lauantai 23. joulukuuta 2017

"A simple makefile" is a unicorn

Whenever there is a discussion online about the tools to build software, there is always That One Person that shows up and claims that all build tools are useless bloated junk and that you should "just write a simple Makefile" because that is lean, efficient, portable and does everything anyone could ever want.

Like every sentence that has the word "just", this is at best horribly simplistic but mostly plain wrong. Let's dive in more detail into this. If you look up simple Makefiles on the Internet, you might find something like this page. It starts with a very simple (but useless) Makefile and eventually improves it to this:

IDIR =../include

LDIR =../lib


_DEPS = hellomake.h
DEPS = $(patsubst %,$(IDIR)/%,$(_DEPS))

_OBJ = hellomake.o hellofunc.o 
OBJ = $(patsubst %,$(ODIR)/%,$(_OBJ))

$(ODIR)/%.o: %.c $(DEPS)
$(CC) -c -o $@ $< $(CFLAGS)

hellomake: $(OBJ)
gcc -o $@ $^ $(CFLAGS) $(LIBS)

.PHONY: clean

rm -f $(ODIR)/*.o *~ core $(INCDIR)/*~ 

Calling this "simple" is a bit of a stretch. This snippet contains four different kinds of magic expansion variables, calls three external commands (two of which are gcc, just with different ways) and one Make's internal command (bonus question: is patsubst a GNU extension or is it available in BSD Make? what about NMake?) and requires the understanding of shell syntax. It is arguable whether this could be called "simple", especially for newcomers. But even so, this is completely broken and unreliable.

As an example, if you change any header files used by the sources, the system will not rebuild the targets. To fix these issues you need to write more Make. Maybe something like this example, described as A Super-Simple Makefile for Medium-Sized C/C++ Projects:

TARGET_EXEC ?= a.out

BUILD_DIR ?= ./build
SRC_DIRS ?= ./src

SRCS := $(shell find $(SRC_DIRS) -name *.cpp -or -name *.c -or -name *.s)
OBJS := $(SRCS:%=$(BUILD_DIR)/%.o)
DEPS := $(OBJS:.o=.d)

INC_DIRS := $(shell find $(SRC_DIRS) -type d)
INC_FLAGS := $(addprefix -I,$(INC_DIRS))


$(CC) $(OBJS) -o $@ $(LDFLAGS)

# assembly
$(BUILD_DIR)/%.s.o: %.s
$(MKDIR_P) $(dir $@)
$(AS) $(ASFLAGS) -c $< -o $@

# c source
$(BUILD_DIR)/%.c.o: %.c
$(MKDIR_P) $(dir $@)
$(CC) $(CPPFLAGS) $(CFLAGS) -c $< -o $@

# c++ source
$(BUILD_DIR)/%.cpp.o: %.cpp
$(MKDIR_P) $(dir $@)
$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@

.PHONY: clean

$(RM) -r $(BUILD_DIR)

-include $(DEPS)

MKDIR_P ?= mkdir -p

It's unclear what the appropriate word to describe this thing is, but simple would not be at the top of the list for many people.

Even this improved version is broken and unreliable. The biggest issue is that changing compiler flags does not cause a recompile, only timestamps do. This is a common reason for silent build failures. It also does not provide for any way to configure the build depending on the OS in use. Other missing pieces that should be considered entry level features for build systems include:

  • No support for multiple build types (debug, optimized), changing build settings requires editing the Makefile
  • Output directory is hardcoded, you can't have many build directories with different setups
  • No install support
  • Does not work with Visual Studio
  • No unit testing support
  • No support for sanitizers apart from manually adding compiler arguments
  • No support for building shared libraries, apart from manually adding compiler arguments (remember to add -shared in your object file compile args ... or was it on link args ... or was it -fPIC)
  • No support for building static libraries at all
  • And so on and so on

As an example of a slightly more advanced feature, cross compilation is not supported at all.

These are all things you can add to this supposedly super simple Makefile, but the result will be a multi-hundred (thousand?) line monster of non-simplicityness.


Simple makefiles are a unicorn. A myth. They are figments of imagination that have not existed, do not exist and will never exist. Every single case of a supposedly simple Makefile has turned out to be a mule with a carrot glued to its forehead. The time has come to let this myth finally die.

3 kommenttia:

  1. That is hell of a wishlist for a build system to call it simple. Make is tool agnostic so, yeah, you should know how to use your toolchain (and where to pass “-shared”). No support for building static libraries?! LIke at all?! I start to think you do not know make to take you setiously.