Re: conditional macro definitions

[John Graham-Cumming]

bill wrote:
> The syntax foo:=CFLAGS=-DFOO for redefining CFLAGS for the target foo
> seems like a great thing, but it's not valid in gnu-make.

Correct.  That syntax is common in the version of Make used on Sun
systems.  What you are looking for are "target-specific variables" in
GNU Make parlance.

The equivalent syntax in GNU Make is:

    foo: CFLAGS=-DFOO

> I'm currently setting up my make files so that I type "make DEBUG=1"
> and then defining CFLAGS conditionally based on the definition of
> DEBUG.  I'd much rather type "make debug" and use the syntax given
> above.  2 questions:
>
> 1)  Is it possible to do something like: if target==debug;    CFLAGS
> = -DDEBUG; end if rather than: ifdef DEBUG; CFLAGS = -DDEBUG; end if

There are a couple of things that you could do.  The first is just have 
a target called 'debug' that sets CFLAGS:

    debug: CFLAGS=-DFOO

and then make everything else depend on debug.  So imagine that you have 
an 'all' target then you'd add:

    debug: all

And when you do 'make debug' it'll build 'all' (and whatever else it 
will need), but CFLAGS will have the special debug value for every 
target 'behind' 'all'.  That's because in GNU Make when you set a 
target-specific variable you are setting it for the target and all its 
prerequisites.

As an aside, you might like to do

    debug: CFLAGS+=-DFOO

which will just append the flags you need since the rest of CFLAGS might 
still be relevant.

The second way to approach this is to use check what target was 
specified on the command-line of Make and then set CFLAGS conditionally. 
 You can do that like this:

    ifeq ($(MAKECMDGOALS),debug)
    CFLAGS+=-DFOO
    endif

But you are still going to need to hook 'debug' up to the rest of your 
build in some way (above I did this with debug: all), so I think the 
approach above is cleaner.

> 2) Why is the conditional macro definition syntax not incorporated
> into gnu-make? It seems like a good idea.

It is; it's just called a 'target-specific variable' instead.

John.
--
John Graham-Cumming
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