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On Wed, Jan 25, 2017 at 3:25 PM, William Ahern <> wrote:
On Wed, Jan 25, 2017 at 01:21:36PM +0000, Joseph Manning wrote:
> Dear All,
>    Have you ever wondered how the Lua Team manage to retain their
>    ever-youthful good looks?
>    Do you want to become mega-rich like Biff in Back To The Future II,
>    by placing bets when you already know the outcome?
>    Or would you simply like to run your programs so fast that they
>    actually finish before they've even started?
>    Well, these are all now possible, by using Lua for Time Travel !!!
>    ( ... takes his medication, and a semblance of sanity returns ... )
>    So, I was timing a section of code by wrapping it in calls to
>    'os.clock( )', and under certain conditions, the second such call
>    returned a value that was substantially smaller than the first --
>    in fact, it returned a negative value, which should never happen.

Here's a defect ticket for POSIX discussing the overflow issue.

The issue was brought up with the C committee. (See DR 437 in But they want to
wait and see what solutions the POSIX people suggest. Alas, issue #686
hasn't seen any activity since then.

In C I just explicitly type cast clock_t values (each operand individually)
to unsigned long, and everything just magically works because of modulo
arithmetic. But I'm making assumptions about the platform--that it's two's
complement and that unsigned long is the corresponding unsigned type for
clock_t. I really only use it for debugging and quick benchmarking. But as
mentioned in that ticket there's no [easy] portable way to figure out which
unsigned type to convert to. For Lua you'd probably have to use a function
with conditional logic, just like a portable C solution would likely use.

Usually for this sort of thing it's best to use the system's monotonic
clock--clock_gettime(CLOCK_MONOTONIC) on POSIX systems. On modern Linux
systems the granularity and overhead will be the same as clock().[1]
clock_gettime returns a struct timespec, which is also now the preferred
type for time intervals in the latest C standard, C11.

[1] clock() is implemented as clock_gettime(CLOCK_PROCESS_CPUTIME_ID). And
both CLOCK_PROCESS_CPUTIME_ID and CLOCK_MONOTONIC perform pretty much the
same operations--query the CPU TSC, read some cached values, perform the

Perhaps it would be ideal for Lua to automatically save the previous result of clock() and subtract it from the next call? Or would that be too different from the underlying API?

Sent from my Game Boy.