Non recursive factorial in C

Question

I have a simple question for you. I made this code to calculate the factorial of a number without recursion.

int fact2(int n){
    int aux=1, total = 1;
    int i;
    int limit = n - 1;
    for (i=1; i<=limit; i+=2){
        aux = i*(i+1);
        total = total*aux;
    }
    for (;i<=n;i++){
        total = total*i;
    }
return total;

}

As you can see, my code uses loop unrolling to optimize clock cycles in the execution. Now I'm asked to add two-way parallelism to the same code, any idea how?

Answer 1

You can use ptherads library to create two separate threads. Each thread should do half of the multiplications. I could put together following solution.

#include <pthread.h>

typedef struct {
    int id;
    int num;
    int *result;
} thread_arg_t;

void* thread_func(void *arg) {
    int i;
    thread_arg_t *th_arg = (thread_arg_t *)arg;
    int start, end;
    if(th_arg->id == 0) {
        start = 1;
        end = th_arg->num/2;
    } else if (th_arg->id == 1) {
        start = th_arg->num / 2;
        end = th_arg->num + 1;
    } else {
        return NULL;
    }
    for(i=start; i < end; i++) {
            th_arg->result[th_arg->id] *= i;
    }
    return NULL;
}

int factorial2(int n) {
    pthread_t threads[2];
    int rc;
    int result[2];
    thread_arg_t th_arg[2];
    for(i=0; i<2; i++) {
        th_arg[i].id = i;
        th_arg[i].num = n;
        th_arg[i].result = result;
        rc = pthread_create(&threads[i], NULL, thread_func, (void *)&th_arg[i]);
        if (rc){
         printf("pthread_create() failed, rc = %d\n", rc);
         exit(1);
      }
    }

    /* wait for threads to finish */
    for(i=0; i<2; i++) {
      pthread_join(thread[i], NULL);

    /* compute final one multiplication */
    return (result[0] * result[1]);
}

The pthread library implementation should take care of parallelizing the work of two threads for you. Also, this example can be generalized for N threads with minor modifications.