Sources from subdirectories in Makefile

Question

I have a C++ library built using a Makefile. Until recently, all the sources were in a single directory, and the Makefile did something like this

SOURCES = $(wildcard *.cpp)

which worked fine.

Now I've added some sources that are in a subdirectory, say subdir. I know I can do this

SOURCES = $(wildcard *.cpp) $(wildcard subdir/*.cpp)

but I'm looking for a way to avoid specifying subdir manually, that is, make wildcard look into subdirectories, or generating a list of subdirectories somehow and expanding it with several wildcard functions. At this point, having a non-recursive solution (that is, expanding only the first level) would be fine.

I haven't found anything - my best guess is using find -type d to list the subdirectories, but it feels like a hack. Is there any built-in way to do this?

Answer 1

Common practice is to put a Makefile in each subdir with sources, then

all: recursive
    $(MAKE) -C componentX
    # stuff for current dir

or

all: recursive
    cd componentX && $(MAKE)
    # stuff for current dir

recursive: true

It may be wise to put settings for each Makefile in a Makefile.inc in the root source directory. The recursive target forces make to go into the subdirectories. Make sure that it doesn't recompile anything in a target requiring recursive.

If you have many subdirectories, use a loop:

SUBDIRS = componentX componentY

all: recursive
    for i in $(SUBDIRS); do \
        cd "$$i" && $(MAKE) $@; \
    done

(Same for install and other major targets.)

Answer 2

This is a side note and do not answer to your question but there is a paper "Recursive Make Considered Harmful". It's worth reading.

Here is the link. http://aegis.sourceforge.net/auug97.pdf

Answer 3

If you don't want to use recursive makefiles, this might give you some ideas:

subdirs := $(wildcard */)
sources := $(wildcard $(addsuffix *.cpp,$(subdirs)))
objects := $(patsubst %.cpp,%.o,$(sources))

$(objects) : %.o : %.cpp

Answer 4

This should do it:

SOURCES = $(wildcard *.cpp) $(wildcard */*.cpp)

If you change you mind and want a recursive solution (i.e. to any depth), it can be done but it involves some of the more powerful Make functions. You know, the ones that allow you to do things you really shouldn't.

EDIT:
Jack Kelly points out that $(wildcard **/*.cpp) works to any depth, at least on some platforms, using GNUMake 3.81. (How he figured that out, I have no idea.)