#include <mutex>
#include <thread>
#include <cstdlib>
#include <cassert>
std::mutex m;
typedef std::chrono::system_clock Clock;
typedef Clock::time_point time_point;
typedef Clock::duration duration;
typedef std::chrono::milliseconds ms;
typedef std::chrono::nanoseconds ns;
void f()
{
time_point t0 = Clock::now();
time_point t1;
{
std::unique_lock<std::mutex> ul(m);
t1 = Clock::now();
}
ns d = t1 - t0 - ms(250);
assert(d < ns(2500000)); }
int main()
{
m.lock();
std::thread t(f);
std::this_thread::sleep_for(ms(250));
m.unlock();
t.join();
}