A small make orientation guide.

The Makefile (capital M) is parsed by GNU Make, which is responsible for generating the various GCC compiler commands, required to compile this source code. For example, I could issue individual compiler commands like this to create my binaries:

gcc -Wall -g -std=gnu11 -O0 -Iinclude -Iinclude/engine `pkg-config --cflags glib-2.0` -c -o main.o src/main.c

However, this is tedious, error prone and slow. I don’t want to have to keep a list of source files I modified, that require rebuilding. Worse, things dependant on changes also need to be rebuilt, forcing me to constantly consider the dependency graph. What a chore. GNU make to the rescue.


# Makefile for transpiling with Babel in a Node app, or in a client- or
# server-side shared library.

.PHONY: all clean

# Install `babel-cli` in a project to get the transpiler.
babel := node_modules/.bin/babel

# Identify modules to be transpiled by recursively searching the `src/`
# directory.
src_files := $(shell find src/ -name '*.js')

# Building will involve copying every `.js` file from `src/` to a corresponding
# file in `lib/` with a `.js.flow` extension. Then we will run `babel` to
# transpile copied files, where the transpiled file will get a `.js` extension.
# This assignment computes the list of transpiled `.js` that we expect to end up;
# and we will work backward from there to figure out how to build them.
transpiled_files := $(patsubst src/%,lib/%,$(src_files))

# Putting each generated file in the same directory with its corresponding
# source file is important when working with Flow: during type-checking Flow
# will look in npm packages for `.js.flow` files to find type definitions. So
# putting `.js` and `.js.flow` files side-by-side is how you export type
# definitions from a shared library.

# Compute the list of type-definition source files that we want to end up with.
# This is done by replacing the `.js` extension from every value in the
# `transpiled_files` list with a `.js.flow` extension.
flow_files := $(patsubst %.js,%.js.flow,$(transpiled_files))

# Ask `make` to build all of the transpiled `.js` and `.js.flow` files that we
# want to end up with in `lib/`.
# This target also depends on the `node_modules/` directory, so that `make`
# automatically runs `yarn install` if `package.json` has changed.
all: node_modules $(flow_files) $(transpiled_files)

# This rule tells `make` how to transpile a source file using `babel`.
# Transpiled files will be written to `lib/`
lib/%: src/%
	mkdir -p $(dir $@)
	$(babel) $< --out-file $@ --source-maps

# Transpiling one file at a time makes incremental transpilation faster:
# `make` will only transpile source files that have changed since the last
# invocation.

# This rule tells `make` how to produce a `.js.flow` file. It is just a copy of
# a source file - the rule copies a file from `src/` to `lib/` and changes the
# extension.
lib/%.js.flow: src/%.js
	mkdir -p $(dir $@)
	cp $< $@

	rm -rf lib

# This rule informs `make` that the `node_modules/` directory is out-of-date
# after changes to `package.json` or `yarn.lock`, and instructs `make` on how to
# install modules to get back up-to-date.
node_modules: package.json yarn.lock
	yarn install

This tiny Makefile showcases many of the useful features of make, such as variables, implicit variables, recipes, path probing with vpath, automatic make variables (e.g. $<, $@, $*), phony targets and utility functions. RTFM for more.

Make essentials

make generates files from other files, using recipes, the syntax is as follows. Please note, thanks to POSIX standardisation the recipe MUST be indented with a tab (not spaces):

target_file: prerequisite_file1 prerequisite_file2
	shell command to build target_file (MUST be indented with tabs, not spaces)
	another shell command (these commands are called the "recipe")

Unless you specify otherwise, Make assumes that the target (target_file above) and prerequisites (prerequisite_file1 and prerequisite_file2) are actual files or directories. You can ask Make to build a target from the command line like this:

$ make target_file

If the target_file does not exist, or if prerequisite_file1 or prerequisite_file2 have been modified since target_file was last built, Make will run the given shell commands. But first Make will check to see if there are recipes in the Makefile for prerequisite_file1 and prerequisite_file2 and build or rebuild those if necessary.

An example of this in action:

OBJECTS= main.o safe_sum.o

all: $(P)

$(P): $(OBJECTS)
    $(CC) $(CFLAGS) -o $(P) $(OBJECTS) $(LDLIBS)

The first target, in this case all is the default target, and is dependent on my project seething. In order to build the binary seething, make moves on looking for the seething target, which is dependent on objects main.o and safe_sum.o. The recipe defines instructions make should use to invoke the C compiler, which will produce an output binary of seething. Luckily we don’t have to explain to make how to build main.o and safe_sum.o, because make is smart enough to infer a default build recipe, because we are dealing with C, will be $(CC) $(CFLAGS) $(LDFLAGS) -o $@ $*.c, concretely for safe_sum.o this will wind up being gcc -Wall -g -std=gnu11 -O0 -Iinclude -o safe_sum.o safe_sum.c

Equal signs

About the equal signs:

:= simple shallow evaluation = recursively expands defined variables

A := Did $(C)
B = Did $(C)
C  = you understand?

    $(info $(A)) # output "Did"
    $(info $(B)) # output "Did you understand?"

Built-in variables

  • $@ current target
  • $< name of the first prerequisite
  • $^ all prerequisites
  • $* target with no suffix (e.g. prog.o, $* is just prog)
  • $(@D) directory part of the file name of the target
  • $(@F) file part of the file name of the target

Phony targets

A phony target prevents make from confusing it with a real file. For example, with .PHONY, even if a file called ‘clean’ is actually created, make clean will still execute.

.PHONY: clean
  rm -f $(P)
  rm -f *.o
  rm -f *.log

Utility functions

make provides a treasure trove of handy utility functions for common tasks such as transforming text. Below uses filter-out to strip out a couple of CFLAG options:

the_day=$(shell date +%A)

.PHONY: stringfun
  $(info $(filter-out -Wall -g -O0,$(CFLAGS)))
  $(info Ben it's $(the_day)!)

Running this:

[ben@think]$ make stringfun
-std=gnu11 -Iinclude -Iinclude/engine `pkg-config --cflags glib-2.0`
Ben it's Sunday!
make: Nothing to be done for 'stringfun'.

Other bits

make -p will dump out implicit rules and variables available

C specifics

Custom variables

OBJECTS= main.o safe_sum.o
the_day=$(shell date +%A)

Implicit variables

make is a generic system for generating files, based on other files. It is in no way, specific to a C compiler, and can be used to drive any compiler. Make is a great platform for driving common tasks, such as producing documentation, or running a test suite. To ease working with common languages and compilers, make does have a general awareness, for example, of how to (agnostically) drive a C compiler. If it detects its working with a C, standard (or implicit) variables such as CC (name of desired C compiler), CFLAGS (compiler flags), LDFLAGS (linker flags) kick in. See implicit variables for more.

Program for compiling C programs

CC=gcc  # CC = C compiler to use
CFLAGS= -Wall -g -std=gnu11 -O3 -Iinclude -Iinclude/engine  # flags to pass to C compiler

If you’re not familiar with GCC:

  • -Wall adds compiler warnings
  • -g adds symbols for debugging
  • -std=gnu11 compiler should allow code conforming to the C11 and POSIX standards
  • -O3 set optimization level three, which applies every clever trick to make your code as fast as possible
  • -I include paths

While in CFLAGS territory, its important to make note of pkg-config:

pkg-config - Return metainformation about installed libraries

Hard coding include paths to third party dependencies works, however it is likely to become a maintenance burden in the future, worse if there are plans to share this Makefile with other developers, it is unlikely to work on their system (i.e. poor portability). For example, my project requires the GNOME core utility library GLib. I could append the hardcoded path to my CFLAG like this:

CFLAGS+= -I/usr/include/glib-2.0

However, a slightly better approach is to shell out to pkg-config which will return the neatly formatted -I, using paths based on my machines installation paths. For example:

CFLAGS+= `pkg-config --cflags glib-2.0`

Linker flags LDFLAGS for everything non-library related (e.g. -L).

  • -L (e.g. -L/usr/local/lib) is where to search for libraries to resolve symbols

Libraries that the linker (ld) needs to, link in.

LDFLAGS+= -lglib-2.0 # hard-coded version

Again, pkg-config can do linker flags, for more robust Makefiles like this:

LDLIBS+= `pkg-config --libs glib-2.0 gobject-2.0 gio-2.0`

LDADD (ADDitional linker/ld) is useful for feeding any other additional terms to the linker, for example LDADD= -Llibpath -Wl,-Rlibpath:

  • -L flag tells the compiler where to search for libraries to resolve symbols
  • -Wl flag passes its flags through from gcc to the linker
  • -R the linker will embed these into the runtime search path for libraries to link to

VPATH specifies a list of directories that make should search: