# make(1)

## Anatomy of `make` rules
```make
target .. : prerequisite ..
	recipe
	..
```

- `target`: an output generated by the rule
- `prerequisite`: an input that is used to generate the target
- `recipe`: 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:
```make
%.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.

```make
# file: Makefile

all: foobar blabla

foo% bla%: aaa bbb bbb
	@echo "@ = $@"
	@echo "< = $<"
	@echo "^ = $^"
	@echo "+ = $+"
	@echo "* = $*"
	@echo "----"

aaa:
bbb:
```

Running above `Makefile` gives:
```text
@ = 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 using `make -C path`

### Multi-line variables
```make
define my_var
@echo foo
@echo bar
endef

all:
	$(my_var)
```
Running above `Makefile` gives:
```text
foo
bar
```

## Arguments

Arguments specified on the command line *override* ordinary variable
assignments in the makefile ([overriding variables][make-var-override]).

```make
VAR = abc
all:
	@echo VAR=$(VAR)
```

```text
# make
VAR=abc

# make VAR=123
VAR=123
```

## Useful functions

### Substitution references
Substitute strings matching pattern in a list.
```make
in  := a.o l.a c.o
out := $(in:.o=.c)
# => out = a.c l.a c.c
```

### `patsubst` ([ref][make-patsubst])
```make
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.
```make
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.
```make
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).
```make
$(abspath fname1 fname2 ..)
```

### `realpath`
Resolve each file name as canonical path.
```make
$(realpath fname1 fname2 ..)
```

### `call`  ([ref][make-call])
Invoke parametrized function, which is an expression saved in a variable.
```make
swap = $(2) $(1)

all:
	@echo "call swap first second -> $(call swap,first,second)"
```
Outputs:
```text
call swap first second -> second first
```

### `eval` ([ref][make-eval])
Allows to define new makefile constructs by evaluating the result of a variable
or function.
```make
define new_rule
$(1):
	@echo "$(1) -> $(2)"
endef

default: rule1 rule2

$(eval $(call new_rule,rule1,foo))
$(eval $(call new_rule,rule2,bar))
```
Outputs:
```text
rule1 -> foo
rule2 -> bar
```

### foreach ([ref][make-foreach])
Repeat a piece of text for a list of values, given the syntax `$(foreach
var,list,text)`.
```make
myfn = x$(1)x

default:
	@echo $(foreach V,foo bar baz,$(call myfn,$(V)))
```
Outputs:
```text
xfoox xbarx xbazx
```

## Examples

### Config based settings
```make
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.
```sh
$ 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
```make
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:
```text
rule1 -> foo
rule2 -> bar
```
> Use `make -R -p` to print the make database including the rules.

[make-var-override]: https://www.gnu.org/software/make/manual/html_node/Overriding.html
[make-patsubst]: https://www.gnu.org/software/make/manual/html_node/Text-Functions.html#index-patsubst-1
[make-call]: https://www.gnu.org/software/make/manual/html_node/Call-Function.html
[make-eval]: https://www.gnu.org/software/make/manual/html_node/Eval-Function.html
[make-foreach]: https://www.gnu.org/software/make/manual/html_node/Foreach-Function.html