How to get one correct value from a function which is parallelized by multi-threads

Question

For faster calculation, trying to make my method parallelized by 4 threads. Where threads are making 4 separate calculation whether I was expecting concurrent operation and on single variable.

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

static int x, j=5;

void *print_count (void *dummy)
{
  for(int i=0;i<1000;i++){
  x+=j;
  }
}

int main ()
{
pthread_t p1, p2, p3, p4;

pthread_create (&p1, NULL, print_count, NULL);
pthread_create (&p2, NULL, print_count, NULL);
pthread_create (&p3, NULL, print_count, NULL);
pthread_create (&p4, NULL, print_count, NULL);

pthread_join (p1, NULL);
pthread_join (p2, NULL);
pthread_join (p3, NULL);
pthread_join (p4, NULL);

printf("Actual output: %d \nExpected output: 5000\n", x);

return 0;

}

I expect the output 5000 as the increment is 5 and loop for 1000 times. But the actual output first of all not static, it always change and it is near 4 times of 5000 because threads are calculating the print_count separately.

Thank you

Answer 1

If you are under C11 you can use _Atomic

Of course each thread needs to work over a range of values (not with the complete set), pass a struct:

#include <stdio.h>
#include <stdatomic.h>
#include <pthread.h>

_Atomic int x;
static int j = 5;

struct range {
    int from, to;
};

void *print_count(void *data)
{
    struct range *range = data;

    for (int i = range->from; i < range->to; i++) {
        x += j;
    }
    return NULL;
}

int main(void)
{
    pthread_t p1, p2, p3, p4;
    struct range ranges[] = {
        {0, 250},
        {250, 500},
        {500, 750},
        {750, 1000}
    };

    pthread_create(&p1, NULL, print_count, &ranges[0]);
    pthread_create(&p2, NULL, print_count, &ranges[1]);
    pthread_create(&p3, NULL, print_count, &ranges[2]);
    pthread_create(&p4, NULL, print_count, &ranges[3]);

    pthread_join(p1, NULL);
    pthread_join(p2, NULL);
    pthread_join(p3, NULL);
    pthread_join(p4, NULL);

    printf("Actual output: %d \nExpected output: 5000\n", x);
    return 0;
}

Or a compound literal:

pthread_create(&p1, NULL, print_count, (int []){  0,  250});
pthread_create(&p2, NULL, print_count, (int []){250,  500});
pthread_create(&p3, NULL, print_count, (int []){500,  750});
pthread_create(&p4, NULL, print_count, (int []){750, 1000});

...

void *print_count(void *data)
{
    int *range = data;

    for (int i = range[0]; i < range[1]; i++) {
        x += j;
    }
    return NULL;
}

in order to divide the task.

Output:

Actual output: 5000 
Expected output: 5000