c++ - std::condition_variable wait_for 无限

Question

我正在尝试使用以下代码对条件变量进行无限等待(只是显示问题的示例):

std::condition_variable cond;
std::mutex mtx;
std::unique_lock<std::mutex> lock(mtx);
cond.wait_for(lock, std::chrono::steady_clock::duration::max());

但是等待会立即退出。深入研究 wait_for 的 (MS) 实现，我发现它实际上使用了 wait_until 函数。但在此之前，它会调用 chrono::system_clock::now() 并仅添加持续时间来转换时间。

当然这会导致整数溢出，因此新时间变为 <= 'now'。因此 wait_until 立即退出。所有其他定时等待函数(例如 std::timed_mutex 类中的 try_lock_for)也是如此。

综上所述，我想问一下，这是否是定时等待函数实现中的错误，如果是，那我在哪里写呢？

此外，由于 wait_until 使用 system_clock，如果在等待期间有时间调整，实际等待时间应该会有所不同(因为 system_clock 不是单调的).因此不信任等待时间。

Answer 1

cppreference documentation说:

Note that rel_time must be small enough not to overflow when added to std::chrono::steady_clock::now().

和:

The clock tied to timeout_time is used, which is not required to be a monotonic clock.There are no guarantees regarding the behavior of this function if the clock is adjusted discontinuously, but the existing implementations convert timeout_time from Clock to std::chrono::system_clock and delegate to POSIX pthread_cond_timedwait so that the wait honors ajustments to the system clock, but not to the the user-provided Clock. In any case, the function also may wait for longer than until after timeout_time has been reached due to scheduling or resource contention delays.

Even if the clock in use is std::chrono::steady_clock or another monotonic clock, a system clock adjustment may induce a spurious wakeup.

如果你想要无限超时，你可以做这样的事情(未测试):

wait_until(lock, std::chrono::sys_time::max());