在特定时间后唤醒线程

Question

I have two threads, say 1 and 2, and have put 2 to sleep. 1 performs some task and sets a timer to wake up 2 't' seconds after the task is done.

So, I need to program to send a signal to 2 't' seconds later. How can I achieve this?

Answer 1

You can use a timed-wait on a condition variable, which will wake up when that time is reached / has elapsed. You can also just make a thread sleep for a period of time.

pthreads (which you specify) only has a wait until an absolute time.

The new standard C++ library and boost give you the option of wait_for(time_period) or wait_until(absolute_time) .

In your case I don't think you need to do any of this but you might use a 3rd thread.

1. Thread 1 performs the task, Thread 2 waits on a condition variable, when thread 1 completes the task it creates a 3rd thread which sleeps and wakes thread 2.

2. Thread 1 performs the task and immediately signals the condition variable held by thread 2, but thread 2 then sleeps before it continues. (So really it wakes up earlier but it simulates the delay). No need for a 3rd thread, However thread 2 might be a "client" thread and you want to enforce the delay so use method 1 if that is the case.

Answer 2

Prior than C++11, the standard library did not offer timers. Even with C++11, you have standrd timing facilities (eg, sleep, condition variables, etc.) but you have to assemble them to have the desired behavior.

So, you roughly have the following otions:

• rely on some specific (cross-platform) library (eg, Qt, Boost, etc.)
• rely on OS-level facilities (eg, Posix on Linux)
• create a third thread acting as timer: once activated, it sleeps for 2 seconds and then signals the second thread through a condition variable
• let the first thread compute the amount of time to wait after the work done, make it sleep and then signal the other thread through a condition variable

I suggest the last option.