4 ms·

I think you are right. 50 days is 4.32e9 ms, which is just a bit under max value of unsigned 32-bit int.

munk-a 7y ago
It's actually a bit over the max value[1] - I agree though that I'd strongly suspect this issue is related to overflowing a millisecond counter stored in a 32-bit int. The numbers are way too close. Hey, maybe <51 was just a off-by-one error... or maybe the actual advisory is to be <50 and some PM decided that number was too round or violated an SLA. 1. 4,294,967,296 or 4.29e9
- 3pt14159 7y ago
  But with engineering we almost always have safety factors. I'd say it's probably a 64-bit int, but that would be way too much of a safety factor.
  - pdpi 7y ago
    Safety factors are a thing, sure. Safety factors of 4.29e9x (which is what you get when you go from 32- to 64-bit ints) are possibly a bit excessive, and not at all worthy of an FAA airworthiness directive.
    - goblin89 7y ago
      My biggest surprise today is from learning that critical aircraft software is left running for days without a full restart. Somehow I assumed everything gets completely shut down every time they refuel or so.
  - im_down_w_otp 7y ago
    Given this appears to be used in the communications layer of the system I expect that the width is a defined segment of some part of the wire protocol.
- tekstar 7y ago
  They should follow the Linux kernel and set all counters to rollover a few minutes after boot.
  - egwor 7y ago
    Does it actually do that? Is it done to ensure that all apps can handle the scenario?
    - tekstar 7y ago
      Yes. Jiffies (the main linux counter) rolls over either 10 or 20 minutes after boot. Yes. It is to ensure kernel modules as code can handle the roll over.
vbezhenar 7y ago
Good old GetTickCount() bug.
- woadwarrior01 7y ago
  That's the first thought that came to my mind. What are the odds that they've got an embedded CPU running Win32 somewhere in the aircraft? :) Context[1] for anyone not familiar with the old Win32 APIs. [1]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount https://docs.microsoft.com/en-us/windows/win32/api/sysinfoap...
  - tyingq 7y ago
    The system in question runs VxWorks. "for reasons the directive did not go into, the 787's common core system (CCS) – a Wind River VxWorks realtime OS product..."

It's actually a bit over the max value[1] - I agree though that I'd strongly suspect this issue is related to overflowing a millisecond counter stored in a 32-bit int. The numbers are way too close. Hey, maybe <51 was just a off-by-one error... or maybe the actual advisory is to be <50 and some PM decided that number was too round or violated an SLA. 1. 4,294,967,296 or 4.29e9

3pt14159 7y ago
But with engineering we almost always have safety factors. I'd say it's probably a 64-bit int, but that would be way too much of a safety factor.
- pdpi 7y ago
  Safety factors are a thing, sure. Safety factors of 4.29e9x (which is what you get when you go from 32- to 64-bit ints) are possibly a bit excessive, and not at all worthy of an FAA airworthiness directive.
  - goblin89 7y ago
    My biggest surprise today is from learning that critical aircraft software is left running for days without a full restart. Somehow I assumed everything gets completely shut down every time they refuel or so.
- im_down_w_otp 7y ago
  Given this appears to be used in the communications layer of the system I expect that the width is a defined segment of some part of the wire protocol.
tekstar 7y ago
They should follow the Linux kernel and set all counters to rollover a few minutes after boot.
- egwor 7y ago
  Does it actually do that? Is it done to ensure that all apps can handle the scenario?
  - tekstar 7y ago
    Yes. Jiffies (the main linux counter) rolls over either 10 or 20 minutes after boot. Yes. It is to ensure kernel modules as code can handle the roll over.

vbezhenar 7y ago

Good old GetTickCount() bug.

woadwarrior01 7y ago
That's the first thought that came to my mind. What are the odds that they've got an embedded CPU running Win32 somewhere in the aircraft? :) Context[1] for anyone not familiar with the old Win32 APIs. [1]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-gettickcount https://docs.microsoft.com/en-us/windows/win32/api/sysinfoap...
- tyingq 7y ago
  The system in question runs VxWorks. "for reasons the directive did not go into, the 787's common core system (CCS) – a Wind River VxWorks realtime OS product..."