Re: [PATCH] up_to_date_p: treat equal mtime as outdated.

[Nick Bowler]

On 2020-04-13, Paul Eggert <address@hidden> wrote:
> I just checked, and GNU Make uses high-resolution file timestamps when
> available, and considers a file to be up-to-date if it has exactly the same
> timestamp as its dependency. I suspect that this is because Makefile rules
> like
> this:
>
> a: b
>       cp -p b a
>
> would otherwise cause needless work if one ran 'make; make'.

If the Autoconf manual's portability notes are to be believed, then
such a rule is really not a good idea because on filesystems suporting
subsecond timestamp precision some "cp -p" implementations can create
the destination file with a strictly older timestamp than the source.

It's worth pointing out that even on filesystems supporting high-
precision timestamps, actual mtime updates usually have significantly
lower resolution (typically on the order of a millisecond or so).
Sufficiently fast make rules can frequently create many files with
exactly the same mtime.  Therefore considering equal timestamps to be
"up-to-date" is really the only reasonable option.

In cases where it is required to create two files with distinct mtimes,
the only real portable option I've found is to touch one of the files
in a loop and compare timestamps until they are different.

Cheers,
  Nick