#define _GNU_SOURCE
#include <asm/prctl.h>
#include <assert.h>
#include <linux/futex.h>
#include <malloc.h>
#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

// 64kB stack
#define FIBER_STACK (1024 * 64)
#define THREADS     2
static int myfutex = 0;
struct atomic_int {
    volatile int counter;
};
static struct atomic_int my_counter;

static inline void atomic_inc(struct atomic_int* v) {
    asm volatile("lock; incl %0" : "+m"(v->counter));
}

static int futex(int* uaddr, int futex_op, int val, const struct timespec* timeout, int* uaddr2,
                 int val3) {
    return syscall(SYS_futex, uaddr, futex_op, val, timeout, uaddr, val3);
}

int thread_function(void* argument) {
    int* ptr = (int*)argument;
    int rv;
    atomic_inc(&my_counter);

    // Sleep on the futex
    rv = futex(&myfutex, FUTEX_WAIT_BITSET, 0, NULL, NULL, *ptr);
    assert(rv == 0);
    // printf("child thread %d awakened\n", getpid());
    return 0;
}

int main(int argc, const char** argv) {
    void* stacks[THREADS];
    pid_t pids[THREADS];
    int varx[THREADS];
    my_counter.counter = 0;

    for (int i = 0; i < THREADS; i++) {
        varx[i] = (1 << i);

        // Allocate the stack
        stacks[i] = malloc(FIBER_STACK);
        if (stacks[i] == 0) {
            perror("malloc: could not allocate stack");
            _exit(1);
        }

        // Call the clone system call to create the child thread
        pids[i] = clone(&thread_function, (void*)stacks[i] + FIBER_STACK,
                        CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_VM, &varx[i]);

        // printf("clone() creates new thread %d\n", pids[i]);

        if (pids[i] == -1) {
            perror("clone");
            _exit(2);
        }
    }

    // Make sure the threads are sleeping
    do {
        sleep(1);
    } while (my_counter.counter != THREADS);

    printf("Waking up kiddos\n");
    /* Wake in reverse order */
    for (int i = THREADS - 1; i >= 0; i--) {
        pid_t pid;
        int rv;
        int var = (1 << i);

        // Wake up the thread
        rv = futex(&myfutex, FUTEX_WAKE_BITSET, 1, NULL, NULL, var);
        assert(rv == 1);

        // Wait for the child thread to exit
        pid = waitpid(pids[i], NULL, __WALL);
        if (pid == -1) {
            perror("waitpid");
            _exit(3);
        }

        // Free the stack
        free(stacks[i]);
    }

    printf("Woke all kiddos\n");

    return 0;
}