[Buildroot] [PATCH 1 of 7 v3] infra: consistently use double dollar signs inside inner-xxx-targets
Thomas Petazzoni
thomas.petazzoni at free-electrons.com
Wed Jun 11 15:56:04 UTC 2014
Dear Thomas De Schampheleire,
On Sun, 8 Jun 2014 16:29:45 +0200, Thomas De Schampheleire wrote:
> In this case the (original) construct is:
> Target case:
> FOO_BAR ?= $(FOO_BAR)
>
> Host case:
> HOST_FOO_BAR ?= $(FOO_BAR)
>
> For the target case there would be a circular reference, and the
> statement doesn't make sense anyway.
> To solve this, an extra check $(ifeq $(4),host) is really needed.
> For the host case, if HOST_FOO_BAR is not yet set, it is set equal to
> the value of FOO_BAR. FOO_BAR may or may not be set previously: there
> is no default value at play here.
>
> A very important thing to understand here, construct (a)
> HOST_FOO_BAR ?= $(FOO_BAR)
> is not equivalent to (b)
> ifndef HOST_FOO_BAR
> HOST_FOO_BAR = $(FOO_BAR)
> endif
>
> because 'ifdef' checks for a *non-empty value* while '?=' checks for
> *set or not set*.
> Since the .mk can have a statement like:
> FOO_PATCH = blaat.patch
> HOST_FOO_PATCH =
> the second form (b) above will still set HOST_FOO_BAR to FOO_BAR which
> is not what we want.
>
>
> I hope the above is more clear to you, please let me know.
>
> It is clear that the code is non-trivial, but I don't know if and how
> we should document all this.
> What do you think?
> Do also let me know if you expect changes in the patch or commit
> message to make any of this clear.
Thanks a lot for the detailed explanation! Definitely very useful, as
I now understand better the logic behind these additional ifeq
($(4),host) tests. We probably need to add more comments in
pkg-generic.mk, but that's clearly unrelated to your patch, and can be
handled later on.
Best regards,
Thomas
--
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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