Question

An example to illustrate my question:

Top level makefile

rootdir = $(realpath .)
export includedir = $(rootdir)/include
default:
    @$(MAKE) --directory=$(rootdir)/src/libs/libfoo

Makefile for src/libfoo

currentdir = $(realpath .)
includedir = $(function or magic to make a relative path
               from $(currentdir) to $(includedir),
               which in this example would be ../../../include)

Another example:

current dir = /home/username/projects/app/trunk/src/libs/libfoo/ 
destination = /home/username/projects/app/build/libfoo/ 
relative    = ../../../../build/libfoo

How can this be done, while trying to be as portable as possible?

Answer 1

Doing what you want does not look easy. It may be possible using a lot of combined $(if in the makefile but not portable (gmake only) and cumbersome.

IMHO, you are trying to solve a problem that you create yourself. Why don’t you send the correct value of includedir as a relative path from the Top-level Makefile? It can be done very easily as follows:

rootdir = $(realpath .)
default:
    @$(MAKE) --directory=$(rootdir)/src/libs/libfoo includedir=../../../include

Then you can use $(includedir) in the sub-makefiles. It is already defined as relative.