Hello all
I am currently coding a starter multi threading program in c. It is supposed to find all the prime numbers from 2 to n. I have no previous experience in thread coding so I am looking for some help and maybe some debugging advise It only calculates primes right for numbers less than 100. I need it to find a larger range but sadly I really don't know any c threading. Any and all help will be greatly appriciated. Here is my code:

#include <unistd.h>

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

#include <pthread.h>

#include <sys/types.h>

#include <sys/signal.h>

#include <sys/socket.h>

#include <sys/time.h>

#include <sys/resource.h>

#include <sys/wait.h>

#include <sys/errno.h>

#include <netinet/in.h>

#define	INTERVAL	5	/* secs */

struct pthread


	pthread_mutex_t	mutex;

	unsigned int	prime;

	int startValue; 

	int endValue;

} shared_global;

void    CalculatePrimes();

int	errexit(const char *format, ...);

int main(int argc, char *argv[])


	pthread_t	th;

	pthread_attr_t	ta;

        int i; 


	shared_global.startValue = 2;

	shared_global.prime = 0;


	switch (argc) 


		case	1: shared_global.endValue =  0;


		case	2:shared_global.endValue = atoi(argv[1]);



			printf("don't do that!\n");



	(void) pthread_attr_init(&ta);

	(void) pthread_attr_setdetachstate(&ta, PTHREAD_CREATE_DETACHED);

	(void) pthread_mutex_init(&shared_global.mutex, 0);

	for(i = 5;i < 9;i=i+1)


		if (pthread_create(&th, &ta, (void * (*)(void *))CalculatePrimes,(void *) i) < 0)


			printf("pthread_create failed\n");



        	printf("Thread created\n");


	printf("There are %d primes.\n", shared_global.prime);



void CalculatePrimes()


	bool isPrime;


	while(shared_global.startValue < shared_global.endValue)


		isPrime = true;

		(void) pthread_mutex_lock(&shared_global.mutex);

		for(int j=2; j<shared_global.startValue; j= j+1)


			if (shared_global.startValue % j ==0)


				isPrime = false;





			printf("%d is a prime number.\n", shared_global.startValue);

			shared_global.prime= shared_global.prime +1;

			printf("this is a %d\n", shared_global.prime);


		shared_global.startValue = shared_global.startValue +1;

		(void) pthread_mutex_unlock(&shared_global.mutex);



	//printf("There are %d prime numbers", shared_global.prime);



*The algorithm used to find the prime numbers is supposed to be inefficent because it is used as a bench mark. Thank you.

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All 6 Replies

Can you please explain me what you actually want to do? I mean I know you are trying to find all the prime numbers but why are you starting 4 different threads?

Can you please explain me what you actually want to do? I mean I know you are trying to find all the prime numbers but why are you starting 4 different threads?

I am trying to create multiple threads to find prime numbers in a range. I want to create multiple threads to solve the problems. I really don't know how to handle this with threads. I would like to find out what I'm doing wrong in this creation of threads. I figured the four threads will go over the four processors I have to work with.

> void CalculatePrimes()
First off, this should be declared as void *CalculatePrimes(void *p) When you do this
if (pthread_create(&th, &ta, (void * (*)(void *))CalculatePrimes,(void *) i) < 0)
You can get the parameter i passed to the thread like this

void *CalculatePrimes(void *p) {
  int param = (int)p;

What exactly is your approach (describe it in words).
At the moment, you have several threads all doing the same thing.

At a guess, you're trying to create a thread for each number, so a given thread will test just ONE number (the parameter passed to it), and then update the shared data appropriately.

That is exactally what I am trying to do to some number n. I will pass it a value to a thread then recieve an answer and start another thread from that until n limit is reached. I can also separate it up to do a number of calcualations and then have the thread return that value.

So start with some simple threads which just print out the passed parameter.

Then build on that.

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