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

#define NUMTHREADS 4
#define  N 3

double *a, *b;

void *dot(void *arg)
{

   int  start, end  ;
   long offset, i;
   double  *mysum = malloc(sizeof(double));
   *mysum=0.0;
   offset = (long)arg;

   start = offset*N;
   end   = start + N;

   for (i=start; i<end ; i++)
    {
      *mysum += (a[i] * b[i]);
    }

   printf("ja   wyliczylem %d, %d,  %f \n", start,  end, *mysum);
     pthread_exit((void *) mysum);
}

int main (int argc, char *argv[])
{
pthread_t   watki[NUMTHREADS];

long i;
double   iloczyn=0.0;
double *loc_iloczyn;
pthread_attr_t  atr;
a= (double*) malloc (NUMTHREADS*N*sizeof(double));
b= (double*) malloc (NUMTHREADS*N*sizeof(double));

for (i=0; i<N*NUMTHREADS; i++) {
  a[i]=1.0;
  b[i]=2.0*a[i];
  }

pthread_attr_init (&atr);
pthread_attr_setdetachstate( &atr, PTHREAD_CREATE_JOINABLE);

for(i=0; i<NUMTHREADS; i++)
  {
     pthread_create(&(watki[i]), NULL, dot, (void *)i);
   }


for(i=0; i<NUMTHREADS; i++) {
  pthread_join(watki[i], (void **)&loc_iloczyn);
  iloczyn = iloczyn + *loc_iloczyn;
}
pthread_attr_destroy(&atr);
printf ("iloczyn =  %f \n", iloczyn);
free(a);
free(b);
free(loc_iloczyn);
pthread_exit(NULL);
}