Calculating pi in pthreads - c

Question

I am trying to calculate pi using the bpp method but my result keeps coming up at 0.The whole idea is for each thread to compute a part of it and the sum of each thread gets summed up using the join method
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <time.h>
#define NUM_THREADS 20
void *pi_function(void *p);//returns the value of pi
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER; //creates a mutex variable
double pi=0,p16=1;int k=0;
double sumvalue=0,sum=0;
main()
{
pthread_t threads[NUM_THREADS]; //creates the number of threads NUM_THREADS
int iret1; //used to ensure that threads are created properly
//pthread_create(thread,attr,start_routine,arg)
int i;
pthread_mutex_init(&mutex1, NULL);
for(i=0;i<NUM_THREADS;i++){
iret1= pthread_create(&threads[i],NULL,pie_function,(void *) i);
if(iret1){
printf("ERROR; return code from pthread_create() is %d\n", iret1);
exit(-1);
}
}
for(i=0;i<NUM_THREADS;i++){
iret1=pthread_join(threads[i],&sumvalue);
if(iret1){
printf("ERROR; return code from pthread_create() is %d\n", iret1);
exit(-1);
}
pi=pi+sumvalue; //my result here keeps returning 0
}
pthread_mutex_destroy(&mutex1);
printf("Main: program completed. Exiting.\n");
printf("The value of pi is : %f\n",pi);
exit(0);
}
void *pie_function(void * p){
int rc;
int k=(int)p;
sumvalue += 1.0/p16 * (4.0/(8* k + 1) - 2.0/(8*k + 4)
- 1.0/(8*k + 5) - 1.0/(8*k+6));
pthread_mutex_lock( &mutex1 ); //locks the share variable pi and p16
p16 *=16;
rc=pthread_mutex_unlock( &mutex1 );
if(rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
}
pthread_exit(&sumvalue);
}

Answer 1

For your purpose you don't need to have a mutex or other complicated structure. Just have every thread compute on its own local variables. Provide to each thread the address of a double where he receives his k and may return the result, in the same way as you already separate the ptread_t variables for each thread.

Answer 2

To avoid getting 0 as the output. Put pi=pi+sumvalue inside the join for loop.
since pi=pi+sumvalue is executed only once when sumvalue is very small and pi=0. pi=0 is the output. Just put pi=pi+sumvalue in the join for loop.
To get consistent value of pi follow below mentioned details.
You have to make sure two thinks:-
Just make pi as an global variable and remove all global variables. limit the use of many global variable as updating them will become a critical section. Update pi as
pthread mutex lock(&mutex1);
pi += pi + my sum;
pthread mutex unlock(&mutex1);
you can also calculate sum_values as:
void pie_function(void rank) {
long my_rank = (long) rank;
//printf("%ld \n",my_rank);
double factor, sumvalue = 0.0;
long long i;
long long n=1000000;
long long my_n = n/thread_count;
long long my_first_i = my_n*my_rank;
long long my_last_i = my_first_i + my_n;
if (my_first_i % 2 == 0)
factor = 1.0;
else
factor = -1.0;
for (i = my_first_i; i < my_last_i; i++, factor = -factor)
sumvalue+= 4factor/(2i+1);
pthread_exit(&sumvalue);
}