Monotonic Clocks - the Right Way to Determine Elapsed Time - Softwariness

Monotonic Clocks – the Right Way to Determine Elapsed Time – Softwariness

If you want to display the time to a user of your program, you query the time of day. However, if your program needs to measure elapsed time, you need a timer that will give the right answer even if the user changes the time on the system clock.

The familiar system clock which tells the time of day is referred to as a real-time clock or a wall-clock. The time on this clock will jump when changed. Relying on the wall-clock to find out how much time has passed since a previous event is a bug waiting to happen.

For example, suppose your application carries out an operation in response to some event, but it only does that operation if it is more than an hour since the last time it carried out that operation, otherwise it does nothing. Maybe the operation is some kind of clean-up task. Suppose the user made a typo and set the wrong month on the system clock before your program started; they might then later fix that. Suppose the date is now a month earlier than when your program started, it will be a month until your program thinks that it should carry out its clean-up operations again!

To safely measure elapsed time in a program, you need a clock that ticks out time continuously, without any jumps when a user sets the system time. This kind of clock is called a monotonic clock. This article takes a look at the different monotonic clock functions available in the Windows and POSIX APIs, and the cross-platform routines introduced in C++11.

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