Building C-program "out of source tree" with GNU make

Question

I would like to build a C-project for my microcontroller with the GNU make tool. I would like to do it in a clean way, such that my source code is not cluttered with object files and other stuff after the build. So imagine that I have a project folder, called "myProject" with two folders in it:

- myProject
     |
     |---+ source
     |
     '---+ build

The build folder only contains a makefile. The figure below shows what should happen when I run the GNU make tool:

So GNU make should create an object file for each .c source file it can find in the source folder. The object files should be structured in a directory tree that is similar to the structure in the source folder.

GNU make should also make a .d dependency file (in fact, a dependency file is some sort of makefile itself) for each .c source file. The dependency file is described in the GNU make manual chapter 4.14 "Generating Prerequisites Automatically":

For each source file name.c there is a makefile name.d which lists what files the object file name.o depends on.

From the following Stackoverflow question About the GNU make dependency files *.d, I learned that adding the options -MMD and -MP to the CFLAGS of the GNU gcc compiler can help to automate that.

So now comes the question. Has anyone a sample makefile that performs such out-of-source build? Or some good advices on how to get started?

I'm pretty sure that most people who have written such a makefile, are Linux-people. But the microcontroller project should build also on a Windows machine. Anyway, even if your makefile is Linux-only, it provides a good starting point ;-)

PS: I would like to avoid extra tools like CMake, Autotools, or anything that has to do with an IDE. Just pure GNU make.

I would be very grateful :-)

---

Updating the dependency files
Please have a look at this question: What is the exact chain of events when GNU make updates the .d files?

Answer 1

Here's the Makefile I've added to the documentation (currently in review so I'll post it here) :

# Set project directory one level above the Makefile directory. $(CURDIR) is a GNU make variable containing the path to the current working directory
PROJDIR := $(realpath $(CURDIR)/..)
SOURCEDIR := $(PROJDIR)/Sources
BUILDDIR := $(PROJDIR)/Build

# Name of the final executable
TARGET = myApp.exe

# Decide whether the commands will be shown or not
VERBOSE = TRUE

# Create the list of directories
DIRS = Folder0 Folder1 Folder2
SOURCEDIRS = $(foreach dir, $(DIRS), $(addprefix $(SOURCEDIR)/, $(dir)))
TARGETDIRS = $(foreach dir, $(DIRS), $(addprefix $(BUILDDIR)/, $(dir)))

# Generate the GCC includes parameters by adding -I before each source folder
INCLUDES = $(foreach dir, $(SOURCEDIRS), $(addprefix -I, $(dir)))

# Add this list to VPATH, the place make will look for the source files
VPATH = $(SOURCEDIRS)

# Create a list of *.c sources in DIRS
SOURCES = $(foreach dir,$(SOURCEDIRS),$(wildcard $(dir)/*.c))

# Define objects for all sources
OBJS := $(subst $(SOURCEDIR),$(BUILDDIR),$(SOURCES:.c=.o))

# Define dependencies files for all objects
DEPS = $(OBJS:.o=.d)

# Name the compiler
CC = gcc

# OS specific part
ifeq ($(OS),Windows_NT)
    RM = del /F /Q 
    RMDIR = -RMDIR /S /Q
    MKDIR = -mkdir
    ERRIGNORE = 2>NUL || true
    SEP=\\
else
    RM = rm -rf 
    RMDIR = rm -rf 
    MKDIR = mkdir -p
    ERRIGNORE = 2>/dev/null
    SEP=/
endif

# Remove space after separator
PSEP = $(strip $(SEP))

# Hide or not the calls depending of VERBOSE
ifeq ($(VERBOSE),TRUE)
    HIDE =  
else
    HIDE = @
endif

# Define the function that will generate each rule
define generateRules
$(1)/%.o: %.c
    @echo Building $$@
    $(HIDE)$(CC) -c $$(INCLUDES) -o $$(subst /,$$(PSEP),$$@) $$(subst /,$$(PSEP),$$<) -MMD
endef

# Indicate to make which targets are not files
.PHONY: all clean directories 

all: directories $(TARGET)

$(TARGET): $(OBJS)
    $(HIDE)echo Linking $@
    $(HIDE)$(CC) $(OBJS) -o $(TARGET)

# Include dependencies
-include $(DEPS)

# Generate rules
$(foreach targetdir, $(TARGETDIRS), $(eval $(call generateRules, $(targetdir))))

directories: 
    $(HIDE)$(MKDIR) $(subst /,$(PSEP),$(TARGETDIRS)) $(ERRIGNORE)

# Remove all objects, dependencies and executable files generated during the build
clean:
    $(HIDE)$(RMDIR) $(subst /,$(PSEP),$(TARGETDIRS)) $(ERRIGNORE)
    $(HIDE)$(RM) $(TARGET) $(ERRIGNORE)
    @echo Cleaning done ! 

---

Main features

• Automatic detection of C sources in specified folders
• Multiple source folders
• Multiple corresponding target folders for object and dependency files
• Automatic rule generation for each target folder
• Creation of target folders when they don't exist
• Dependency management with gcc : Build only what is necessary
• Works on Unix and DOS systems
• Written for GNU Make

---

How to use this Makefile

To adapt this Makefile to your project you have to :

1. Change the TARGET variable to match your target name
2. Change the name of the Sources and Build folders in SOURCEDIR and BUILDDIR
3. Change the verbosity level of the Makefile in the Makefile itself or in make call (make all VERBOSE=FALSE)
4. Change the name of the folders in DIRS to match your sources and build folders
5. If required, change the compiler and the flags

In this Makefile Folder0, Folder1 and Folder2 are the equivalent to your FolderA, FolderB and FolderC.

Note that I have not had the opportunity to test it on a Unix system at the moment but it works correctly on Windows.

---

Explanation of a few tricky parts :

Ignoring Windows mkdir errors

ERRIGNORE = 2>NUL || true

This has two effects : The first one, 2>NUL is to redirect the error output to NUL, so as it does not comes in the console.

The second one, || true prevents the command from rising the error level. This is Windows stuff unrelated with the Makefile, it's here because Windows' mkdir command rises the error level if we try to create an already-existing folder, whereas we don't really care, if it does exist that's fine. The common solution is to use the if not exist structure, but that's not UNIX-compatible so even if it's tricky, I consider my solution more clear.

---

Creation of OBJS containing all object files with their correct path

OBJS := $(subst $(SOURCEDIR),$(BUILDDIR),$(SOURCES:.c=.o))

Here we want OBJS to contain all the object files with their paths, and we already have SOURCES which contains all the source files with their paths. $(SOURCES:.c=.o) changes *.c in *.o for all sources, but the path is still the one of the sources. $(subst $(SOURCEDIR),$(BUILDDIR), ...) will simply subtract the whole source path with the build path, so we finally have a variable that contains the .o files with their paths.

---

Dealing with Windows and Unix-style path separators

SEP=\\
SEP = /
PSEP = $(strip $(SEP))

This only exist to allow the Makefile to work on Unix and Windows, since Windows uses backslashes in path whereas everyone else uses slashes.

SEP=\\ Here the double backslash is used to escape the backslash character, which make usually treats as an "ignore newline character" to allow writing on multiple lines.

PSEP = $(strip $(SEP)) This will remove the space char of the SEP variable, which has been added automatically.

---

Automatic generation of rules for each target folder

define generateRules
$(1)/%.o: %.c
    @echo Building $$@
    $(HIDE)$(CC) -c $$(INCLUDES) -o $$(subst /,$$(PSEP),$$@)   $$(subst /,$$(PSEP),$$<) -MMD
endef

That's maybe the trick that is the most related with your usecase. It's a rule template that can be generated with $(eval $(call generateRules, param)) where param is what you can find in the template as $(1). This will basically fill the Makefile with rules like this for each target folder :

path/to/target/%.o: %.c
    @echo Building $@
    $(HIDE)$(CC) -c $(INCLUDES) -o $(subst /,$(PSEP),$@)   $(subst /,$(PSEP),$<) -MMD