Re: Side effects of pruning when doing 'clean all'.

[Paul Smith]

On Thu, 2010-07-29 at 11:43 +0200, Michele Zamparelli wrote:
> I have a makefile which works flawlessly when doing 'clean' and 'all', 
> but which fails miserably when doing
> 'clean all'.
> 
> The reasons is pruning.
> A clean subgoal deletes a generated C file (stack.c), which had been 
> generated earlier on, at dependency inclusion time.
> When executing the all target, make eventually tries to build an object 
> file for a library, assuming that the generated C file is still there.

Yes, that's true.  Make always assumes that once it builds a target,
that target stays built.  It will never try to build the same target
twice in the same invocation of the makefile.  Your "clean" target has
side-effects: it deletes some files that make believes exist... so
you're going to run into problems.

The simplest way to solve your problem is to use one level of recursion:
have top-level rules like "clean" and "all" invoke a sub-make to do
their work (actually, you only really need the sub-make for the "all"
rule).  When you invoke a sub-make to create the "all" target, it will
not know that a different make deleted those files just a minute ago.

> You will probably have spotted another flaw in this: the dependencies 
> (gcc --MM) are automatically generated and included even when only 'make 
> clean' is called, on a directory which is clean already.

This can be solved by putting the includes inside a make conditional
using the $(MAKECMDGOALS) special variable to see what commands the user
ran from the command line.  If it's "clean", then don't include the
dependencies.

> Ideally I would like the 'include' directives to be called only as part 
> of the 'all' target, but I understand this is conceptually not possible. 
> Or is it?

It's almost impossible to get "make clean all" to work right in a single
makefile with a single invocation of make.  You can use the sub-make
invocation idea I mention above to make this work.  Or you can just
require people to always run it in two steps "make clean && make all"
(you can again check this with MAKECMDGOALS if you want to enforce it).

Of course, you could just stop using the auto-generating dependency
trick in the first place: it's got a lot of downsides.  For an
alternative you can start with my document "Advanced Auto-Dependency
Generation" on the website below.

-- 
-------------------------------------------------------------------------------
 Paul D. Smith <address@hidden>          Find some GNU make tips at:
 http://www.gnu.org                      http://make.mad-scientist.net
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist