Introduction to making Makefiles
Johannes Franken
<jfranken@jfranken.de>
make is an interpreter for Makefiles. A Makefile is much like a shell script with additional directions,
which qualify make to focus on the required lines. This will save you a lot of time.
Unfortunately common documentation stresses on compiling C programs. This guide shows how to use make for
workaday tasks, that have nothing to do with the C programming language.
Contents
- What's in those Makefiles?
- Explicit rules
- Prerequisites
- Referencing existing files
- Implicit rules
- User-defined variables and functions
- Combining multiple Makefiles
- Double-colon rules
- Comparing timestamps
- Patterns
- Built-in functions
- Automatic variables
- Links
A Makefile basically consists of
- Definitions of variables and functions,
e.g. myshell=bash
- Comments,
e.g. # converting postscript to pdf
- Includes,
e.g. -include Makefile.local
- Rules,
e.g. %.pdf: %.ps; -@ps2pdf $<
Basics
The generic anatomy of a rule is as follows:
target [ more targets] :[:] [ prerequisites ] [; commands]
[ <tab> commands ]
[ <tab> commands ]
...
|
Everything in square brackets is optional.
Rules that have got commands are called explicit, those which haven't are called implicit. This
chapter is about explicit rules.
Mind the tab key (<tab>) prefacing the commands past the first line.
Use a semicolon to stuff multiple commands into one single line, or a backslash to join split lines.
Example
The following Makefile:
hello:
@echo hello \
world
diskfree:; df -h /
|
defines two rules (with targets hello and diskfree) and (explicitly...) assigns each of them a
short command. Now we can specifically call those commands:
$ make hello
hello world
$ make diskfree
df -h /
Filesystem Size Used Avail Use% Mounted on
/dev/hda6 43G 37G 3.6G 92% /
|
BTW: The at-sign (@) in front of the command prevents make from announcing the command it's
going to do.
Observation 1: If you don't tell make a target, it will simply do the first
rule:
Observation 2: If you tell make multiple targets, it will do them all in
order:
$ make diskfree hello
df -h /
Filesystem Size Used Avail Use% Mounted on
/dev/hda6 43G 37G 3.6G 92% /
hello world
|
When getting dressed every morning, the order of installations should result from the following dependency tree:
It makes sure you're already wearing socks and trousers before getting into the shoes. We pronounce:
Socks and trousers are prerequisites for shoes.
and minute for the Makefile:
shoes: socks trousers
@echo put on shoes
|
Since make will substitute the target's name wherever it reads $@, we can generalize to:
shoes: socks trousers
@echo put on $@
|
If you pursue this method for the full tree in one-lined notation, you will get the following Makefile:
.PHONY: coat shoes mobile sweater socks\
trousers shirt pants undershirt
# target prerequisite command
# ------------------------------------------------
coat: shoes mobile sweater; @echo put on $@
shoes: socks trousers; @echo put on $@
mobile: trousers; @echo put on $@
sweater: shirt; @echo put on $@
socks: ; @echo put on $@
trousers: pants shirt; @echo put on $@
shirt: undershirt; @echo put on $@
pants: ; @echo put on $@
undershirt: ; @echo put on $@
|
BTW: The .PHONY-directive tells make which target names have nothing to do with potentially
existing files of the same name. The next example will work out on that.
The result looks quite promising:
$ make coat | nl
1 put on socks
2 put on pants
3 put on undershirt
4 put on shirt
5 put on trousers
6 put on shoes
7 put on mobile
8 put on sweater
9 put on coat
|
Just in case I just wanted to get into my sweater and everything what's neccessary for it:
$ make sweater | nl
1 put on undershirt
2 put on shirt
3 put on sweater
|
BTW: That |nl doesn't belong to make. This little unix tool just numbers make's output
lines.
Task: In the last example make assumed there always was no prerequisite satisfied at
invocation time, means you have to be naked before getting dressed. Well, I prefer having a mug of coffee after
installing the basics and before putting on the uncomfortable articles. Now, how can I allow the following sequence to
work?
$ make shirt trousers socks
$ drink coffee
$ make coat
|
Basic approach: Ascertaining the steps to the coat must be split in two parts. For this to work,
make has to save the state after the first run. So I create an empty file after finishing each target, with
the filename equal to the target's name. When I now take out the .PHONY-line, make will conclude
from an existing file to a satisfied target. Exactly speaking, it will compare the timestamps of target's and
prerequisite's files. See next chapter for details on this.
Solution: The following Makefile:
# target prerequisite command
# ----------------------------------------------------------
coat: shoes mobile sweater; @echo put on $@; touch $@
shoes: socks trousers; @echo put on $@; touch $@
mobile: trousers; @echo put on $@; touch $@
sweater: shirt; @echo put on $@; touch $@
socks: ; @echo put on $@; touch $@
trousers: pants shirt; @echo put on $@; touch $@
shirt: undershirt; @echo put on $@; touch $@
pants: ; @echo put on $@; touch $@
undershirt: ; @echo put on $@; touch $@
|
will do the job:
$ make shirt trousers socks | nl
1 put on undershirt
2 put on shirt
3 put on pants
4 put on trousers
5 put on socks
$ ls
trousers undershirt shirt socks pants
$ # drink coffee
$ make coat | nl
1 put on shoes
2 put on mobile
3 put on pullover
4 put on coat
$ ls
shirt coat shoes undershirt mobile
trousers pullover socks pants
|
I can split each rule into two parts:
- An implicit rule stating the prerequisits, and
- an explicit one for the commands.
Any rules in the last example lead to the same commands and differed in their prerequisites only. For those rules that
have no prerequisites (e.g. socks) I don't even need an implicit rule. The explicit ones can be pooled,
because of their commands being all identic. Thus I get a shorter and pretty clear Makefile:
# An explicit rule assigns the commands for several targets
coat shoes mobile sweater socks trousers\
shirt pants undershirt: ; @echo put on $@; touch $@
# Implicit rules state the prerequisites
coat: shoes mobile sweater
shoes: socks trousers
mobile: trousers
sweater: shirt
trousers: pants shirt
shirt: undershirt
|
Use = or := to assign values to variables, depending on if potentially contained variables and
functions should be expanded at using or declaration time. To retrieve the stored value, write
$(myvar). To have make executing the value (like a function), write $(call
myvar).
# Declaration of a variable
articles = coat shoes mobile sweater socks\
trousers shirt pants undershirt
# An explicit rule assigns the commands for several targets
$(articles) :; @echo put on $@; touch $@
# Implicit rules state the prerequisites
coat: shoes mobile sweater
shoes: socks trousers
mobile: trousers
sweater: shirt
trousers: pants shirt
shirt: undershirt
# Additional feature
.PHONY: naked
naked: ; @-rm $(articles)
|
BTW: The minus sign (-) in front of the rm causes make to ignore any errors that
occure while doing rm. This way a make naked coat will end at the coat, even for people who're
already naked at starting time.
Divide and konquer! For heavy projects (e.g. compiling the Linux kernel) you need giant Makefiles. They will become
much clearer, when you split them into separate, smaller parts.
When make encounters an include-command, it will stop processing the current Makefile, read
the included Makefile and then continue where it left off. If you don't want it to abort when the included Makefile's
missing, just say -include Makefile(s).
Example:
# tying the cravat
-include Makefile.tie
|
The following Makefile erroneously defines a target socks twice:
socks: ; @echo get into left sock
socks: ; @echo get into right sock
|
make will show up with a warning message and run the last target's command only:
$ make
Makefile:2: warning: overriding commands for target `socks'
Makefile:1: warning: ignoring old commands for target `socks'
get into right sock
|
Particularly when working with includes, you might want make to run the commands of any rule with a certain
target name. For this to work, you just need to duplicate the colon:
socks:: ; @echo get into left sock
socks:: ; @echo get into right sock
|
It will work as expected:
$ make
get into left sock
get into right sock
|
Here's a little Makefile I use for converting Postscript-files (tiger.ps) into PDF-files
(tiger.pdf):
tiger.pdf: tiger.ps; ps2pdf $<
|
make will only call ps2pdf when tiger.pdf is non-existing or older than
tiger.ps. BTW: make will automatically substitute the filename of the prerequisite
(tiger.ps) for $<.
Here's what you get:
$ ls tig*
tiger.ps
$ make tiger.pdf
ps2pdf tiger.ps
$ ls tig*
tiger.pdf tiger.ps
$ make tiger.pdf
make: `tiger.pdf' is up to date.
$
$ cp tiger2.ps tiger.ps
$ make tiger.pdf
ps2pdf tiger.ps
|
Thanks to the pattern-notation I need not write a separate rule for each postscript file. Generalizing the last
example:
The make tiger.pdf will work as before. But now a simple make (no targets) won't work any more.
make must be told a target's name.
$ make
make: *** No targets. Stop.
$ make tiger.pdf
ps2pdf tiger.ps
|
The following Makefile will create the file tiger.pdf even when you call make without any
parameters:
all : tiger.pdf
%.pdf: %.ps; -ps2pdf $<
|
Now, this works fine for the tiger. What if I want to convert any files in the current directory? No
problem - the $(wildcard)function can hand me a list of all *.ps files, and the
$(patsubst)-function will change it to a list of PDF filenames:
all: $(patsubst %.ps,%.pdf,$(wildcard *.ps))
%.pdf: %.ps; -ps2pdf $<
|
| $(subst from,to,text) |
Replace from with to in text. |
| $(patsubst pattern,replacement,text) |
Replace words matching pattern with replacement in text. |
| $(strip string) |
Remove excess whitespace characters from string. |
| $(findstring find,text) |
Locate find in text. |
| $(filter pattern...,text) |
Select words in text that match one of the pattern words. |
| $(filter-out pattern...,text) |
Select words in text that do not match any of the pattern words. |
| $(sort list) |
Sort the words in list lexicographically, removing duplicates. |
| $(dir names...) |
Extract the directory part of each file name. |
| $(notdir names...) |
Extract the non-directory part of each file name. |
| $(suffix names...) |
Extract the suffix (the last dot and following characters) of each file name. |
| $(basename names...) |
Extract the base name (name without suffix) of each file name. |
| $(addsuffix suffix,names...) |
Append suffix to each word in names. |
| $(addprefix prefix,names...) |
Prepend prefix to each word in names. |
| $(join list1,list2) |
Join two parallel lists of words. |
| $(word n,text) |
Extract the nth word (one-origin) of text. |
| $(words text) |
Count the number of words in text. |
| $(wordlist s,e,text) |
Returns the list of words in text from s to e. |
| $(firstword names...) |
Extract the first word of names. |
| $(wildcard pattern...) |
Find file names matching a shell file name pattern (not a `%' pattern). |
| $(error text...) |
When this function is evaluated, make generates a fatal error with the message
text. |
| $(warning text...) |
When this function is evaluated, make generates a warning with the
message text. |
| $(shell command) |
Execute a shell command and return its output. |
| $(origin variable) |
Return a string describing how the make variable variable was
defined. |
| $(foreach var,words,text) |
Evaluate text with var bound to each word in words, and concatenate
the results. |
| $(call var,param,...) |
Evaluate the variable var replacing any references to $(1),$(2) with the
first, second, etc. param values. |
| $@ |
The name of the target. |
| $% |
The target member name, when the target is an archive member. |
| $< |
The name of the first (or only) prerequisite. |
| $? |
The names of all the prerequisites that are newer than the target, with spaces between them. |
$^
$+ |
The names of all the prerequisites, with spaces between them. The value of
$^ omits duplicate prerequisites, while $+ retains them and preserves their order. |
| $* |
The stem with which an implicit rule matches. |
$(@D)
$(@F) |
The directory part and the file-within-directory part of $@ |
$(*D)
$(*F) |
The directory part and the file-within-directory part of $* |
$(%D)
$(%F) |
The directory part and the file-within-directory part of $% |
$(<D)
$(<F) |
The directory part and the file-within-directory part of $< |
$(^D)
$(^F) |
The directory part and the file-within-directory part of $^ |
$(+D)
$(+F) |
The directory part and the file-within-directory part of $+ |
$(?D)
$(?F) |
The directory part and the file-within-directory part of $? |
