Question

Write a multithreaded C program that outputs prime numbers. This program should work as follows: the user will run the program and will enter a number on the command line. The program will then create a separate thread that outputs all the prime numbers less than or equal to the number entered by the user. ADD  "comments" to the source code please and a screenshot of the output with steps on how to compile

Answer 1

Compiling instruction:

gcc file.c -o output.out -lpthread

./output.out

Code with comments

#include <stdio.h>

#include <pthread.h>

int N = 100;  //number of promes to be generated

int prime_arr[100000] = {0};  //prime arrray

void *printprime(void *ptr)  //thread function

{

    int j, flag;

    int i = (int)(long long int)ptr;  //getting thread number

    //for thread 0, we check for all primes 0,4,8,12

    //for thread 1, we check for all primes 1,5,9,13

    while (i < N) {  //while number in range

        flag = 0;  //check if i has factor

        for (j = 2; j <= i / 2; j++)  //factor can be at max i/2 value

        {

            if (i % j == 0)  //factor found

            {

                flag = 1;

                break;

            }

        }

        if (flag == 0 && (i > 1))  //prime found, no factor

        {

            prime_arr[i] = 1;

        }

        i += 4;  //increase by interval of 4

    }

}

int main()

{

    printf("Enter N: ");

    scanf("%d", &N);  //input N

    pthread_t tid[4] = {{0}};  //create an array of 4 threads

    int count = 0;

    for (count = 0; count < 4; count++)  //initialize threads and start

    {

        printf("\r\n CREATING THREADS %d", count);

        pthread_create(&tid[count], NULL, printprime, (void *)count);  //count is passed as argument, target = printprime

    }

    printf("\n");

    for (count = 0; count < 4; count++)

    {

        pthread_join(tid[count], NULL);  //while all thread havent finished

    }

    int c = 0;

    for (count = 0; count < N; count++)  //print primes

        if (prime_arr[count] == 1)

            printf("%d ", count);

    printf("\n");

   return 0;

}