Makefiles with source files in different directories

I have a project where the directory structure is like this:

                         $projectroot
                              |
              +---------------+----------------+
              |               |                |
            part1/          part2/           part3/
              |               |                |
       +------+-----+     +---+----+       +---+-----+
       |      |     |     |        |       |         |
     data/   src/  inc/  src/     inc/   src/       inc/

How should I write a makefile that would be in part/src (or wherever really) that could comple/link on the c/c++ source files in part?/src ?

Can I do something like -I$projectroot/part1/src -I$projectroot/part1/inc -I$projectroot/part2/src ...

If that would work, is there an easier way to do it. I've seen projects where there is a makefile in each of the corresponding part? folders. [in this post I used the question mark like in bash syntax]

The traditional way is to have a Makefile in each of the subdirectories (part1, part2, etc.) allowing you to build them independently. Further, have a Makefile in the root directory of the project which builds everything. The "root" Makefile would look something like the following:

all:
    +$(MAKE) -C part1
    +$(MAKE) -C part2
    +$(MAKE) -C part3

Since each line in a make target is run in its own shell, there is no need to worry about traversing back up the directory tree or to other directories.

I suggest taking a look at the GNU make manual section 5.7; it is very helpful.

If you have code in one subdirectory dependent on code in another subdirectory, you are probably better off with a single makefile at top-level.

See Recursive Make Considered Harmful for the full rationale, but basically you want make to have the full information it needs to decide whether or not a file needs to be rebuilt, and it won't have that if you only tell it about a third of your project.

The link above seems to be not reachable. The same document is reachable here:

• aegis.sourceforge.net (archived)
• lcgapp.cern.ch

The VPATH option might come in handy, which tells make what directories to look in for source code. You'd still need a -I option for each include path, though. An example:

CXXFLAGS=-Ipart1/inc -Ipart2/inc -Ipart3/inc
VPATH=part1/src:part2/src:part3/src

OutputExecutable: part1api.o part2api.o part3api.o

This will automatically find the matching partXapi.cpp files in any of the VPATH specified directories and compile them. However, this is more useful when your src directory is broken into subdirectories. For what you describe, as others have said, you are probably better off with a makefile for each part, especially if each part can stand alone.