Re: make dependency on parts of a file

[David Holland <dholland-tech%netbsd.org@localhost>]

On Fri, Apr 04, 2014 at 01:51:09PM +0200, Edgar Fu? wrote:
 > Suppose I have a file A and another file B depending on only parts
 > of A.  That is, if (the time stamp of) A changes, it is quite
 > likely that B (derived from A) will not.  Suppose it's cheap to
 > build B from A (or to check whether B actually changed).  Then
 > suppose there's C, which depends on B but is expensive to build.
 > 
 > Is there a sane way to rebuild C only if the relevant parts of A changed?

Not that tidily, but you can do something like this:

all: C

B: A
        $(MKB) A
B.reference: B
        if diff -q B.reference B; then cp B B.reference; fi
C: B.reference
        $(MKC) B.reference

If that doesn't work, you can also do

all: B.reference

B: A
        $(MKB) A
B.reference: B
        if diff -q B.reference B; then cp B B.reference && $(MAKE) C; fi
C: B
        $(MKC) B

The netbsd build does something like this for installing header files;
I don't remember offhand if it's a mess or a decent example.

-- 
David A. Holland
dholland%netbsd.org@localhost