The smart way to use make in a directory tree

[Josh Bialkowski]

I'm going to start with a little story and then describe the question 
that it leads me to:

I started of with one c++ project. I started a second project but wanted 
to reuse some of that code, so I copied the directory structure and 
started my second project. Now I've started a third project for which I 
would like to reuse some of the code from both of the previous projects. 
I would like to setup these projects modularly using libraries so that I 
can use the same backend libraries in all three projects, but when I 
find bugs I can fix them in the library and I dont have to fix it two or 
three times.

So each of the libraries is located in it's own subdirectory and has 
it's own makefile. Kind of like this:


src
|-- libA
|-- libB
|-- libC
|-- libD
|-- projA (depends on libA, libB, libD)
|-- projB (depends on libB, libC, libD)
|-- projC (depends on libC, libD, libD)


Now my question: I'm looking for a "smart" way to handle the make 
structure for the different projects, so that when I "make" a project 
(i.e. projA) it will remake any of the libraries it depends on (i.e. 
libs A, B, D) if any of their dependencies have changed. Right now I 
have one makefile in each subdirectory (i.e. libA/makefile libB/makefile 
projA/makefile). In order to build project A I have to manually cd into 
each of the libraries and make them (if I've changed any code in there) 
and then cd into the project directory and rebuild that project.

The biggest problem is that these libraries aren't stable enough to 
really justify keeping them separate. As I'm working on the different 
projects, I'm frequently making changes to the libraries as well. I'd 
like to treat this as a unified "code base" composed of several modules 
which are combined into a couple of different programs.

I have come up with a couple of different ideas but they are too 
involved and hacky... I have to believe there is a more "standard" way 
of doing this kind of thing. There's probably even a way of dealing with 
trees with arbitrary depth (though my example is only depth 1).

Any suggestions would be greatly appreciated.



--
Josh Bialkowski
Ph.D. Candidate
Aerospace Robotics and Embedded Systems Group
Laboratory for Information and Decision Systems
Massachusetts Institute of Technology