Re: variable scope

[Philip Guenther]

On Wed, Mar 10, 2010 at 1:59 AM, Massimiliano Cialdi
<address@hidden> wrote:
> Hi I have a problem with variable definition

Not quite.  You have a problem with $(eval) being Too Subtle For
Mortals.  The result is that people *think* they understand $(eval),
but really their programs work only by coincidence...until they stop
working.


To be particular, this definition is broken:

> define asdrubale
> $(info  marker0 $(0) ** $(1))
> $(1)_VAR1 = -param_$$($(1)_MCU)
> $(foreach z,$($(1)_COMPILE_SET),$(eval $(call build_target,$(1),$(z))))
> endef

This is apparently meant to be used with $(eval $(call
asdrubale,target)).  When make sees that expression, it first
COMPLETELY EXPANDS the $(call) by setting the number variables and
performing variable expansion, BUT NOT MAKEFILE EVALUATION.

So, it expands the $(info ...) call, resulting in an empty line and
the side-effect of some output to stdout.  Then it expands the second
line and gets
    target_VAR1 = -param_$(target_MCU)

Note that at this point, that's just text that's the result of
variable expansion.  It's *not* evaluated as a makefile fragment!
Then make expands the third line, which involves expanding the
build_target definition *and* evaluating the result of that expansion.
 That has the side-effect of various variable assignments and rule
definitions, but doesn't generate any text, so the complete expansion
of
$(call asdrubale,target) is
    target_VAR1 = -param_$(target_MCU)
...and a bunch of side-effects.  That text is the argument to $(eval),
so it *finally* gets evaluated and the variable assignment occurs.


There are probably a dozen ways to "fix" this code.  My favorite would
be to not use $(eval) at all, because I don't want to spend my life
maintaining Makefiles that give me headaches and which none of my
co-workers can help me with.  Instead, I would use includes that could
be pulled in repeatedly, because the order of evaluation in those
doesn't stab you in the back.

If you insist on using $(eval), ask yourself why you're using $(eval)
both around the call to asdrubale *and* _inside_ of asdrubale.

...and if you get it working, then ask yourself how sure you are that
stuff isn't *still* being expanded and/or evaluated more times than it
should.  And whether you'll understand why this works next week.  And
whether the guy in the next cube will understand it next month...


Philip Guenther