Re: Remaking when some "parameter" changes value

[Greg Chicares]

On 2006-2-12 23:54 UTC, Nick Patavalis wrote:
> My problem, in a nutshell, is this: How can I force a project be
> remade, when one of the parameters passed to Make changes?  When, for
> example, the flags to the compiler, or to the linker, change?

I don't think there's a much better method than the one you show.

> But if, after having built everything, or after I have
> built a part of the project, if I say:
> 
>   $ make ARCH=ppc-linux-
> 
> Then, instead of rebuilding everything---which would be the only safe
> and reasonable thing to do---I either get "Nothing to be done...", or
> a partial build that mixes "arm" and "ppc" objects and binaries.

In the particular case of $(ARCH), I'd rather use different build
directories.

> The
> same goes if I say, for instance:
> 
>   $ make CFLAGS=-Wall -Winline -O2
> 
> I think you can understand the trouble this can lead to.

A really bad case would be CFLAGS=-DNDEBUG. Then again, changing
  make CFLAGS='-Wall'
to
  make CFLAGS='-W -Wall -Werror -pedantic-errors'
might legitimately mean "don't compile that one source file I just
changed unless it elicits no warnings", even though other source
files aren't all warning-free. "Do What I Mean" isn't just hard,
it's ambiguous.

> Here's the best solution I could find to this problem. I rewrite the
> Makefile above, adding the following lines at the end:
> 
>   $(OBJS1) $(OBJS2) : signature
>   $(shell ./mksignature.sh \
>       "$(ARCH)" \
>       "$(CC)" \
>       "$(CFLAGS)" \
>       "$(LD)" \
>       "$(LDFLAGS)" \
>       "$(LDLIBS) \
>       ... etc ... )
[...]
> This solves the problem in a reliable, though not terribly efficient,
> way: It forces make to rebuild everything when some of the "hidden"
> dependencies change (hidden in the sense that they are not captured in
> files, hence Make does not normally know about them).

That implementation captures changes to explicitly-listed variables.
You could use $(MAKEOVERRIDES) to capture changes to command-line
variable definitions.

Some variations with ordering or whitespace differences
  CFLAGS='-Wall -Winline'
  CFLAGS='  -Wall -Winline'
  CFLAGS='-Winline -Wall'
may be equivalent, while others may not be
  CFLAGS='-O3 -O0'
  CFLAGS='-O0 -O3'
but cost quickly exceeds benefit for exquisite refinements.

Similarly, your makefile might say
  CFLAGS='-g'
and overriding it with a value identical to that default
  make CFLAGS='-g'
ideally would be recognized as no different.

> What would,
> though, be ideal is a way to force make rebuild what is absolutely
> necessary. Like for example, if I change LDFLAGS, to relink the
> executables, without recompiling the objects. The technique shown
> above can be easily extended to accommodate this (by using multiple
> "signature" files) but, to my taste, the resulting makefile gets way
> too complicated, and it doesn't worth the trouble for most real-world
> cases.

Agreed. Probably it's best to be mindful of when you need to
make clean first.