What is the difference between std::condition_variable::wait_for and std::condition_variable::wait_until?

Question

The reference I'm using explains the two in the following way:

• wait_for "blocks the current thread until the condition variable is woken up or after the specified timeout duration"

• wait_until "blocks the current thread until the condition variable is woken up or until specified time point has been reached"

What is the difference? Will wait_until spin so that the thread can continue exactly (more or less) when it is signaled, whereas wait_for just adds the thread back into scheduling at that point?

Answer 1

The difference is in how the wait duration is represented: wait_for takes a relative time ("wait for up to 10 seconds"), whereas wait_until takes an absolute time ("wait until 12:00 on October 30, 2012").

Compare the declarations of the time parameters:

// wait_for: const std::chrono::duration<Rep, Period>& rel_time  // wait_until: const std::chrono::time_point<Clock, Duration>& abs_time 

Answer 2

Your question and other answers have all repeated the difference; that wait_for waits for a specified amount of time and wait_until waits until a specified point in time, but the implications are not spelled out.

A time_point has an associated clock, and that clock is what's used to determine if the appropriate time has come. That means that clock adjustments are taken into account by the wait_until function. wait_until(..., system_clock::now() + std::chrono::seconds(10)) could end up waiting an hour and 10 seconds if the clock happens to be adjusted back by an hour before the wait is up.

A duration does not have any associated clock and therefore wait_for chooses its own clock. The standard specifies that it uses std::steady_clock, which cannot be adjusted and advances at a steady rate relative to real time. This means that wait_for will wait for the specified time regardless of any adjustments made to any clocks. wait_for(..., std::chrono::seconds(10)) is guaranteed to wait 10 seconds (+ some time for the implementation to work and for scheduling issues).

There is no difference with regard to spinning vs. sleeping the thread; as wait_for is specified to behave as if it called wait_until with steady_clock::now() + duration.

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Here's the part of the standard where this is spelled out:

[thread.req.timing] 30.2.4/2-4

² Implementations necessarily have some delay in returning from a timeout. Any overhead in interrupt response, function return, and scheduling induces a “quality of implementation” delay, expressed as duration D_i. Ideally, this delay would be zero. Further, any contention for processor and memory resources induces a “quality of management” delay, expressed as duration D_m. The delay durations may vary from timeout to timeout, but in all cases shorter is better.

³ The member functions whose names end in _for take an argument that specifies a duration. These functions produce relative timeouts. Implementations should use a steady clock to measure time for these functions. Given a duration argument D_t, the real-time duration of the timeout is D_t + D_i + D_m.

⁴ The member functions whose names end in _until take an argument that specifies a time point. These functions produce absolute timeouts. Implementations should use the clock specified in the time point to measure time for these functions. Given a clock time point argument C_t, the clock time point of the return from timeout should be C_t + D_i + D_m when the clock is not adjusted during the timeout. If the clock is adjusted to the time C_a during the timeout, the behavior should be as follows:
         — if C_a > C_t, the waiting function should wake as soon as possible, i.e. C_a + D_i + D_m, since the timeout is already satisfied. [ Note: This specification may result in the total duration of the wait decreasing when measured against a steady clock. —end note ]

        — if C_a <= C_t, the waiting function should not time out until Clock::now() returns a time C_n >= C_t, i.e. waking at C_t + D_i + D_m. [ Note: When the clock is adjusted backwards, this specification may result in the total duration of the wait increasing when measured against a steady clock. When the clock is adjusted forwards, this specification may result in the total duration of the wait decreasing when measured against a steady clock. —end note ]

An implementation shall return from such a timeout at any point from the time specified above to the time it would return from a steady-clock relative timeout on the difference between C_t and the time point of the call to the _until function. [ Note: Implementations should decrease the duration of the wait when the clock is adjusted forwards. —end note ]