Re: 6.2 The Two Flavors of Variables

[grischka]

> > CFLAGS = $(CFLAGS) -O
> >
> > Why it causes an infinite loop? The first time $(CFLAGS) is null so
> > the right side will be expanded to "null -O". There will be no second
> > time as null is a final value and CFLAGS becomes -O
>
> Your statement "the first time $(CFLAGS) is null" indicates that you've
> missing the critical aspect of recursive variables.
> [...]
> Now make wants to expand $(CFLAGS) and it sees that the value of that
> variable is '$(CFLAGS) -O'.  So it then tries to expand that, and sees
> that $(CFLAGS) expands to '$(CFLAGS) -O'.  And so on until you run out
> of memory.

I'd still argue that this is nothing more than a pretty accurate
description of an implementation bug.  Which is also the only way
to explain why gnu-make invented the term "recursive variable" to
denote a variable that must not be used recursively.

For example, there is no difference semantically whether you write
    CFLAGS = $(CFLAGS) -O
or
    CFLAGS += -O

Now since gnu-make does support the latter there cannot be a quasi
natural logical reason why it can't support the former, because +=
also implicitly references an value of the variable which if it was
expanded like you describe above would cause the same memory overflow.

See also:
    http://lists.gnu.org/archive/html/help-make/2008-04/msg00034.html
which basically discusses a -= operator as in
    CFLAGS -= -O
.

--- grischka