Make

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Getting Started Why do Makefiles exist?

Makefiles are used to help decide which parts of a large program need to be recompiled. In the vast majority of cases, C or C++ files are compiled. Other languages typically have their own tools that serve a similar purpose as Make. It can be used beyond programs too, when you need a series of instructions to run depending on what files have changed. This tutorial will focus on the C/C++ compilation use case.

Here's an example dependency graph that you might build with Make. If any file's dependencies changes, then the file will get recompiled: What alternatives are there to Make?

Popular C/C++ alternative build systems are SCons, CMake, Bazel, and Ninja. Some code editors like Microsoft Visual Studio have their own built in build tools. For Java, there's Ant, Maven, and Gradle. Other languages like Go and Rust have their own build tools.

Interpreted languages like Python, Ruby, and Javascript don't require an analogue to Makefiles. The goal of Makefiles is to compile whatever files need to be compiled, based on what files have changed. But when files in interpreted languages change, nothing needs to get recompiled. When the program runs, the most recent version of the file is used. Running the Examples

To run these examples, you'll need a terminal and "make" installed. For each example, put the contents in a file called Makefile, and in that directory run the command make. Let's start with the simplest of Makefiles:

hello:

   echo "hello world"

Here is the output of running the above example:

$ make echo "hello world" hello world

That's it! If you're a bit confused, here's a video that goes through these steps, along with describing the basic structure of Makefiles. Makefile Syntax

A Makefile consists of a set of rules. A rule generally looks like this:

targets: prerequisites

  command
  command
  command
   The targets are file names, separated by spaces. Typically, there is only one per rule.
   The commands are a series of steps typically used to make the target(s). These need to start with a tab character, not spaces.
   The prerequisites are also file names, separated by spaces. These files need to exist before the commands for the target are run. These are also called dependencies

Beginner Examples

The following Makefile has three separate rules. When you run make blah in the terminal, it will build a program called blah in a series of steps:

   Make is given blah as the target, so it first searches for this target
   blah requires blah.o, so make searches for the blah.o target
   blah.o requires blah.c, so make searches for the blah.c target
   blah.c has no dependencies, so the echo command is run
   The cc -c command is then run, because all of the blah.o dependencies are finished
   The top cc command is run, because all the blah dependencies are finished
   That's it: blah is a compiled c program

blah: blah.o

   cc blah.o -o blah # Runs third

blah.o: blah.c

   cc -c blah.c -o blah.o # Runs second

blah.c:

   echo "int main() { return 0; }" > blah.c # Runs first

This makefile has a single target, called some_file. The default target is the first target, so in this case some_file will run.

some_file:

   echo "This line will always print"

This file will make some_file the first time, and the second time notice it's already made, resulting in make: 'some_file' is up to date.

some_file:

   echo "This line will only print once"
   touch some_file

Here, the target some_file "depends" on other_file. When we run make, the default target (some_file, since it's first) will get called. It will first look at its list of dependencies, and if any of them are older, it will first run the targets for those dependencies, and then run itself. The second time this is run, neither target will run because both targets exist.

some_file: other_file

   echo "This will run second, because it depends on other_file"
   touch some_file

other_file:

   echo "This will run first"
   touch other_file

This will always run both targets, because some_file depends on other_file, which is never created.

some_file: other_file

   touch some_file

other_file:

   echo "nothing"

clean is often used as a target that removes the output of other targets, but it is not a special word in make.

some_file:

   touch some_file

clean:

   rm -f some_file

Variables

Variables can only be strings. Here's an example of using them:

files = file1 file2 some_file: $(files)

   echo "Look at this variable: " $(files)
   touch some_file

file1:

   touch file1

file2:

   touch file2

clean:

   rm -f file1 file2 some_file

Reference variables using ${} or $()

x = dude

all:

   echo $(x)
   echo ${x}
   # Bad practice, but works
   echo $x

Targets The all target

Making multiple targets and you want all of them to run? Make an all target.

all: one two three

one:

   touch one

two:

   touch two

three:

   touch three

clean:

   rm -f one two three

Multiple targets

When there are multiple targets for a rule, the commands will be run for each target $@ is an automatic variable that contains the target name.

all: f1.o f2.o

f1.o f2.o:

   echo $@
  1. Equivalent to:
  2. f1.o
  3. echo $@
  4. f2.o
  5. echo $@

Automatic Variables and Wildcards

  • Wildcard

Both * and % are called wildcards in Make, but they mean entirely different things. * searches your filesystem for matching filenames. I suggest that you always wrap it in the wildcard function, because otherwise you may fall into a common pitfall described below. It's oddly unhelpful and I find it more confusing than useful.

  1. Print out file information about every .c file

print: $(wildcard *.c)

   ls -la  $?
  • may be used in the target, prerequisites, or in the wildcard function.

Danger: * may not be directly used in a variable definitions

Danger: When * matches no files, it is left as it is (unless run in the wildcard function)

thing_wrong := *.o # Don't do this! '*' will not get expanded thing_right := $(wildcard *.o)

all: one two three four

  1. Fails, because $(thing_wrong) is the string "*.o"

one: $(thing_wrong)

  1. Stays as *.o if there are no files that match this pattern :(

two: *.o

  1. Works as you would expect! In this case, it does nothing.

three: $(thing_right)

  1. Same as rule three

four: $(wildcard *.o)

% Wildcard

% is really useful, but is somewhat confusing because of the variety of situations it can be used in.

   When used in "matching" mode, it matches one or more characters in a string. This match is called the stem.
   When used in "replacing" mode, it takes the stem that was matched and replaces that in a string.
   % is most often used in rule definitions and in some specific functions.

See these sections on examples of it being used:

   Static Pattern Rules
   Pattern Rules
   String Substitution
   The vpath Directive

Automatic Variables

There are many automatic variables, but often only a few show up:

hey: one two

   # Outputs "hey", since this is the first target
   echo $@
   # Outputs all prerequisites newer than the target
   echo $?
   # Outputs all prerequisites
   echo $^
   touch hey

one:

   touch one

two:

   touch two

clean:

   rm -f hey one two

Fancy Rules Static Pattern Rules

Make loves c compilation. And every time it expresses its love, things get confusing. Here's the syntax for a new type of rule called a static pattern:

targets ...: target-pattern: prereq-patterns ...

  commands

The essence is that the given target is matched by the target-pattern (via a % wildcard). Whatever was matched is called the stem. The stem is then substituted into the prereq-pattern, to generate the target's prereqs.

A typical use case is to compile .c files into .o files. Here's the manual way:

objects = foo.o bar.o all.o all: $(objects)

  1. These files compile via implicit rules

foo.o: foo.c bar.o: bar.c all.o: all.c

all.c:

   echo "int main() { return 0; }" > all.c

%.c:

   touch $@

clean:

   rm -f *.c *.o all

Here's the more efficient way, using a static pattern rule:

objects = foo.o bar.o all.o all: $(objects)

  1. These files compile via implicit rules
  2. Syntax - targets ...: target-pattern: prereq-patterns ...
  3. In the case of the first target, foo.o, the target-pattern matches foo.o and sets the "stem" to be "foo".
  4. It then replaces the '%' in prereq-patterns with that stem

$(objects): %.o: %.c

all.c:

   echo "int main() { return 0; }" > all.c

%.c:

   touch $@

clean:

   rm -f *.c *.o all

Static Pattern Rules and Filter

While I introduce functions later on, I'll forshadow what you can do with them. The filter function can be used in Static pattern rules to match the correct files. In this example, I made up the .raw and .result extensions.

obj_files = foo.result bar.o lose.o src_files = foo.raw bar.c lose.c

all: $(obj_files)

$(filter %.o,$(obj_files)): %.o: %.c

   echo "target: $@ prereq: $<"

$(filter %.result,$(obj_files)): %.result: %.raw

   echo "target: $@ prereq: $<" 

%.c %.raw:

   touch $@

clean:

   rm -f $(src_files)

Implicit Rules

Perhaps the most confusing part of make is the magic rules and variables that are made. Here's a list of implicit rules:

   Compiling a C program: n.o is made automatically from n.c with a command of the form $(CC) -c $(CPPFLAGS) $(CFLAGS)
   Compiling a C++ program: n.o is made automatically from n.cc or n.cpp with a command of the form $(CXX) -c $(CPPFLAGS) $(CXXFLAGS)
   Linking a single object file: n is made automatically from n.o by running the command $(CC) $(LDFLAGS) n.o $(LOADLIBES) $(LDLIBS)

As such, the important variables used by implicit rules are:

   CC: Program for compiling C programs; default cc
   CXX: Program for compiling C++ programs; default G++
   CFLAGS: Extra flags to give to the C compiler
   CXXFLAGS: Extra flags to give to the C++ compiler
   CPPFLAGS: Extra flags to give to the C preprocessor
   LDFLAGS: Extra flags to give to compilers when they are supposed to invoke the linker

CC = gcc # Flag for implicit rules CFLAGS = -g # Flag for implicit rules. Turn on debug info

  1. Implicit rule #1: blah is built via the C linker implicit rule
  2. Implicit rule #2: blah.o is built via the C compilation implicit rule, because blah.c exists

blah: blah.o

blah.c:

   echo "int main() { return 0; }" > blah.c

clean:

   rm -f blah*

Pattern Rules

Pattern rules are often used but quite confusing. You can look at them as two ways:

   A way to define your own implicit rules
   A simpler form of static pattern rules

Let's start with an example first:

  1. Define a pattern rule that compiles every .c file into a .o file

%.o : %.c

       $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@

Pattern rules contain a '%' in the target. This '%' matches any nonempty string, and the other characters match themselves. ‘%’ in a prerequisite of a pattern rule stands for the same stem that was matched by the ‘%’ in the target.

Here's another example:

  1. Define a pattern rule that has no pattern in the prerequisites.
  2. This just creates empty .c files when needed.

%.c:

  touch $@

Double-Colon Rules

Double-Colon Rules are rarely used, but allow multiple rules to be defined for the same target. If these were single colons, a warning would be printed and only the second set of commands would run.

all: blah

blah::

   echo "hello"

blah::

   echo "hello again"

Commands and execution Command Echoing/Silencing

Add an @ before a command to stop it from being printed You can also run make with -s to add an @ before each line

all:

   @echo "This make line will not be printed"
   echo "But this will"

Command Execution

Each command is run in a new shell (or at least the effect is as such)

all:

   cd ..
   # The cd above does not affect this line, because each command is effectively run in a new shell
   echo `pwd`
   # This cd command affects the next because they are on the same line
   cd ..;echo `pwd`
   # Same as above
   cd ..; \
   echo `pwd`

Default Shell

The default shell is /bin/sh. You can change this by changing the variable SHELL:

SHELL=/bin/bash

cool:

   echo "Hello from bash"

Error handling with -k, -i, and -

Add -k when running make to continue running even in the face of errors. Helpful if you want to see all the errors of Make at once. Add a - before a command to suppress the error Add -i to make to have this happen for every command.

one:

   # This error will be printed but ignored, and make will continue to run
   -false
   touch one

Interrupting or killing make

Note only: If you ctrl+c make, it will delete the newer targets it just made. Recursive use of make

To recursively call a makefile, use the special $(MAKE) instead of make because it will pass the make flags for you and won't itself be affected by them.

new_contents = "hello:\n\ttouch inside_file" all:

   mkdir -p subdir
   printf $(new_contents) | sed -e 's/^ //' > subdir/makefile
   cd subdir && $(MAKE)

clean:

   rm -rf subdir

Use export for recursive make

The export directive takes a variable and makes it accessible to sub-make commands. In this example, cooly is exported such that the makefile in subdir can use it.

Note: export has the same syntax as sh, but they aren't related (although similar in function)

new_contents = "hello:\n\\techo \$$(cooly)"

all:

   mkdir -p subdir
   echo $(new_contents) | sed -e 's/^ //' > subdir/makefile
   @echo "---MAKEFILE CONTENTS---"
   @cd subdir && cat makefile
   @echo "---END MAKEFILE CONTENTS---"
   cd subdir && $(MAKE)
  1. Note that variables and exports. They are set/affected globally.

cooly = "The subdirectory can see me!" export cooly

  1. This would nullify the line above: unexport cooly

clean:

   rm -rf subdir

You need to export variables to have them run in the shell as well.

one=this will only work locally export two=we can run subcommands with this

all:

   @echo $(one)
   @echo $$one
   @echo $(two)
   @echo $$two

.EXPORT_ALL_VARIABLES exports all variables for you.

.EXPORT_ALL_VARIABLES: new_contents = "hello:\n\techo \$$(cooly)"

cooly = "The subdirectory can see me!"

  1. This would nullify the line above: unexport cooly

all:

   mkdir -p subdir
   echo $(new_contents) | sed -e 's/^ //' > subdir/makefile
   @echo "---MAKEFILE CONTENTS---"
   @cd subdir && cat makefile
   @echo "---END MAKEFILE CONTENTS---"
   cd subdir && $(MAKE)

clean:

   rm -rf subdir

Arguments to make

There's a nice list of options that can be run from make. Check out --dry-run, --touch, --old-file.

You can have multiple targets to make, i.e. make clean run test runs the clean goal, then run, and then test. Variables Pt. 2 Flavors and modification

There are two flavors of variables:

   recursive (use =) - only looks for the variables when the command is used, not when it's defined.
   simply expanded (use :=) - like normal imperative programming -- only those defined so far get expanded
  1. Recursive variable. This will print "later" below

one = one ${later_variable}

  1. Simply expanded variable. This will not print "later" below

two := two ${later_variable}

later_variable = later

all:

   echo $(one)
   echo $(two)

Simply expanded (using :=) allows you to append to a variable. Recursive definitions will give an infinite loop error.

one = hello

  1. one gets defined as a simply expanded variable (:=) and thus can handle appending

one := ${one} there

all:

   echo $(one)

?= only sets variables if they have not yet been set

one = hello one ?= will not be set two ?= will be set

all:

   echo $(one)
   echo $(two)

Spaces at the end of a line are not stripped, but those at the start are. To make a variable with a single space, use $(nullstring)

with_spaces = hello # with_spaces has many spaces after "hello" after = $(with_spaces)there

nullstring = space = $(nullstring) # Make a variable with a single space.

all:

   echo "$(after)"
   echo start"$(space)"end

An undefined variable is actually an empty string!

all:

   # Undefined variables are just empty strings!
   echo $(nowhere)

Use += to append

foo := start foo += more

all:

   echo $(foo)

String Substitution is also a really common and useful way to modify variables. Also check out Text Functions and Filename Functions. Command line arguments and override

You can override variables that come from the command line by using override. Here we ran make with make option_one=hi

  1. Overrides command line arguments

override option_one = did_override

  1. Does not override command line arguments

option_two = not_override all:

   echo $(option_one)
   echo $(option_two)

List of commands and define

"define" is actually just a list of commands. It has nothing to do with being a function. Note here that it's a bit different than having a semi-colon between commands, because each is run in a separate shell, as expected.

one = export blah="I was set!"; echo $$blah

define two export blah=set echo $$blah endef

  1. One and two are different.

all:

   @echo "This prints 'I was set'"
   @$(one)
   @echo "This does not print 'I was set' because each command runs in a separate shell"
   @$(two)

Target-specific variables

Variables can be assigned for specific targets

all: one = cool

all:

   echo one is defined: $(one)

other:

   echo one is nothing: $(one)

Pattern-specific variables

You can assign variables for specific target patterns

%.c: one = cool

blah.c:

   echo one is defined: $(one)

other:

   echo one is nothing: $(one)

Conditional part of Makefiles Conditional if/else

foo = ok

all: ifeq ($(foo), ok)

   echo "foo equals ok"

else

   echo "nope"

endif

Check if a variable is empty

nullstring = foo = $(nullstring) # end of line; there is a space here

all: ifeq ($(strip $(foo)),)

   echo "foo is empty after being stripped"

endif ifeq ($(nullstring),)

   echo "nullstring doesn't even have spaces"

endif

Check if a variable is defined

ifdef does not expand variable references; it just sees if something is defined at all

bar = foo = $(bar)

all: ifdef foo

   echo "foo is defined"

endif ifdef bar

   echo "but bar is not"

endif

$(makeflags)

This example shows you how to test make flags with findstring and MAKEFLAGS. Run this example with make -i to see it print out the echo statement.

bar = foo = $(bar)

all:

  1. Search for the "-i" flag. MAKEFLAGS is just a list of single characters, one per flag. So look for "i" in this case.

ifneq (,$(findstring i, $(MAKEFLAGS)))

   echo "i was passed to MAKEFLAGS"

endif

Functions First Functions

Functions are mainly just for text processing. Call functions with $(fn, arguments) or ${fn, arguments}. You can make your own using the call builtin function. Make has a decent amount of builtin functions.

bar := ${subst not, totally, "I am not superman"} all:

   @echo $(bar)

If you want to replace spaces or commas, use variables

comma := , empty:= space := $(empty) $(empty) foo := a b c bar := $(subst $(space),$(comma),$(foo))

all:

   @echo $(bar)

Do NOT include spaces in the arguments after the first. That will be seen as part of the string.

comma := , empty:= space := $(empty) $(empty) foo := a b c bar := $(subst $(space), $(comma) , $(foo))

all:

   # Output is ", a , b , c". Notice the spaces introduced
   @echo $(bar)

String Substitution

$(patsubst pattern,replacement,text) does the following:

"Finds whitespace-separated words in text that match pattern and replaces them with replacement. Here pattern may contain a ‘%’ which acts as a wildcard, matching any number of any characters within a word. If replacement also contains a ‘%’, the ‘%’ is replaced by the text that matched the ‘%’ in pattern. Only the first ‘%’ in the pattern and replacement is treated this way; any subsequent ‘%’ is unchanged." (GNU docs)

The substitution reference $(text:pattern=replacement) is a shorthand for this.

There's another shorthand that that replaces only suffixes: $(text:suffix=replacement). No % wildcard is used here.

Note: don't add extra spaces for this shorthand. It will be seen as a search or replacement term.

foo := a.o b.o l.a c.o one := $(patsubst %.o,%.c,$(foo))

  1. This is a shorthand for the above

two := $(foo:%.o=%.c)

  1. This is the suffix-only shorthand, and is also equivalent to the above.

three := $(foo:.o=.c)

all:

   echo $(one)
   echo $(two)
   echo $(three)

The foreach function

The foreach function looks like this: $(foreach var,list,text). It converts one list of words (separated by spaces) to another. var is set to each word in list, and text is expanded for each word. This appends an exclamation after each word:

foo := who are you

  1. For each "word" in foo, output that same word with an exclamation after

bar := $(foreach wrd,$(foo),$(wrd)!)

all:

   # Output is "who! are! you!"
   @echo $(bar)

The if function

if checks if the first argument is nonempty. If so runs the second argument, otherwise runs the third.

foo := $(if this-is-not-empty,then!,else!) empty := bar := $(if $(empty),then!,else!)

all:

   @echo $(foo)
   @echo $(bar)

The call function

Make supports creating basic functions. You "define" the function just by creating a variable, but use the parameters $(0), $(1), etc. You then call the function with the special call function. The syntax is $(call variable,param,param). $(0) is the variable, while $(1), $(2), etc. are the params.

sweet_new_fn = Variable Name: $(0) First: $(1) Second: $(2) Empty Variable: $(3)

all:

   # Outputs "Variable Name: sweet_new_fn First: go Second: tigers Empty Variable:"
   @echo $(call sweet_new_fn, go, tigers)

The shell function

shell - This calls the shell, but it replaces newlines with spaces!

all:

   @echo $(shell ls -la) # Very ugly because the newlines are gone!

Other Features Include Makefiles

The include directive tells make to read one or more other makefiles. It's a line in the makefile makefile that looks like this:

include filenames...

This is particularly useful when you use compiler flags like -M that create Makefiles based on the source. For example, if some c files includes a header, that header will be added to a Makefile that's written by gcc. I talk about this more in the Makefile Cookbook The vpath Directive

Use vpath to specify where some set of prerequisites exist. The format is vpath <pattern> <directories, space/colon separated> <pattern> can have a %, which matches any zero or more characters. You can also do this globallyish with the variable VPATH

vpath %.h ../headers ../other-directory

some_binary: ../headers blah.h

   touch some_binary

../headers:

   mkdir ../headers

blah.h:

   touch ../headers/blah.h

clean:

   rm -rf ../headers
   rm -f some_binary

Multiline

The backslash ("\") character gives us the ability to use multiple lines when the commands are too long

some_file:

   echo This line is too long, so \
       it is broken up into multiple lines

.phony

Adding .PHONY to a target will prevent make from confusing the phony target with a file name. In this example, if the file clean is created, make clean will still be run. .PHONY is great to use, but I'll skip it in the rest of the examples for simplicity.

some_file:

   touch some_file
   touch clean

.PHONY: clean clean:

   rm -f some_file
   rm -f clean

.delete_on_error

The make tool will stop running a rule (and will propogate back to prerequisites) if a command returns a nonzero exit status. DELETE_ON_ERROR will delete the target of a rule if the rule fails in this manner. This will happen for all targets, not just the one it is before like PHONY. It's a good idea to always use this, even though make does not for historical reasons.

.DELETE_ON_ERROR: all: one two

one:

   touch one
   false

two:

   touch two
   false

Makefile Cookbook

Let's go through a really juicy Make example that works well for medium sized projects.

The neat thing about this makefile is it automatically determines dependencies for you. All you have to do is put your C/C++ files in the src/ folder.

  1. Thanks to Job Vranish (https://spin.atomicobject.com/2016/08/26/makefile-c-projects/)

TARGET_EXEC := final_program

BUILD_DIR := ./build SRC_DIRS := ./src

  1. Find all the C and C++ files we want to compile

SRCS := $(shell find $(SRC_DIRS) -name *.cpp -or -name *.c)

  1. String substitution for every C/C++ file.
  2. As an example, hello.cpp turns into ./build/hello.cpp.o

OBJS := $(SRCS:%=$(BUILD_DIR)/%.o)

  1. String substitution (suffix version without %).
  2. As an example, ./build/hello.cpp.o turns into ./build/hello.cpp.d

DEPS := $(OBJS:.o=.d)

  1. Every folder in ./src will need to be passed to GCC so that it can find header files

INC_DIRS := $(shell find $(SRC_DIRS) -type d)

  1. Add a prefix to INC_DIRS. So moduleA would become -ImoduleA. GCC understands this -I flag

INC_FLAGS := $(addprefix -I,$(INC_DIRS))

  1. The -MMD and -MP flags together generate Makefiles for us!
  2. These files will have .d instead of .o as the output.

CPPFLAGS := $(INC_FLAGS) -MMD -MP

  1. The final build step.

$(BUILD_DIR)/$(TARGET_EXEC): $(OBJS)

   $(CC) $(OBJS) -o $@ $(LDFLAGS)
  1. Build step for C source

$(BUILD_DIR)/%.c.o: %.c

   mkdir -p $(dir $@)
   $(CC) $(CPPFLAGS) $(CFLAGS) -c $< -o $@
  1. Build step for C++ source

$(BUILD_DIR)/%.cpp.o: %.cpp

   mkdir -p $(dir $@)
   $(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@


.PHONY: clean clean:

   rm -r $(BUILD_DIR)
  1. Include the .d makefiles. The - at the front suppresses the errors of missing
  2. Makefiles. Initially, all the .d files will be missing, and we don't want those
  3. errors to show up.

-include $(DEPS)