c_primes.c

/* Compile with:
export CFLAGS="-g -Wall -O3 --std=c11 -pthread -latomic"
export LDLIBS="-lm"
make c_primes
*/
#include <stdio.h>
#include <stdatomic.h>
#include <stdlib.h>  //malloc
#include <stdbool.h>
#include <pthread.h>
#include <math.h> //sqrt

typedef struct {
    long int *plist;
    long int length;
    long int max;
} prime_s;

int add_a_prime(_Atomic prime_s *pin, long int new_prime){
    prime_s p = atomic_load(pin);
    p.length++;
    p.plist = realloc(p.plist, sizeof(long int) * p.length);
    if (!p.plist) return 1;
    p.plist[p.length-1] = new_prime;
    if (new_prime > p.max) p.max = new_prime;
    atomic_store(pin, p);
    return 0;
}

typedef struct{
    long int i;
    _Atomic prime_s *prime_list;
    pthread_mutex_t *mutex;
} test_s;

void* test_a_number(void *vin){
    test_s *in = vin;
    long int i = in->i;
    prime_s pview;
    do {
        pview = atomic_load(in->prime_list);
    } while (pview.max*2 < i);

    bool is_prime = true;
    for (int j=0; j <  pview.length && pview.plist[j] <= sqrt(i); j++)
        if (!(i % pview.plist[j])){
            is_prime = false;
            break;
        }

    if (is_prime){
        pthread_mutex_lock(in->mutex);
        int retval = add_a_prime(in->prime_list, i);
        if (retval) {printf("Too many primes.\n"); exit(0);}
        pthread_mutex_unlock(in->mutex);
    }
    return NULL;
}

int main(){
    prime_s inits = {.plist=NULL, .length=0, .max=0};
    _Atomic prime_s prime_list = ATOMIC_VAR_INIT(inits);

    pthread_mutex_t m;
    pthread_mutex_init(&m, NULL);

    int thread_ct = 3;
    test_s ts[thread_ct];
    pthread_t threads[thread_ct];

    add_a_prime(&prime_list, 2);
    long int max = 1e6;
    for (long int i=3; i< max; i+=thread_ct){
        for (int t=0; t < thread_ct && t+i < max; t++){
            ts[t] = (test_s) {.i = i+t, .prime_list=&prime_list, .mutex=&m};
            pthread_create(threads+t, NULL, test_a_number,  ts+t);
        }
        for (int t=0; t< thread_ct && t+i <max; t++)
            pthread_join(threads[t], NULL);
    }

    prime_s pview = atomic_load(&prime_list);
    for (int j=0; j < pview.length; j++)
        printf("%li\n", pview.plist[j]);
}