#include <stdlib.h>
#include <stdio.h>

#include <condition_variable>
#include <mutex>
#include <thread>

std::mutex contended_mutex;
std::mutex control_mutex;
std::mutex thread_started_mutex;

std::unique_lock<std::mutex> *contended_lock = nullptr;
std::unique_lock<std::mutex> *control_lock = nullptr;
std::unique_lock<std::mutex> *thread_started_lock = nullptr;

std::condition_variable  control_condition;
std::condition_variable  thread_started_condition;

// This function runs in a thread.  The locking dance is to make sure that 
// by the time the main thread reaches the pthread_join below, this thread
// has for sure acquired the contended_mutex.  So then the call_me_to_get_lock
// function will block trying to get the mutex, and only succeed once it
// signals this thread, then lets it run to wake up from the cond_wait and
// release the mutex.

void *
lock_acquirer_1 ()
{
    contended_lock->lock();
  
    // Grab this mutex, that will ensure that the main thread
    // is in its cond_wait for it (since that's when it drops the mutex.
    thread_started_lock->lock();
    thread_started_lock->unlock();

    // Now signal the main thread that it can continue, we have the contended lock
    // so the call to call_me_to_get_lock won't make any progress till  this
    // thread gets a chance to run.
    control_lock->lock();

    thread_started_condition.notify_all();
    control_condition.wait(*control_lock);

    return NULL;
}

int
call_me_to_get_lock ()
{
    control_condition.notify_all();
    contended_lock->lock();
    return 567;
}

int main ()
{
    contended_lock = new std::unique_lock<std::mutex>(contended_mutex, std::defer_lock);
    control_lock = new std::unique_lock<std::mutex>(control_mutex, std::defer_lock);
    thread_started_lock = new std::unique_lock<std::mutex>(thread_started_mutex, std::defer_lock);

    thread_started_lock->lock();

    std::thread thread_1(lock_acquirer_1);
  
    thread_started_condition.wait(*thread_started_lock);

    control_lock->lock();
    control_lock->unlock();

    // Break here.  At this point the other thread will have the contended_mutex,
    // and be sitting in its cond_wait for the control condition.  So there is
    // no way that our by-hand calling of call_me_to_get_lock will proceed
    // without running the first thread at least somewhat.

    call_me_to_get_lock();
    thread_1.join();

  return 0;

}