Core · Module 5 · Lesson 12 of 12
Clocks, Timers, and Timeouts
Choose the right POSIX clock, measure elapsed time monotonically, and design deadlines that remain correct across interruptions.
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By the end you can
- Distinguish wall-clock time from monotonic elapsed time
- Build deadlines from clock readings
- Avoid extending a timeout after EINTR
- Select timer mechanisms for blocking and periodic work
Builds on System Call Errors, errno, and EINTR.
01 · A question first
If the system clock is adjusted backward while a request is waiting, should a five-second timeout become longer? Reliable systems separate calendar time from elapsed time.
Make a guess before you read on.
02 · The idea
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What's inside
- Start with the problem
- Wall-clock time and monotonic time
- Use absolute deadlines
- Sleep, poll, and timers
- What it costs
- Pitfalls and variations
- Where it is used
- Operational checklist
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