epoll_wait and changing system time back

Question

My app uses epoll_wait to perform a timed wait for IO events. If no event happens, epoll_wait is supposed to return after the timeout and my app continues.

During testing, someone turn the system clock back by a day and the part of my app that uses epoll_wait stopped working for 24 hours. Obviously, this is a problem.

I've rummaged around looking for something that might allow my app to know that the time has changed (for example, a signal) but I haven't found anything.

Is there any way to deal with abrupt time changes like this?

Answer 1

使用基于单调时间的计时器（例如timer_create（2））来生成信号并执行阻塞epoll_wait，并检查-1返回码和errno是否设置为EINTR。

Answer 2

The Linux kernel implementation of epoll_wait uses CLOCK_MONOTONIC so it should be immune to system clock changes. (I investigated this since I was seeing a similar problem and wanted to discount epoll_wait as a possible cause.) Before v2.6.37 it used to use jiffies which are also monotonic.

I've submitted a patch to the epoll_wait man page to clarify this.