Re: conditional macro definitions

[John Graham-Cumming]

bill wrote:
> Thanks to everyone who responded to that question.   I'm a little 
> confused about how to deal with the issue that the conditional
> definition seems to occur too late. In the following example, I
> modify the value of $(DIR) when the target is debug. But the target
> $(DIR) maintains the old value.  So when I "make debug", there is no 
> rule to build /tmp/debug.  How do I make the conditional values be
> targets?
>
> % rmdir /tmp/foo % cat makefile
>
> DIR = /tmp/foo
>
> debug : DIR = /tmp/debug
>
> all: $(DIR)
>
> debug: all
>
> $(DIR): mkdir -p $@ ...

You can't do that and have it work!  All the rule definitions get parsed 
and set when the Makefile is read.  Hence the value of DIR used from the 
Makefile is first read (/tmp/foo) is used to define the rule for $(DIR) 
at the end of you Makefile.

My recommendation is that you live with the 'make DEBUG=1' style and 
then set DIR based on an ifeq before any rules that use $(DIR).  You 
could use the MAKECMDGOALS that I also mentioned previously but to be 
honest I think that would be odd, and the 'make DEBUG=1' style is an 
idiom which most people understand.

John.
--
John Graham-Cumming
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