【编程】Makefile教程Makefile Tutorial By Example

jason246 发表于 2023-2-5 08:54:45

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I built this guide because I could never quite wrap my head around Makefiles. They seemed awash with hidden rules and esoteric symbols, and asking simple questions didn’t yield simple answers. To solve this, I sat down for several weekends and read everything I could about Makefiles. I've condensed the most critical knowledge into this guide. Each topic has a brief description and a self contained example that you can run yourself.
If you mostly understand Make, consider checking out the <a href="#makefile-cookbook">Makefile Cookbook</a>, which has a template for medium sized projects with ample comments about what each part of the Makefile is doing.
Good luck, and I hope you are able to slay the confusing world of Makefiles!
<h1 id="getting-started">Getting Started</h1>
<h2 id="why-do-makefiles-exist">Why do Makefiles exist?</h2>
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. Make can also be used beyond compilation 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:
<div align="center">
<img src="https://tongyici.net:5556/static/data/program/makefile/dependency_graph.png">
</div>

<h2 id="what-alternatives-are-there-to-make">What alternatives are there to Make?</h2>
Popular C/C++ alternative build systems are <a href="https://scons.org/">SCons</a>, <a href="https://cmake.org/">CMake</a>, <a href="https://bazel.build/">Bazel</a>, and <a href="https://ninja-build.org/">Ninja</a>. Some code editors like <a href="https://visualstudio.microsoft.com/">Microsoft Visual Studio</a> have their own built in build tools. For Java, there's <a href="https://ant.apache.org/">Ant</a>, <a href="https://maven.apache.org/what-is-maven.html">Maven</a>, and <a href="https://gradle.org/">Gradle</a>. 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.
<h2 id="the-versions-and-types-of-make">The versions and types of Make</h2>
There are a variety of implementations of Make, but most of this guide will work on whatever version you're using. However, it's specifically written for GNU Make, which is the standard implementation on Linux and MacOS. All the examples work for Make versions 3 and 4, which are nearly equivalent other than some esoteric differences.
<h2 id="running-the-examples">Running the Examples</h2>
To run these examples, you'll need a terminal and "make" installed. For each example, put the contents in a file called <code>Makefile</code>, and in that directory run the command <code>make</code>. Let's start with the simplest of Makefiles:
<pre><code class="hljs makefile">hello:
echo "Hello, World"</code></pre>
<blockquote>
Note: Makefiles must be indented using TABs and not spaces or <code>make</code> will fail.
</blockquote>
Here is the output of running the above example:
<pre><code class="hljs shell">$ make
echo "Hello, World"
Hello, World</code></pre>
That's it! 

<h2 id="makefile-syntax">Makefile Syntax</h2>
A Makefile consists of a set of rules. A rule generally looks like this:
<pre><code class="hljs makefile">targets: prerequisites
command
command
command</code></pre>
<ul>
<li>The targets are file names, separated by spaces. Typically, there is only one per rule.</li>
<li>The commands are a series of steps typically used to make the target(s). These need to start with a tab character, not spaces.</li>
<li>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</li>
</ul>
<h2 id="the-essence-of-make">The essence of Make</h2>
Let's start with a hello world example:
<pre><code class="hljs makefile">hello:
echo "Hello, World"
echo "This line will always print, because the file hello does not exist."</code></pre>
There's already a lot to take in here. Let's break it down:
<ul>
<li>We have one target called <code>hello</code></li>
<li>This target has two commands</li>
<li>This target has no prerequisites</li>
</ul>
We'll then run <code>make hello</code>. As long as the <code>hello</code> file does not exist, the commands will run. If <code>hello</code> does exist, no commands will run.
It's important to realize that I'm talking about <code>hello</code> as both a target and a file. That's because the two are directly tied together. Typically, when a target is run (aka when the commands of a target are run), the commands will create a file with the same name as the target. In this case, the <code>hello</code> target does not create the <code>hello</code> file.
Let's create a more typical Makefile - one that compiles a single C file. But before we do, make a file called <code>blah.c</code> that has the following contents:
<pre><code class="hljs c">// blah.c
int main() { return 0; }</code></pre>
Then create the Makefile (called <code>Makefile</code>, as always):
<pre><code class="hljs makefile">blah:
cc blah.c -o blah</code></pre>
This time, try simply running <code>make</code>. Since there's no target supplied as an argument to the <code>make</code> command, the first target is run. In this case, there's only one target (<code>blah</code>). The first time you run this, <code>blah</code> will be created. The second time, you'll see <code>make: 'blah' is up to date</code>. That's because the <code>blah</code> file already exists. But there's a problem: if we modify <code>blah.c</code> and then run <code>make</code>, nothing gets recompiled.
We solve this by adding a prerequisite:
<pre><code class="hljs makefile">blah: blah.c
cc blah.c -o blah</code></pre>
When we run <code>make</code> again, the following set of steps happens:
<ul>
<li>The first target is selected, because the first target is the default target</li>
<li>This has a prerequisite of <code>blah.c</code></li>
<li>Make decides if it should run the <code>blah</code> target. It will only run if <code>blah</code> doesn't exist, or <code>blah.c</code> is newer than <code>blah</code></li>
</ul>
This last step is critical, and is the essence of make. What it's attempting to do is decide if the prerequisites of <code>blah</code> have changed since <code>blah</code> was last compiled. That is, if <code>blah.c</code> is modified, running <code>make</code> should recompile the file. And conversely, if <code>blah.c</code> has not changed, then it should not be recompiled.
To make this happen, it uses the filesystem timestamps as a proxy to determine if something has changed. This is a reasonable heuristic, because file timestamps typically will only change if the files are
modified. But it's important to realize that this isn't always the case. You could, for example, modify a file, and then change the modified timestamp of that file to something old. If you did, Make would incorrectly guess that the file hadn't changed and thus could be ignored.
Whew, what a mouthful. Make sure that you understand this. It's the crux of Makefiles, and might take you a few minutes to properly understand. Play around with the above examples or watch the video above if things are still confusing.
<h2 id="more-quick-examples">More quick examples</h2>
The following Makefile ultimately runs all three targets. When you run <code>make</code> in the terminal, it will build a program called <code>blah</code> in a series of steps:
<ul>
<li>Make selects the target <code>blah</code>, because the first target is the default target</li>
<li><code>blah</code> requires <code>blah.o</code>, so make searches for the <code>blah.o</code> target</li>
<li><code>blah.o</code> requires <code>blah.c</code>, so make searches for the <code>blah.c</code> target</li>
<li><code>blah.c</code> has no dependencies, so the <code>echo</code> command is run</li>
<li>The <code>cc -c</code> command is then run, because all of the <code>blah.o</code> dependencies are finished</li>
<li>The top <code>cc</code> command is run, because all the <code>blah</code> dependencies are finished</li>
<li>That's it: <code>blah</code> is a compiled c program</li>
</ul>
<pre><code class="hljs makefile">blah: blah.o
cc blah.o -o blah # Runs third

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

# Typically blah.c would already exist, but I want to limit any additional required files
blah.c:
echo "int main() { return 0; }" > blah.c # Runs first</code></pre>
If you delete <code>blah.c</code>, all three targets will be rerun. If you edit it (and thus change the timestamp to newer than <code>blah.o</code>), the first two targets will run. If you run <code>touch blah.o</code> (and thus change the timestamp to newer than <code>blah</code>), then only the first target will run. If you change nothing, none of the targets will run. Try it out!
This next example doesn't do anything new, but is nontheless a good additional example. It will always run both targets, because <code>some_file</code> depends on <code>other_file</code>, which is never created.
<pre><code class="hljs makefile">some_file: other_file
echo "This will always run, and runs second"
touch some_file

other_file:
echo "This will always run, and runs first"</code></pre>
<h2 id="make-clean">Make clean</h2>
<code>clean</code> is often used as a target that removes the output of other targets, but it is not a special word in Make. You can run <code>make</code> and <code>make clean</code> on this to create and delete <code>some_file</code>.
Note that <code>clean</code> is doing two new things here:
<ul>
<li>It's a target that is not first (the default), and not a prerequisite. That means it'll never run unless you explicitly call <code>make clean</code></li>
<li>It's not intended to be a filename. If you happen to have a file named <code>clean</code>, this target won't run, which is not what we want. See <code>.PHONY</code> later in this tutorial on how to fix this</li>
</ul>
<pre><code class="hljs makefile">some_file: 
touch some_file

clean:
rm -f some_file</code></pre>
<h2 id="variables">Variables</h2>
Variables can only be strings. You'll typically want to use <code>:=</code>, but <code>=</code> also works. See <a href="#variables-pt-2">Variables Pt 2</a>.
Here's an example of using variables:
<pre><code class="hljs makefile">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</code></pre>
Single or double quotes have no meaning to Make. They are simply characters that are assigned to the variable. Quotes are useful to shell/bash, though, and you need them in commands like <code>printf</code>. In this example, the two commands behave the same:
<pre><code class="hljs makefile">a := one two # a is assigned to the string "one two"
b := 'one two' # Not recommended. b is assigned to the string "'one two'"
all:
printf '$a'
printf $b</code></pre>
Reference variables using either <code>${}</code> or <code>$()</code>
<pre><code class="hljs makefile">x := dude

all:
echo $(x)
echo ${x}

# Bad practice, but works
echo $x </code></pre>
<h1 id="targets">Targets</h1>
<h2 id="the-all-target">The all target</h2>

Making multiple targets and you want all of them to run? Make an <code>all</code> target.
Since this is the first rule listed, it will run by default if <code>make</code> is called without specifying a target.
<pre><code class="hljs makefile">all: one two three

one:
touch one
two:
touch two
three:
touch three

clean:
rm -f one two three
</code></pre>
<h2 id="multiple-targets">Multiple targets</h2>

When there are multiple targets for a rule, the commands will be run for each target. <code>$@</code> is an <a href="#automatic-variables">automatic variable</a> that contains the target name.
<pre><code class="hljs makefile">all: f1.o f2.o

f1.o f2.o:
echo $@
# Equivalent to:
# f1.o:
# echo f1.o
# f2.o:
# echo f2.o
</code></pre>
<h1 id="automatic-variables-and-wildcards">Automatic Variables and Wildcards</h1>
<h2 id="-wildcard">* Wildcard</h2>

Both <code>*</code> and <code>%</code> are called wildcards in Make, but they mean entirely different things. <code>*</code> searches your filesystem for matching filenames. I suggest that you always wrap it in the <code>wildcard</code> function, because otherwise you may fall into a common pitfall described below.
<pre><code class="hljs makefile"># Print out file information about every .c file
print: $(wildcard *.c)
ls -la$?</code></pre>
<code>*</code> may be used in the target, prerequisites, or in the <code>wildcard</code> function.
Danger: <code>*</code> may not be directly used in a variable definitions
Danger: When <code>*</code> matches no files, it is left as it is (unless run in the <code>wildcard</code> function)
<pre><code class="hljs makefile">thing_wrong := *.o # Don't do this! '*' will not get expanded
thing_right := $(wildcard *.o)

all: one two three four

# Fails, because $(thing_wrong) is the string "*.o"
one: $(thing_wrong)

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

# Works as you would expect! In this case, it does nothing.
three: $(thing_right)

# Same as rule three
four: $(wildcard *.o)</code></pre>
<h2 id="-wildcard-1">% Wildcard</h2>
<code>%</code> is really useful, but is somewhat confusing because of the variety of situations it can be used in.
<ul>
<li>When used in "matching" mode, it matches one or more characters in a string. This match is called the stem.</li>
<li>When used in "replacing" mode, it takes the stem that was matched and replaces that in a string.</li>
<li><code>%</code> is most often used in rule definitions and in some specific functions.</li>
</ul>
See these sections on examples of it being used:
<ul>
<li><a href="#static-pattern-rules">Static Pattern Rules</a></li>
<li><a href="#pattern-rules">Pattern Rules</a></li>
<li><a href="#string-substitution">String Substitution</a></li>
<li><a href="#the-vpath-directive">The vpath Directive</a></li>
</ul>
<h2 id="automatic-variables">Automatic Variables</h2>

There are many <a href="https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html">automatic variables</a>, but often only a few show up:
<pre><code class="hljs makefile">hey: one two
# Outputs "hey", since this is the target name
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
</code></pre>
<h1 id="fancy-rules">Fancy Rules</h1>
<h2 id="implicit-rules">Implicit Rules</h2>

Make loves c compilation. And every time it expresses its love, things get confusing. Perhaps the most confusing part of Make is the magic/automatic rules that are made. Make calls these "implicit" rules. I don't personally agree with this design decision, and I don't recommend using them, but they're often used and are thus useful to know. Here's a list of implicit rules:
<ul>
<li>Compiling a C program: <code>n.o</code> is made automatically from <code>n.c</code> with a command of the form <code>$(CC) -c $(CPPFLAGS) $(CFLAGS) $^ -o $@</code></li>
<li>Compiling a C++ program: <code>n.o</code> is made automatically from <code>n.cc</code> or <code>n.cpp</code> with a command of the form <code>$(CXX) -c $(CPPFLAGS) $(CXXFLAGS) $^ -o $@</code></li>
<li>Linking a single object file: <code>n</code> is made automatically from <code>n.o</code> by running the command <code>$(CC) $(LDFLAGS) $^ $(LOADLIBES) $(LDLIBS) -o $@</code></li>
</ul>
The important variables used by implicit rules are:
<ul>
<li><code>CC</code>: Program for compiling C programs; default <code>cc</code></li>
<li><code>CXX</code>: Program for compiling C++ programs; default <code>g++</code></li>
<li><code>CFLAGS</code>: Extra flags to give to the C compiler</li>
<li><code>CXXFLAGS</code>: Extra flags to give to the C++ compiler</li>
<li><code>CPPFLAGS</code>: Extra flags to give to the C preprocessor</li>
<li><code>LDFLAGS</code>: Extra flags to give to compilers when they are supposed to invoke the linker</li>
</ul>
Let's see how we can now build a C program without ever explicitly telling Make how to do the compililation:
<pre><code class="hljs makefile">CC = gcc # Flag for implicit rules
CFLAGS = -g # Flag for implicit rules. Turn on debug info

# Implicit rule #1: blah is built via the C linker implicit rule
# 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*</code></pre>
<h2 id="static-pattern-rules">Static Pattern Rules</h2>

Static pattern rules are another way to write less in a Makefile, but I'd say are more useful and a bit less "magic". Here's their syntax:
<pre><code class="hljs makefile">targets...: target-pattern: prereq-patterns ...
commands</code></pre>
The essence is that the given <code>target</code> is matched by the <code>target-pattern</code> (via a <code>%</code> wildcard). Whatever was matched is called the stem. The stem is then substituted into the <code>prereq-pattern</code>, to generate the target's prereqs.
A typical use case is to compile <code>.c</code> files into <code>.o</code> files. Here's the manual way:
<pre><code class="hljs makefile">objects = foo.o bar.o all.o
all: $(objects)

# 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</code></pre>
Here's the more efficient way, using a static pattern rule:
<pre><code class="hljs makefile">objects = foo.o bar.o all.o
all: $(objects)

# These files compile via implicit rules
# Syntax - targets ...: target-pattern: prereq-patterns ...
# In the case of the first target, foo.o, the target-pattern matches foo.o and sets the "stem" to be "foo".
# 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</code></pre>
<h2 id="static-pattern-rules-and-filter">Static Pattern Rules and Filter</h2>

While I introduce functions later on, I'll foreshadow what you can do with them. The <code>filter</code> function can be used in Static pattern rules to match the correct files. In this example, I made up the <code>.raw</code> and <code>.result</code> extensions.
<pre><code class="hljs makefile">obj_files = foo.result bar.o lose.o
src_files = foo.raw bar.c lose.c

all: $(obj_files)
# Note: PHONY is important here. Without it, implicit rules will try to build the executable "all", since the prereqs are ".o" files.
.PHONY: all 

# Ex 1: .o files depend on .c files. Though we don't actually make the .o file.
$(filter %.o,$(obj_files)): %.o: %.c
echo "target: $@ prereq: $<"

# Ex 2: .result files depend on .raw files. Though we don't actually make the .result file.
$(filter %.result,$(obj_files)): %.result: %.raw
echo "target: $@ prereq: $<"

%.c %.raw:
touch $@

clean:
rm -f $(src_files)</code></pre>
<h2 id="pattern-rules">Pattern Rules</h2>
Pattern rules are often used but quite confusing. You can look at them as two ways:
<ul>
<li>A way to define your own implicit rules</li>
<li>A simpler form of static pattern rules</li>
</ul>
Let's start with an example first:
<pre><code class="hljs makefile"># Define a pattern rule that compiles every .c file into a .o file
%.o : %.c
$(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@</code></pre>
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:
<pre><code class="hljs makefile"># Define a pattern rule that has no pattern in the prerequisites.
# This just creates empty .c files when needed.
%.c:
touch $@</code></pre>
<h2 id="double-colon-rules">Double-Colon Rules</h2>

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.
<pre><code class="hljs makefile">all: blah

blah::
echo "hello"

blah::
echo "hello again"</code></pre>
<h1 id="commands-and-execution">Commands and execution</h1>
<h2 id="command-echoingsilencing">Command Echoing/Silencing</h2>

Add an <code>@</code> before a command to stop it from being printed You can also run make with <code>-s</code> to add an <code>@</code> before each line
<pre><code class="hljs makefile">all: 
@echo "This make line will not be printed"
echo "But this will"</code></pre>
<h2 id="command-execution">Command Execution</h2>

Each command is run in a new shell (or at least the effect is as such)
<pre><code class="hljs makefile">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`
</code></pre>
<h2 id="default-shell">Default Shell</h2>

The default shell is <code>/bin/sh</code>. You can change this by changing the variable SHELL:
<pre><code class="hljs makefile">SHELL=/bin/bash

cool:
echo "Hello from bash"</code></pre>
<h2 id="double-dollar-sign">Double dollar sign</h2>
If you want a string to have a dollar sign, you can use <code>$$</code>. This is how to use a shell variable in <code>bash</code> or <code>sh</code>.
Note the differences between Makefile variables and Shell variables in this next example.
<pre><code class="hljs makefile">make_var = I am a make variable
all:
# Same as running "sh_var='I am a shell variable'; echo $sh_var" in the shell
sh_var='I am a shell variable'; echo $$sh_var

# Same as running "echo I am a amke variable" in the shell
echo $(make_var)</code></pre>
<h2 id="error-handling-with--k--i-and--">Error handling with <code>-k</code>, <code>-i</code>, and <code>-</code></h2>

Add <code>-k</code> 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 <code>-</code> before a command to suppress the error Add <code>-i</code> to make to have this happen for every command.

<pre><code class="hljs makefile">one:
# This error will be printed but ignored, and make will continue to run
-false
touch one
</code></pre>
<h2 id="interrupting-or-killing-make">Interrupting or killing make</h2>

Note only: If you <code>ctrl+c</code> make, it will delete the newer targets it just made.
<h2 id="recursive-use-of-make">Recursive use of make</h2>

To recursively call a makefile, use the special <code>$(MAKE)</code> instead of <code>make</code> because it will pass the make flags for you and won't itself be affected by them.
<pre><code class="hljs makefile">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
</code></pre>
<h2 id="export-environments-and-recursive-make">Export, environments, and recursive make</h2>

When Make starts, it automatically creates Make variables out of all the environment variables that are set when it's executed.
<pre><code class="hljs makefile"># Run this with "export shell_env_var='I am an environment variable'; make"
all:
# Print out the Shell variable
echo $$shell_env_var

# Print out the Make variable
echo $(shell_env_var)</code></pre>
The <code>export</code> directive takes a variable and sets it the environment for all shell commands in all the recipes:
<pre><code class="hljs makefile">shell_env_var=Shell env var, created inside of Make
export shell_env_var
all:
echo $(shell_env_var)
echo $$shell_env_var</code></pre>
As such, when you run the <code>make</code> command inside of make, you can use the <code>export</code> directive to make it accessible to sub-make commands. In this example, <code>cooly</code> is exported such that the makefile in subdir can use it.
<pre><code class="hljs makefile">new_contents = "hello:\n\techo \$$(cooly)"

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

# Note that variables and exports. They are set/affected globally.
cooly = "The subdirectory can see me!"
export cooly
# This would nullify the line above: unexport cooly

clean:
rm -rf subdir</code></pre>

You need to export variables to have them run in the shell as well.
<pre><code class="hljs makefile">one=this will only work locally
export two=we can run subcommands with this

all: 
@echo $(one)
@echo $$one
@echo $(two)
@echo $$two</code></pre>

<code>.EXPORT_ALL_VARIABLES</code> exports all variables for you.
<pre><code class="hljs makefile">.EXPORT_ALL_VARIABLES:
new_contents = "hello:\n\techo \$$(cooly)"

cooly = "The subdirectory can see me!"
# This would nullify the line above: unexport cooly

all:
mkdir -p subdir
printf $(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</code></pre>
<h2 id="arguments-to-make">Arguments to make</h2>


There's a nice <a href="http://www.gnu.org/software/make/manual/make.html#Options-Summary">list of options</a> that can be run from make. Check out <code>--dry-run</code>, <code>--touch</code>, <code>--old-file</code>. 
You can have multiple targets to make, i.e. <code>make clean run test</code> runs the <code>clean</code> goal, then <code>run</code>, and then <code>test</code>.
<h1 id="variables-pt-2">Variables Pt. 2</h1>
<h2 id="flavors-and-modification">Flavors and modification</h2>

There are two flavors of variables:
<ul>
<li>recursive (use <code>=</code>) - only looks for the variables when the command is used, not when it's defined.</li>
<li>simply expanded (use <code>:=</code>) - like normal imperative programming -- only those defined so far get expanded</li>
</ul>
<pre><code class="hljs makefile"># Recursive variable. This will print "later" below
one = one ${later_variable}
# Simply expanded variable. This will not print "later" below
two := two ${later_variable}

later_variable = later

all: 
echo $(one)
echo $(two)</code></pre>
Simply expanded (using <code>:=</code>) allows you to append to a variable. Recursive definitions will give an infinite loop error.
<pre><code class="hljs makefile">one = hello
# one gets defined as a simply expanded variable (:=) and thus can handle appending
one := ${one} there

all: 
echo $(one)</code></pre>
<code>?=</code> only sets variables if they have not yet been set
<pre><code class="hljs makefile">one = hello
one ?= will not be set
two ?= will be set

all: 
echo $(one)
echo $(two)</code></pre>
Spaces at the end of a line are not stripped, but those at the start are. To make a variable with a single space, use <code>$(nullstring)</code>
<pre><code class="hljs makefile">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</code></pre>
An undefined variable is actually an empty string!
<pre><code class="hljs makefile">all: 
# Undefined variables are just empty strings!
echo $(nowhere)</code></pre>
Use <code>+=</code> to append
<pre><code class="hljs makefile">foo := start
foo += more

all: 
echo $(foo)</code></pre>
<a href="#string-substitution">String Substitution</a> is also a really common and useful way to modify variables. Also check out <a href="https://www.gnu.org/software/make/manual/html_node/Text-Functions.html#Text-Functions">Text Functions</a> and <a href="https://www.gnu.org/software/make/manual/html_node/File-Name-Functions.html#File-Name-Functions">Filename Functions</a>.
<h2 id="command-line-arguments-and-override">Command line arguments and override</h2>

You can override variables that come from the command line by using <code>override</code>.
Here we ran make with <code>make option_one=hi</code>
<pre><code class="hljs makefile"># Overrides command line arguments
override option_one = did_override
# Does not override command line arguments
option_two = not_override
all: 
echo $(option_one)
echo $(option_two)</code></pre>
<h2 id="list-of-commands-and-define">List of commands and define</h2>

The <a href="https://www.gnu.org/software/make/manual/html_node/Multi_002dLine.html">define directive</a> is not a function, though it may look that way. I've seen it used so infrequently that I won't go into details, but it's mainly used for defining <a href="https://www.gnu.org/software/make/manual/html_node/Canned-Recipes.html#Canned-Recipes">canned recipes</a> and also pairs well with the <a href="https://www.gnu.org/software/make/manual/html_node/Eval-Function.html#Eval-Function">eval function</a>.
<code>define</code>/<code>endef</code> simply creates a variable that is assigned to a list of commands. 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.
<pre><code class="hljs makefile">one = export blah="I was set!"; echo $$blah

define two
export blah="I was set!"
echo $$blah
endef

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)</code></pre>
<h2 id="target-specific-variables">Target-specific variables</h2>

Variables can be assigned for specific targets
<pre><code class="hljs makefile">all: one = cool

all: 
echo one is defined: $(one)

other:
echo one is nothing: $(one)</code></pre>
<h2 id="pattern-specific-variables">Pattern-specific variables</h2>

You can assign variables for specific target patterns
<pre><code class="hljs makefile">%.c: one = cool

blah.c: 
echo one is defined: $(one)

other:
echo one is nothing: $(one)</code></pre>
<h1 id="conditional-part-of-makefiles">Conditional part of Makefiles</h1>
<h2 id="conditional-ifelse">Conditional if/else</h2>

<pre><code class="hljs makefile">foo = ok

all:
ifeq ($(foo), ok)
echo "foo equals ok"
else
echo "nope"
endif</code></pre>
<h2 id="check-if-a-variable-is-empty">Check if a variable is empty</h2>

<pre><code class="hljs makefile">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</code></pre>
<h2 id="check-if-a-variable-is-defined">Check if a variable is defined</h2>

ifdef does not expand variable references; it just sees if something is defined at all
<pre><code class="hljs makefile">bar =
foo = $(bar)

all:
ifdef foo
echo "foo is defined"
endif
ifndef bar
echo "but bar is not"
endif
</code></pre>
<h2 id="makeflags">$(makeflags)</h2>

This example shows you how to test make flags with <code>findstring</code> and <code>MAKEFLAGS</code>. Run this example with <code>make -i</code> to see it print out the echo statement.
<pre><code class="hljs makefile">bar =
foo = $(bar)

all:
# 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</code></pre>
<h1 id="functions">Functions</h1>
<h2 id="first-functions">First Functions</h2>

Functions are mainly just for text processing. Call functions with <code>$(fn, arguments)</code> or <code>${fn, arguments}</code>. You can make your own using the <a href="https://www.gnu.org/software/make/manual/html_node/Call-Function.html#Call-Function">call</a> builtin function. Make has a decent amount of <a href="https://www.gnu.org/software/make/manual/html_node/Functions.html">builtin functions</a>.
<pre><code class="hljs makefile">bar := ${subst not, totally, "I am not superman"}
all: 
@echo $(bar)
</code></pre>
If you want to replace spaces or commas, use variables
<pre><code class="hljs makefile">comma := ,
empty:=
space := $(empty) $(empty)
foo := a b c
bar := $(subst $(space),$(comma),$(foo))

all: 
@echo $(bar)</code></pre>
Do NOT include spaces in the arguments after the first. That will be seen as part of the string.
<pre><code class="hljs makefile">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)
</code></pre>


<h2 id="string-substitution">String Substitution</h2>
<code>$(patsubst pattern,replacement,text)</code> does the following:
"Finds whitespace-separated words in text that match pattern and replaces them with replacement. Here pattern may contain a ‘%’ which acts as a wildcard, matching any number of any characters within a word. If replacement also contains a ‘%’, the ‘%’ is replaced by the text that matched the ‘%’ in pattern. Only the first ‘%’ in the pattern and replacement is treated this way; any subsequent ‘%’ is unchanged." (<a href="https://www.gnu.org/software/make/manual/html_node/Text-Functions.html#Text-Functions">GNU docs</a>)
The substitution reference <code>$(text:pattern=replacement)</code> is a shorthand for this.
There's another shorthand that that replaces only suffixes: <code>$(text:suffix=replacement)</code>. No <code>%</code> wildcard is used here.
Note: don't add extra spaces for this shorthand. It will be seen as a search or replacement term.
<pre><code class="hljs makefile">foo := a.o b.o l.a c.o
one := $(patsubst %.o,%.c,$(foo))
# This is a shorthand for the above
two := $(foo:%.o=%.c)
# This is the suffix-only shorthand, and is also equivalent to the above.
three := $(foo:.o=.c)

all:
echo $(one)
echo $(two)
echo $(three)</code></pre>
<h2 id="the-foreach-function">The foreach function</h2>

The foreach function looks like this: <code>$(foreach var,list,text)</code>. It converts one list of words (separated by spaces) to another. <code>var</code> is set to each word in list, and <code>text</code> is expanded for each word. This appends an exclamation after each word:
<pre><code class="hljs makefile">foo := who are you
# 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)</code></pre>
<h2 id="the-if-function">The if function</h2>

<code>if</code> checks if the first argument is nonempty. If so runs the second argument, otherwise runs the third.
<pre><code class="hljs makefile">foo := $(if this-is-not-empty,then!,else!)
empty :=
bar := $(if $(empty),then!,else!)

all:
@echo $(foo)
@echo $(bar)</code></pre>
<h2 id="the-call-function">The call function</h2>

Make supports creating basic functions. You "define" the function just by creating a variable, but use the parameters <code>$(0)</code>, <code>$(1)</code>, etc. You then call the function with the special <code>call</code> function. The syntax is <code>$(call variable,param,param)</code>. <code>$(0)</code> is the variable, while <code>$(1)</code>, <code>$(2)</code>, etc. are the params.
<pre><code class="hljs makefile">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)</code></pre>
<h2 id="the-shell-function">The shell function</h2>

shell - This calls the shell, but it replaces newlines with spaces!
<pre><code class="hljs makefile">all: 
@echo $(shell ls -la) # Very ugly because the newlines are gone!</code></pre>
<h1 id="other-features">Other Features</h1>
<h2 id="include-makefiles">Include Makefiles</h2>
The include directive tells make to read one or more other makefiles. It's a line in the makefile that looks like this:
<pre><code class="hljs makefile">include filenames...</code></pre>
This is particularly useful when you use compiler flags like <code>-M</code> 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 <a href="#makefile-cookbook">Makefile Cookbook</a>
<h2 id="the-vpath-directive">The vpath Directive</h2>

Use vpath to specify where some set of prerequisites exist. The format is <code>vpath <pattern> <directories, space/colon separated></code> <code><pattern></code> can have a <code>%</code>, which matches any zero or more characters. You can also do this globallyish with the variable VPATH
<pre><code class="hljs makefile">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
</code></pre>
<h2 id="multiline">Multiline</h2>
The backslash ("\") character gives us the ability to use multiple lines when the commands are too long
<pre><code class="hljs makefile">some_file: 
echo This line is too long, so \
it is broken up into multiple lines</code></pre>
<h2 id="phony">.phony</h2>
Adding <code>.PHONY</code> to a target will prevent Make from confusing the phony target with a file name. In this example, if the file <code>clean</code> is created, make clean will still be run. Technically, I should have have used it in every example with <code>all</code> or <code>clean</code>, but I didn't to keep the examples clean. Additionally, "phony" targets typically have names that are rarely file names, and in practice many people skip this.
<pre><code class="hljs makefile">some_file:
touch some_file
touch clean

.PHONY: clean
clean:
rm -f some_file
rm -f clean</code></pre>
<h2 id="delete_on_error">.delete_on_error</h2>


The make tool will stop running a rule (and will propogate back to prerequisites) if a command returns a nonzero exit status. <code>DELETE_ON_ERROR</code> 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.
<pre><code class="hljs makefile">.DELETE_ON_ERROR:
all: one two

one:
touch one
false

two:
touch two
false</code></pre>
<h1 id="makefile-cookbook">Makefile Cookbook</h1>
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 <code>src/</code> folder.
<pre><code class="hljs makefile"># Thanks to Job Vranish (https://spin.atomicobject.com/2016/08/26/makefile-c-projects/)
TARGET_EXEC := final_program

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

# Find all the C and C++ files we want to compile
# Note the single quotes around the * expressions. Make will incorrectly expand these otherwise.
SRCS := $(shell find $(SRC_DIRS) -name '*.cpp' -or -name '*.c' -or -name '*.s')

# String substitution for every C/C++ file.
# As an example, hello.cpp turns into ./build/hello.cpp.o
OBJS := $(SRCS:%=$(BUILD_DIR)/%.o)

# String substitution (suffix version without %).
# As an example, ./build/hello.cpp.o turns into ./build/hello.cpp.d
DEPS := $(OBJS:.o=.d)

# 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)
# Add a prefix to INC_DIRS. So moduleA would become -ImoduleA. GCC understands this -I flag
INC_FLAGS := $(addprefix -I,$(INC_DIRS))

# The -MMD and -MP flags together generate Makefiles for us!
# These files will have .d instead of .o as the output.
CPPFLAGS := $(INC_FLAGS) -MMD -MP

# The final build step.
$(BUILD_DIR)/$(TARGET_EXEC): $(OBJS)
$(CXX) $(OBJS) -o $@ $(LDFLAGS)

# Build step for C source
$(BUILD_DIR)/%.c.o: %.c
mkdir -p $(dir $@)
$(CC) $(CPPFLAGS) $(CFLAGS) -c $< -o $@

# 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)

# Include the .d makefiles. The - at the front suppresses the errors of missing
# Makefiles. Initially, all the .d files will be missing, and we don't want those
# errors to show up.
-include $(DEPS)</code></pre>

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