Date: Tue, 2 Aug 2016 16:33:29 -0700 From: Ward Willats <musl@...dco.com> To: musl@...ts.openwall.com Subject: std::condition_variable::wait_for() breakage when system_clock changes C++11 MIPS Hello. This may be a little too C++ for this list, but I wasn't sure where to turn. Also, I thought I should at least document it for the Google, and see if you folks think I should take this problem elsewhere, or, perhaps more likely, if I have made some stupid error (!) We are running a C++ 11 app on a MIPS-based Linux/OpenWRT platform and linking against MUSCL 1.1.12 and libstdc++ v3. On cold boot, our system clock is often some random time in the future until ntpd or other software corrects it. This led to odd behavior and this experiment: If I use the busybox "date --set" to move our system's clock 24 hours into the future, and then call: std::condition_variable::wait_for() with the pre-canned std::chrono::duration, std::chrono::milliseconds( 120000 ), as a 2 minute wait timeout AND then... ...set the clock BACK 24 hours to the correct/current time with ntpd, wait_for() doesn't return after 2 minutes (assuming no notify() of course). (Presumably, wait_for() is using system_clock, and not steady_clock, and will fire in 24 hours + 2 minutes -- but I haven't waited to find out.) However, if I repeat this experiment and call std::condition_variable::wait_until() with a timeout time_point calculated as std::chrono::steady_clock::now() + std::chrono::milliseconds( 120000 ), which neatly binds a steady_clock ref into the time_point, it does indeed fire in 2 minutes after the clock is set back, as expected. In short, the std::condition_variable API that takes a std::chrono:duration does not work, but the one that takes a std::chrono::time_point does. This is strange to me since sites like cppreference.com <http://cppreference.com/> indicate that both methods use a steady_clock to calculate duration AND imply that wait_for() may even implemented internally in terms of wait_until(). However, in this implementation, both of these assertions seem incorrect. Anyway, it seems like a bug. Personally, I am fine, as I can implement a wrapper that does our wait_for() in terms of wait_until(), but it took a few days of chasing through our code to pin this down, so figured I share and maybe save someone some pain. -- Ward Content of type "text/html" skipped
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.