[Interest] qmake: Test if environment variable is set.

[Thiago Macieira]

On Wednesday 27 May 2015 00:10:19 Thiago Macieira wrote:
> On Wednesday 27 May 2015 09:03:24 Carel Combrink wrote:
> > Am I doing something wrong?
> 
> Yes, you're expecting qmake to observe environment variables. It does not.
> Make honours and processes them, but that's run after qmake has finished
> running and has written the Makefile. Two separate and independent steps.

Actually, I was partially right, but the explanation is wrong.

When you write:

	SOURCES = $(FOO)

Then qmake will write to the Makefile:

SOURCES = $(FOO)

and like I said in the email above, Make will expand and do its job.

When you write:
isEmpty(SOURCES)

then qmake looks at SOURCES and sees it's *not* empty. It contains one item: 
$(FOO) (literally).

When you write:
	message("Abc $$SOURCES def")

Then qmake will do the replacement and get the string "Abc $(FOO) def". At 
that point, it notices the environment expansion and expands that, at that 
time. If you run with debugging enabled, you'll see it saying:

DEBUG 2: literal "Abc $(FOO) def"
DEBUG 2: evaluated expression => "Abc $(FOO) def"
DEBUG 1: /dev/stdin:1: calling built-in message("Abc $(FOO) def")
Project MESSAGE: Abc  def

So TEST_3 isn't empty. Only the output from message was empty because it 
performed an extra expansion.

The rest of the email was right:

> > Is this intended behaviour?
> 
> Yes.
> 
> > How can I proceed to get the correct result?
> 
> You cannot with qmake. You need to either make the variable a pure qmake one
> by passing it on the qmake command-line instead of the environment, or you
> skip doing the qmake checks and rely on Make rules only.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center