k<=T; k++)
    {
        pthread_join(tids[k], NULL);
    }


    printf("pi computed with %d terms in %d threads is %.20f\n", N, T, gpie);

    pthread_mutex_destroy(&mutex);    
    pthread_attr_destroy(&attr);

    free(tids);

    return 0;
}

Answer 1

You are indexing out of range of an array. You have allocated an array for T elements here

pthread_t *tids = (pthread_t *) calloc(T, sizeof(pthread_t));

but you index it incorrectly, here

for(int k = 1; k<=T; k++)
    {
        pthread_join(tids[k], NULL);
    }

and other instances too. In C you index an array from 0 so the loop should be

for(int k=0; k<T; k++)

Answer 2

You have a division by zero in

if(i % 2 == 0){
  myPartialSum -= 4.0/((2*i)*(2*i+1)*(2*i+2));

for j=0.

Fixing this

for(long i = lower; i <= upper; i++)
{        
  if(i % 2 == 0){
    if ( ((2*i)*(2*i+1)*(2*i+2)) == 0)
      myPartialSum = 0.0;
    else 
      myPartialSum -= 4.0/((2*i)*(2*i+1)*(2*i+2)); 
  }
  else{
    myPartialSum += 4.0/((2*i)*(2*i+1)*(2*i+2));
  }
}

and changing the indices, the program works out of the box