Oftentimes, when dealing with complex projects, there are several components we need to build and link to produce the final product.

When dealing with such projects, it is common to organize components source code into subdirectories of the main project, for example:

project_main_dir
      |
      +------------ subdir-1
      |
      +------------ subdir-2
      |
      +------------ ...
      |
      +------------ subdir-n

When building code in such situations we need a way to call subdirectories Makefiles from the project root directory, namely project_main_dir.

We can do that with the following Makefile, which:

1. Declares a list of all subdirectories that needs to be built.
2. Loops over the list to call the make command from each subdirectory with the make -C <subdir> option, which basically tells the make command to change to the specified directory before executing.

Here’s the source code for the project_main_dir Makefile:

SUBDIRS = subdir-1 subdir-2 subdir-n

all:
	for dir in $(SUBDIRS); do	\
		$(MAKE) -C $$dir;	\
	done

clean:
	for dir in $(SUBDIRS); do	\
		$(MAKE) -C $$dir clean;	\
	done