Re-evaluate date of file after updating?

[Norbert Nemec]

Hi there,

is there a way to define dependencies that re-evaluate the date of a 
file *after* updating it? Currently, the MAKE program blindly assumes 
that the target is actually modified its commands. Sometimes, however, 
you want the commands themselves determine whether a file has changed. 
MAKE should then be able to check this by the file date after the 
commands have completed.

To give my use-case in more detail (strongly simplified):

===============

I am currently trying to set up an intelligent build system for a large 
Fortran project. In Fortran, when a file lib.f90 is compiled by
   f90 -c lib.f90
the interface is written into a file lib.mod, the code itself into lib.o

Very often, when lib.f90 has changed, the object file lib.o is modified 
but the interface lib.mod remains the same.

Other files using the module only depend on the interface. If lib.mod 
has not changed, they do not need to be recompiled.

My best attempt at implementing this kind of dependencies in a Makefile 
is as follows

------------

prog: prog.o lib.o
       f90 -o prog prog.o lib.o

prog.o: lib.mod
       f90 -c prog.f90

lib.mod: lib.o
lib.o: lib.f90
       mv lib.mod lib.mod.old
       f90 -c lib.f90
       diff -qN lib.mod.old lib.mod > /dev/null && mv lib.mod.old lib.mod
       rm -f lib.mod.old

------------

This behaves nearly as needed, but unfortunately, the rule for prog.o 
does not care whether lib.mod has actually changed. It always recreates 
prog.o when lib.f90 was modified.

An alternative attempt was to modify the rule for lib.mod into

------------

lib.mod: | lib.o

------------

This, however, did not work either: Now, the prog.o would not be 
recompiled, even if lib.mod was modified when re-compiling lib.f90. The 
only solution here was to run MAKE twice.

It seems that the MAKE program reads in all the file timestamps at the 
beginning of the build process and then never checks again. What I would 
need is some mechanism that checks the file dates of the prerequisites 
*after* they have been rebuilt.

===============

Greetings,
Norbert Nemec

--
_________________________________________Norbert Nemec
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