gnuMake, How to have an environment variable override

Question

Currently I am working with Makefiles that have definitions like

MYLIB=/.../mylib-1.2.34

The problem is that these are different for different developers, and it is a pain having to re-edit the file after every checkout.

So I tried exporting a specific environment variable, and then doing

MYLIBX:=$(MYLIB_ENV)
MYLIBX?=MYLIB

Trouble is that if MYLIB_ENV is not defined, it still creates an empty MYLIBX, so the ?= does not work. Is there a clean way to do this very basic thing?

I am working with a "rich" set of make files developed over many years that do all sorts of things like make and call each other, so changing things deeply is not an option.

SOLUTION

Double shuffle. MYLIB already defined.

MYLIB_ENV?=MYLIB
MYLIB:=MYLIB_ENV

Answer 1

The problem with

MYLIB:=$(MYLIB_ENV)
MYLIB?=/.../mylib-1.2.34

is that MYLIB is always defined in the first line, so the second never applies.

The typical approach in this situation is just

MYLIB?=/.../mylib-1.2.34

That way individual developers can specify their own value from the shell, either on the make command line

make MYLIB=...

or in their environment before running make

export MYLIB=...
make

(so they can set it once, e.g. in their shell startup scripts, and forget about it).

If you just run

make

without specifying a value for MYLIB, the default /.../mylib-1.2.34 is used.

Another option is to determine where the Makefile is stored, but that doesn't work in all cases (in particular if the path to the Makefile contains spaces).

Answer 2

It's not very clear what it is exactly that you are after. But in case you want that you need to override the settings of the make variable MYLIB to something different from what it is specified in the Makefile WITHOUT having to edit the Makefile for that, then it is very simple: In your make invocations, specify MYLIB on the commandline with a value of your choice , like as:

make -f yourmakefile yourtarget MYLIB="I_need_my_personal_value"

Another method :

MYLIB := ORIGINAL_VALUE

ifneq '$(MYLIB_ENV)' ''
MYLIB := $(MYLIB_ENV)
endif