Question

I’m using (GNU) Make in my project. I’m currently putting one makefile per directory and specify the subdirectories using SUBDIRS. It’s been suggested to me that this is not the ideal way of using make, that using a one toplevel make file (or several, split up using include). I’ve tried migrating/using this layout in the past, but it appears to me that it’s unnecessary complicated.

Which are the benefits/drawbacks of using recursive makefiles?

Answer 1

The first thing you should keep in mind (just to eliminate any misunderstanding) is that we’re not talking about a single vs. multiple makefiles. Splitting your makefile in one per subdirectory is probably a good idea in any case.

Recursive makefiles are bad primarily because you partition your dependency tree into several trees. This prevents dependencies between make instances from being expressed correctly. This also causes (parts of) the dependency tree to be recalculated multiple times, which is a performance issue in the end (although usually not a big one.)

There are a couple of tricks you need to use in order to properly use the single-make approach, especially when you have a large code base:

First, use GNU make (you already do, I see). GNU make has a number of features which simplifies things, and you won’t have to worry about compatibilities.

Second, use target-specific variable values. This will allow you to have, for example, different values of CFLAGS for different targets, instead of forcing you to have a single CFLAGS in your entire make:

 main: CFLAGS=-O2  lib: CFLAGS=-O2 -g 

Third, make sure you use VPATH/vpath to the full extent supported by GNU make.

You also want to make sure that you do not have multiple source files with the same name. One limitation of VPATH is that it does not allow you to have target-specific VPATH definitions, so the names of your source files will have to co-exist in a single ‘VPATH namespace’.