test_64.c

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

volatile uint64_t x = 0;

void *writeThread(void *arg) {
    for (uint64_t i = 0; i < 10000000; i++) {
        while(!__sync_bool_compare_and_swap(&x,x,i+(i<<32))); // x = i+(i<<32)
    }
    return NULL;
}

void *readThread(void *arg) {
    while(1) {
        uint64_t aux = __sync_sub_and_fetch(&x,0); // aux = x
        if ((aux >> 32) != (aux & UINT32_MAX)) {
            printf("Wrong\n");
        }
    }
}

int main(void) {
    pthread_t t1, t2;
    pthread_create(&t1,NULL,writeThread,NULL);
    pthread_create(&t2,NULL,readThread,NULL);
    pthread_join(t1,NULL);
    printf("%llu\n", (unsigned long long)x);
}