In a makefile, how to get the relative path from one absolute path to another?

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.

Answer 2

Python is portable! So, I would suggest you this simple example in your submakefile

With current_dir and destination_dir paths os.path.relpath() does the job for you so you do not have to re-invent the wheel.

submakefile.mk

current_dir=$(CURDIR)
makefile_target:
    (echo "import os"; echo "print os.path.relpath('$(current_dir)', '$(destination_dir)')" )| python

Answer 3

Didier's answer is the best one, but the following might give you some ideas:

includedir=/a/b/c/d
currentdir=/a/b/e/f/g
up=; while ! expr $includedir : $currentdir >/dev/null; do up=../$up; currentdir=`dirname $currentdir`; done; relative=$up`expr $includedir : $currentdir'/*\(.*\)'`
echo "up=$up  currentdir=$currentdir, relative=$relative"

Sorted!

(no-one said it had to be pretty...)