Question

Your assignment is to write a program ‘hw3.c’, which uses multiple threads to improve the performance (save time). An easy approach is splitting the array into pieces and each piece is handled by different thread. Following is the example of how to compile and run ‘hw3.c’. Don’t mind the warning message. Note that each thread is working on different parts of the array.

#define BUFFER_SIZE 4000000
int countPrime=0;
int numbers[BUFFER_SIZE];

int isPrime(int n)
{
   int i;
   for(i=2;i<n;i++)
       if (n%i==0)
           return 0;
   return 1;
}
int main()
{
   int i;

   // fill the buffer
   for(i=0;i<BUFFER_SIZE;i++)
       numbers[i] = (i+100)%100000;
  
   // Computation
   for(i=0;i<BUFFER_SIZE;i++)
       if (isPrime(numbers[i]))
           countPrime++;

   printf("count of prime numbers = %d\n",countPrime);
}

Answer 1

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

#define MIN_USER_INPUT 2
#define BUFFER_SIZE 4000000

void *getUserInput(){
    int *number_from_user=(int *)malloc(sizeof(int));

    puts("Enter a number to print all prime numbers in range [0 - number]");

    while(1){
        printf("Input : ");
        scanf("%d",number_from_user);

        if(*number_from_user<MIN_USER_INPUT || *number_from_user>BUFFER_SIZE){
            printf("Please Enter a positive number from %d to %d\n",MIN_USER_INPUT,BUFFER_SIZE);
            ///rewinds the input buffer (if user gives character type input it prevents infinite while loop
            rewind(stdin);
        }else break;
    }
    puts("Input thread received user input.");
    ///writing number_from_user to the second argument (a generic pointer) passed in thread join call
    pthread_exit(number_from_user);
}
int checkPrime(int number){
    if(number==1) return 0; ///1 is not prime
    int i;
     ///(int) casting on the next line is not necessary.
     ///just making sure that it does not cause a bug with other compilers
     /// for a different architecture in which int division might return floating point number
    for(i=2; i<=(int)(number/2); i++)
        if(!(number%i)) return 0; ///Given number is not prime.
    ///Control reached here which means given number is prime
    return 1;
}
void *printPrimeUntil(void *argument){
    int number = *((int *)argument),i,count=0;

    printf("Thread two received : %d\n",number);

    for(i=2; i<=number; i++){
        if(checkPrime(i)){
            printf("%5d",i);
            if(!(++count%5))       ///print 5 prime number on a single line then move to next line
                printf("\n");
        }
    }
    pthread_exit(NULL);
}
int main(){
    pthread_t input_thread, output_thread;
    void *ret_number;

    //first thread
    if(pthread_create(&input_thread,NULL,getUserInput,NULL))
        return -1*printf("Error in thread creation!!!\n");///return a negative integer

    pthread_join(input_thread,&ret_number);

    int *number = (int *)ret_number;

    //second thread
    if(pthread_create(&output_thread,NULL,&printPrimeUntil,(void *)number))
       return -1*printf("Error in thread creation!!!\n");///return a negative integer

    return pthread_join(output_thread,NULL); ///wait for thread to finish
}