Pi calculator with mutex Synchronization - c

Question

Finishing up an assignment here. Got the code to work and calculate pie except for random values where I receive the following errors:
./piesync 10 3
pi computed with 10 terms in 3 threads is 3.14183961892940200045
* Error in `./piesync': free(): invalid next size (fast): 0x0000000001ca3010 *
./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