snipplets.dev/code/C/atomic.c

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

atomic_int acnt;
int cnt;

int f(void *thr_data) {
  (void)thr_data;
  for (int n = 0; n < 1000; ++n) {
    ++cnt;
    ++acnt;
    // for this example, relaxed memory order is sufficient, e.g.
    // atomic_fetch_add_explicit(&acnt, 1, memory_order_relaxed);
  }
  return 0;
}

int main(void) {
  thrd_t thr[10];
  for (int n = 0; n < 10; ++n)
    thrd_create(&thr[n], f, NULL);
  for (int n = 0; n < 10; ++n)
    thrd_join(thr[n], NULL);

  printf("The atomic counter is %u\n", acnt);
  printf("The non-atomic counter is %u\n", cnt);
}