make(1)
Anatomy of make
rules
target .. : prerequisite ..
recipe
..
target
: an output generated by the ruleprerequisite
: an input that is used to generate the targetrecipe
: list of actions to generate the output from the input
Use
make -p
to print all rules and variables (implicitly + explicitly defined).
Pattern rules & variables
Pattern rules
A pattern rule contains the %
char (exactly one of them) and look like this example:
%.o : %.c
$(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@
The target matches files of the pattern %.o
, where %
matches any none-empty
substring and other character match just them self.
The substring matched by %
is called the stem
.
%
in the prerequisite stands for the matched stem
in the target.
Automatic variables
As targets and prerequisites in pattern rules can't be spelled explicitly in the recipe, make provides a set of automatic variables to work with:
$@
: Name of the target that triggered the rule.$<
: Name of the first prerequisite.$^
: Names of all prerequisites (without duplicates).$+
: Names of all prerequisites (with duplicates).$*
: Stem of the pattern rule.
# file: Makefile
all: foobar blabla
foo% bla%: aaa bbb bbb
@echo "@ = $@"
@echo "< = $<"
@echo "^ = $^"
@echo "+ = $+"
@echo "* = $*"
@echo "----"
aaa:
bbb:
Running above Makefile
gives:
@ = foobar
< = aaa
^ = aaa bbb
+ = aaa bbb bbb
* = bar
----
@ = blabla
< = aaa
^ = aaa bbb
+ = aaa bbb bbb
* = bla
----
Variables related to filesystem paths:
$(CURDIR)
: Path of current working dir after usingmake -C path
Multi-line variables
define my_var
@echo foo
@echo bar
endef
all:
$(my_var)
Running above Makefile
gives:
foo
bar
Arguments
Arguments specified on the command line override ordinary variable assignments in the makefile (overriding variables).
VAR = abc
all:
@echo VAR=$(VAR)
# make
VAR=abc
# make VAR=123
VAR=123
Useful functions
Substitution references
Substitute strings matching pattern in a list.
in := a.o l.a c.o
out := $(in:.o=.c)
# => out = a.c l.a c.c
patsubst
(ref)
in := a.c b.c
out := $(patsubst %.c, build/%.o, $(in))
# => out = build/a.o build/b.o
# This is actually equivalent to $(in:%.c=build/%.o)
filter
Keep strings matching a pattern in a list.
in := a.a b.b c.c d.d
out := $(filter %.b %.c, $(in))
# => out = b.b c.c
filter-out
Remove strings matching a pattern from a list.
in := a.a b.b c.c d.d
out := $(filter-out %.b %.c, $(in))
# => out = a.a d.d
abspath
Resolve each file name as absolute path (don't resolve symlinks).
$(abspath fname1 fname2 ..)
realpath
Resolve each file name as canonical path.
$(realpath fname1 fname2 ..)
call
(ref)
Invoke parametrized function, which is an expression saved in a variable.
swap = $(2) $(1)
all:
@echo "call swap first second -> $(call swap,first,second)"
Outputs:
call swap first second -> second first
eval
(ref)
Allows to define new makefile constructs by evaluating the result of a variable or function.
define new_rule
$(1):
@echo "$(1) -> $(2)"
endef
default: rule1 rule2
$(eval $(call new_rule,rule1,foo))
$(eval $(call new_rule,rule2,bar))
Outputs:
rule1 -> foo
rule2 -> bar
foreach (ref)
Repeat a piece of text for a list of values, given the syntax $(foreach var,list,text)
.
myfn = x$(1)x
default:
@echo $(foreach V,foo bar baz,$(call myfn,$(V)))
Outputs:
xfoox xbarx xbazx
Examples
Config based settings
conf-y := default
conf-$(FOO) := $(conf-y) foo
conf-$(BAR) := $(conf-y) bar
libs-y := libdef
libs-$(FOO) += libfoo
libs-$(BAR) += libbar
all:
@echo "conf-y: $(conf-y)"
@echo "libs-y: $(libs-y)"
Yields the following results.
$ make
conf-y: default
libs-y: libdef
$ make FOO=y
conf-y: default foo
libs-y: libdef libfoo
$ make BAR=y
conf-y: default bar
libs-y: libdef libbar
$ make FOO=y BAR=y
conf-y: default foo bar
libs-y: libdef libfoo libbar
Using foreach / eval / call
to generate new rules
define new_rule
$(1):
@echo "$(1) -> $(2)"
endef
arg-rule1 = foo
arg-rule2 = bar
RULES = rule1 rule2
all: $(RULES)
$(foreach R,$(RULES),$(eval $(call new_rule,$(R),$(arg-$(R)))))
# equivalent to
# $(eval $(call new_rule,rule1,foo))
# $(eval $(call new_rule,rule2,bar))
Outputs:
rule1 -> foo
rule2 -> bar
Use
make -R -p
to print the make database including the rules.