Question

• package_version := $(version)x0d$(date)

  what is the x0d part between version and date vars? is it just string?

• What $(dotin_files:.in=) does below

code

dotin_files       := $(shell find . -type f -name \*.in)
dotin_files       := $(dotin_files:.in=)

• what this means $(dotin_files:=.in)

code

$(dotin_files): $(dotin_files:=.in)
 $(substitute) $@.in > $@

• can target contain multiple files?

• what is the meaning of declaring target variable as PHONY?

code

.PHONY: $(dotin_files)

• In the regex replacement code below

code

substitute := perl -p -e 's/@([^@]+)@/defined $$ENV{$$1} ? $$ENV{$$1} : $$&/ge'

what are $$ENV{$$1} and $$&? I guess it’s Perl scope…

thanks for your time

Answer 1

Variable Expansion

$() is variable expansion in make, this should just be string substitution – if your makefile is

version=1
date=1.1.10
package_version:=$(version)x0d$(date)

then the variable package_version will expand to 1x0d1.1.10.

Substitution

The syntax $(var:a=b) is a substitution reference and will expand to var with a suffix a substituted with b.

For example, in

foobar:= foo bar
faabar:=$(foobar:oo=aa)

$(faabar) will expand to the string faa bar.

Multiple Targets

Multiple targets in a make rule is equivalent to having n rules with a single target, eg

foo bar:foo.c bar.c
     $(CC) -o $@ $^

is equivalent to

foo:foo.c bar.c
     $(CC) -o $@ $^
bar:foo.c bar.c
     $(CC) -o $@ $^

remember that any variables here are expanded.

Phony Targets

The .PHONY target declares that a rule doesn’t produce an actual file, so it will always be built. As always, variables are expanded first. In your case this will expand to something like

.PHONY: foo bar

Escaping

A dollar sign is an escape character in makefiles, the $$ in your perl example is a literal $, eg substitute will be the string

perl -p -e 's/@([^@]+)@/defined $ENV{$1} ? $ENV{$1} : $&/ge'

The dollar signs here are processed by perl, and probably give environment variables (I don’t know perl).