Re: prerequisite staleness criteria

[Paul D. Smith]

%% "Lawrence, Lynne" <address@hidden> writes:

  ll> Is there a way, with GNU make, to alter the criteria it uses to
  ll> test whether a prerequisite is stale?

No.  This is something that is often requested, but is still on the TODO
list.

  ll> For instance, when determining whether to copy a file from a build
  ll> directory to an install directory it would be convenient to copy
  ll> only if the file checksums differ, regardless of whether the file
  ll> in the build directory is newer.

Although on the surface it might seem like it's simple to implement this
just by replacing the current stat() of the file with something else, if
you think carefully you'll see that this is actually a major change.

Currently we compare the timestamp of the target with the timestamp of
the prerequisite.  The key thing to understand about this is that these
timestamps are stored by the filesystem, and we're comparing different
files to each other so they all exist at the same time: make just asks
the filesystem for them every time it is invoked.


If you wanted to switch to another method, say comparing checksums, now
you're comparing the current state of a given file with its PREVIOUS
STATE... so now that previous state has to be stored somewhere!

I call this "stateful make"; it really doesn't matter what the contents
of the "state" is: now you have to have make creating and maintaining
some kind of database every time it is invoked so that the next time
it's invoked it can see what's changed.  Obviously this "database" can
be very simple; any implementation capable of a standard key/value
lookup is acceptable (even something like one state file or symlink per
target).

Nevertheless, there's a lot of additional complexity here.  What if the
DB is non-existent or corrupted?  What do we do about multiple instances
of make running in the same directory and updating files there--even in
parallel if you use -j (concurrency/locking/etc.)


Still, this would be a great feature.  Someone will certainly implement
it, someday ... :-/ :-).

-- 
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