// PrimesThreads.c

// threads-based program to find the number of primes between 2 and n;
// uses the Sieve of Eratosthenes, deleting all multiples of 2, all
// multiples of 3, all multiples of 5, etc. 

// for illustration purposes only; NOT claimed to be efficient

// Linux compilation:  gcc -g -o primesthreads PrimesThreads.c -lpthread -lm

// usage:  primesthreads n num_threads

#include <stdio.h>
#include <math.h>
#include <pthread.h>  // required for threads usage

#define MAX_N 100000000
#define MAX_THREADS 25

extern void *worker(int tn);

// shared variables
int nthreads,  // number of threads (not counting main())
    n,  // range to check for primeness
    prime[MAX_N+1],  // in the end, prime[i] = 1 if i prime, else 0
    nextbase;  // next sieve multiplier to be used
// lock for the shared variable nextbase
pthread_mutex_t nextbaselock = PTHREAD_MUTEX_INITIALIZER;
// ID structs for the threads
pthread_t id[MAX_THREADS];

main(int argc, char **argv)
{  int nprimes,  // number of primes found 
       i,work;
   n = atoi(argv[1]);
   nthreads = atoi(argv[2]);
   // mark all even numbers nonprime, and the rest "prime until
   // shown otherwise"
   for (i = 3; i <= n; i++)  {
      if (i%2 == 0) prime[i] = 0;
      else prime[i] = 1;
   }
   nextbase = 3;
   // get threads started
   for (i = 0; i < nthreads; i++)  {
      // this call says create a thread, record its ID in the array
      // id, and get the thread started executing the function worker(), 
      // passing the argument i to that function
      pthread_create(&id[i],NULL,worker,i);
   }

   // wait for all done
   for (i = 0; i < nthreads; i++)  {
      // this call says wait until thread number id[i] finishes
      // execution, and to assign the return value of that thread to our
      // local variable work here
      pthread_join(id[i],&work);
      printf("%d values of base done\n",work);
   }
   
   // report results
   nprimes = 1;
   for (i = 3; i <= n; i++)
      if (prime[i])  {
         nprimes++;
      }
   printf("the number of primes found was %d\n",nprimes);

}