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NAME


makepp_variables -- How to use variables in makepp

DESCRIPTION


?: $*,
$+,
$/,
$<,
$?,
$@,
$^,
=,
!=,
&=,
+=,
:=,
;=,
?=, A: AR,
ARFLAGS,
AS, C: CC,
CFLAGS,
"changed_dependencies",
changed_inputs,
CURDIR,
CXX,
CXXFLAGS, D: "dependencies",
"dependency", F: F77,
FC,
"foreach", I: input,
inputs, L: LD,
LEX,
LIBTOOL, M: MAKE,
MAKECMDGOALS,
MAKEFLAGS,
MAKEINFO,
MAKEPP_DEBUG,
MAKEPPFLAGS,
_MAKEPPFLAGS,
MAKEPP_LN_CP,
"makepp_percent_subdirs",
"makepp_require_phony",
makepp_signature_C_flat,
"makepp_simple_concatenation",
MAKEPP_VERSION, O: "output",
"outputs", P: PERL,
PWD, R: RM,
ROOT, S: SHELL,
"sorted_dependencies",
sorted_inputs,
"stem", T: target,
targets, V: VPATH, Y: YACC

Makefiles typically use variables in many places. One important reason for using
variables is to ensure that information is contained in only one place in the makefile, so
that if it changes, there is no danger of the two copies of the information getting out of
sync.

Variable names are case sensitive. In theory, variable names can be made of many
characters, but makepp will probably not be able to grok it if you do anything other than
alphanumeric characters, "_", and "-".

Each makefile has its own set of variables, and setting a variable in one makefile will
have no effect on its value in any other makefile. If you want to have variables set in
many makefiles, the best way to do it is to have each of them include a common definitions
file (see the include statement) or use "global" variables.

Variable Assignment
A variable can assume a value in several different ways:

· A variable may be set inside a makefile. There are a number of different ways to do
this; see below.

· A variable's value may be specified on the command line, like this:

makepp CFLAGS=-O2 my_program

If more than one makefile is loaded, the CFLAGS variable is propagated to all of the
makefiles. Variables set on the command line automatically override any setting of
the variable in any of the makefiles.

If ever needed, the makefile must in turn explicitly override command line settings.
The intention is not to ignore what the user requests, but rather a way to modify it.
The "override" modifier may precede any assignment statement. But in the case of
keyword statements, the order is important, which is why the override variant is
always shown below. The "override" modifier applies only to any assignments where it
is present, and does not influence later assignments to the variable.

· If a variable is set in the environment, it can be referenced as a makepp variable.
Ordinarily assignments to variables inside a makefile override settings from the
environment, but you can change this by using the "-e" or "--environment-overrides"
command line option.

Variables are assigned with one of several assignment expressions, like this

X = 1
MODULES := a b c d
CC ?= gcc
CFLAGS += -Wall
define VAR
var line 1
var line 2
enddef
export PATH := $(PWD):$(PATH)
global MYPROJECT.INFO = info to be seen in all makefiles

Leading and trailing whitespace around values is always stripped off.

The different assignment operators have somewhat different meanings.

Simple assignment operators

=
VARIABLE = text string
override VARIABLE = text string

This is the usual assignment statement that all implementations of make support. The
expression on the right hand side is not evaluated until the value of "$(VARIABLE)" is
actually used somewhere. Thus, if you do the following:

X = 1
Y = $(X)
X = 2

Then "$(Y)" later in the makefile will evaluate to "2".

In general, you usually want to use ":=" (see below) instead of "=" because it
provides more predictable variable evaluation. However, there are times when you need
to defer the variable evaluation. Also, if you're writing a makefile that must be
backwards-compatible with some version of make other than GNU make, then you have no
choice: you may only use "=".

:=
VARIABLE := expr
override VARIABLE := expr

This is the same as "VARIABLE = expr" except that the right hand side is evaluated
once and for all at the time of the assignment. Thus if

X := 1
Y := $(X)
X := 2

then "$(Y)" later in the makefile will evaluate to "1" since that's what "$(X)" was
when "$(Y)" was defined.

;=
VARIABLE ;= expr
override VARIABLE ;= expr

This is the same as "VARIABLE := expr" except that the right hand side is evaluated
only at the time of the first use and then remembered. This is useful for expensive
commands, which always return the same value, but which you don't want to perform when
building unrelated targets:

VAR1 ;= $(perl expensive calculations)
VAR2 ;= $(shell external command)

Note that old makefiles will usually use ":=" here, to at least do this only once.
But with this operator you can even additionally not do it, if you currently don't
need the value. For values which are identical in several directories, you can
optimize this further with "global", discussed below.

However this is not intended as a clever way to force order of evaluation. If a
variable defined like this includes the value of another variable, and that other one
has a target specific value, and the first expansion is for that target, then the
target specific value will stick for all other contexts as well. This is a bug and
will hopefully be fixed in the future.

+=
VARIABLE += expr
override VARIABLE += expr

Appends the string to the previous contents of the variable, separated by a space. If
the variable was previously assigned with ":=", then the right hand side is evaluated
before appending.

&=
VARIABLE &= expr
override VARIABLE &= expr

Prepends the string to the previous contents of the variable, separated by a space.
If the variable was previously assigned with ":=", then the right hand side is
evaluated before appending.

For example one way of guaranteeing that "CFLAGS", whatever else the user may put in,
always starts with "-Wall" are these two lines:

CFLAGS = -O2 # Possibly overridden on the command line
override CFLAGS &= -Wall # Unconditionally prepended

In old makefiles you typically had to do something like this, which had the side
effect of forcing the type to ":=" to prevent endless recursion:

VARIABLE := expr $(VARIABLE)

?=
VARIABLE ?= expr
override VARIABLE ?= expr # Useless, but legal

Sets the value of the variable, but only if the variable is not specified earlier in
the makefile, on the command line, or in the environment. The above assignment is
exactly equivalent to

ifndef VARIABLE
VARIABLE = expr
endif

!=
VARIABLE != shell command
override VARIABLE != shell command

Runs the shell command and sets the variable to contain the command's standard output.
This is exactly equivalent to

VARIABLE := $(shell command)

Multiline variables

The "define" statement is the multiline equivalent of the simple statements above. The
operator after the variable is optional. If missing, it is equivalent to "define VARIABLE
=". The "&=" and "+=" operators are slightly different here, in that they glue this to
the old value with a newline, rather than a space. There must not be anything except a
comment after the statement, i.e. the value starts on the next line.

define VARIABLE :=
first line of variable's value
second line of variable's value
third line of variable's value
endef

override define VARIABLE
...
enddef

Keywords before "define" can be combinations of either one of "export" or "global" and
"override".

If you need a variable's value to contain newlines, you must use the "define" statement as
shown (or you can assign the value directly in Perl). ("endef" was chosen for
compatibility with GNU make. You may also use "enddef".) This is primarily useful for
"canned command sequences", e.g., something like this:

define COMPILE_C_PROGRAM
@&echo "Compiling $(input)"
@$(CC) $(CFLAGS) $(CPPFLAGS) $(INCLUDES) -c $(input) -o $(output)
endef

Then you can use this multi-line variable in several rules, like this:

%.o : %.c
$(COMPILE_C_PROGRAM)

$(ARCH)/%.o : $(ARCH)/%.c
$(COMPILE_C_PROGRAM)

Note that you can often achieve the same effect by using a semicolon instead of a newline,
because the shell interprets that as a command delimeter too. For example,

COMPILE_C_PROGRAM = @echo "Compiling $(input)"; \
$(CC) $(CFLAGS) $(CPPFLAGS) $(INCLUDES) -c $(input) -o $(output)

will have the same effect, except that the semicolon forces Perl to pass it to the shell,
instead of executing the command directly and more efficiently. You also have to put each
builtin on a line of its own, meaning that you have to switch to the external echo in the
semicolon case.

There is one speciality when expanding within "define", i.e. "define X :=" or on a
variable that was already ":=", "define X &=" and "define X +=". In this case the
"$(shell command ...)" or builtin "$(&command ...)" do not get newlines transformed into
spaces.

Exporting variables to subprocesses

export VAR ...
export VAR = value
override export VAR += value

The first form marks the given variables for export to subprocesses, with whatever value
the variable then has. The second form, which applies only to one variable, additionally
assigns a value right away. All the variants of assignment are allowed.

Sharing variables across makefiles

global VAR ...
global VAR = value
override global VAR &= value

The first form marks the given variables as global to all makefiles. If any of them
already had a value, that value is moved from the makefile local variable to the global
one. However if, at the time this statement is seen, any other makefiles already had a
value for any of them, then those makefiles will not see the global one.

The second form, which applies only to one variable, additionally assigns a value right
away. All the variants of assignment are allowed. Note that ":=" will expand right away
to the values in the local makefile. On the contrary "=" variables will expand to the
values at the point of use.

Global variables can be tricky to deal with, because makepp may load makefiles in any
order, as the need arises to build some target for which no rule or makefiles is loaded.
For this reason it is recommended to have a RootMakeppfile and to explicitly load all
others which modify or use the variable with "load-makefile".

global GLIBLIBS ;= $(shell pkg-config --libs glib-2.0)

Also note that your project may some day be built together with other projects. For this
reason it is recommended to always make the project name part of any global variable name.

Target-specific assignments

target: VARIABLE = string
target: VARIABLE := string
target: override VARIABLE += string

Sets a target-specific value of the variable. A target-specific value is in effect only
in an action which produces the given target. This is primarily used for things like
this:

CFLAGS := -O2

my_prog: file1.o file2.o special_file.o

special_file.o : CFLAGS := -g

%.o: %.c
$(CC) $(CFLAGS) -c $(input) -o $(output)

What happens here is that all ".c" files will be compiled with optimization ("-O2") except
"special_file.c", which is compiled in debug mode ("-g"). This is a convenient way to
specify different compilation options for only a few files.

Target-specific variable assignments like this apply only to the actions of the rule; they
are not in effect when evaluating the targets or the dependencies of a rule. If a rule
has more than one target, target-specific variable assignments are taken only from the
first target. Also note that makepp's target-specific variables are slightly different
from GNU make's in that they only apply to the rule for the one file mentioned, and not to
any of its predecessors.

Wildcard expansion is performed on the target, so you can do something like this:

test_*.o : CFLAGS += -DTEST

For compatibility with GNU make, "%" may be used in place of "*".

Variable Substitution
Makepp's variable substitution rules are similar to those of other makes, but somewhat
more powerful. As in all makes, "$(CC)" or "${CC}" both represent the value of the
variable CC. If you need a literal dollar sign, put in a double dollar sign ($$), like
this:

target: dep1 dep2 dep3 dep4
&rm -f $(output)
for file in $(inputs); do cat $$file >> $(output); done

Additionally makepp has a "$[VARIABLE]" syntax, which does the same thing as the other
two, but before makepp groks anything else. This allows using it for complete rules
and/or conditionals:

define bracket_rule =
ifdef SOME_VAR
bracket:
&echo this is a rule -o $(output)
endif
enddef

$[bracket_rule]

rc-style substitution

By default, makepp uses rc-style substitution (so called because it was pioneered by the
rc shell). This is best illustrated by an example:

MODULES = a b c d

mylib.a : module_dir/$(MODULES).o $(OTHER_OBJECTS)
$(CXX) $(dependencies) -o $(target)

The prefix "module_dir/" is prepended to each word in MODULES, and the suffix ".o" is
appended to each word.

You can also use rc-style substitution without even putting the list of words into a
variable; the syntax is "$( word1 word2)". Note the space between the parenthesis and the
first word. So the above example could have been written as:

mylib.a : module_dir/$( a b c d).o $(OTHER_OBJECTS)
$(CXX) $(dependencies) -o $(target)

A variable will give rc-style substitution only when there is more than one word in it.
With one word it is like traditional make. Alas, when the variable is empty, there is a
conflict. Traditional makes simply expand it to the empty string. But when you think of
it as a list, you'd want "-I$(DIRLIST)" to disapear, not to give a lonely "-I". The
solution is to wrap it into a list that starts with a space: "-I$( $(DIRLIST))" gives you
exactly as many options, as there are words in the list.

If you put several variables in the same word which expand to arrays of words, rc-style
substitution actually takes the cartesian product, so you can do something like this if
you want:

DIRS = s1 s2
MODULES = a b c
SUFFIXES = .o .c
FILES := $(DIRS)/$(MODULES)$(SUFFIXES)

and FILES will contain the string

s1/a.o s1/a.c s1/b.o s1/b.c s1/c.o s1/c.c s2/a.o s2/a.c s2/b.o s2/b.c s2/c.o s2/c.c

Besides white space, rc-style substitution stops at any quote, any round, curly or square
parentheses, and any of ", : ; = # @". Note that this may be surprising, and again need
an explicit list. Say you want to look for headers by wildcard in all directories
starting with an upper-case letter:

BAD := [A-Z]*/**/*.$( hpp h) # [A-Z]*/**/*.hpp */**/*.h
GOOD := $( [A-Z]*/**/*.)$( hpp h) # [A-Z]*/**/*.hpp [A-Z]*/**/*.h

If rc-style substitution gets in the way, or if you need to have leading or trailing
whitespace in your make variables, then you can turn off rc-style substitution by setting
the variable "makepp_simple_concatenation=1". You can turn it off globally on the command
line or as an environment variable. Or on a per-makefile basis by setting the variable in
your makefile. You should do this near the top of the makefile, or else you may run into
funny situations where rc-style substitution is used for some evaluations and not others.
(All expressions evaluated before the assignment will use rc-style substitutions, and all
expressions evaluated after will not. Since the time of evaluation of expressions in
makefiles is complicated and not always obvious from the order of statements in the
makefile, it's best to set "makepp_simple_concatenation" as early as possible.) You can
even set it just for one target:

target: makepp_simple_concatenation = 1
target:
&echo before_first$(LIST)after_last -o $(output)

Because "$[VARIABLE]" is evaluated earlier than "$(VARIABLE)", combining the two in rc-
substitution will not give the result you may expect, unless you wrap it in an explicit
list:

A = a b
N = 1 2
BAD := $(A)$[N]
GOOD := $(A)$( $[N])

The last lines get read as

BAD := $(A)1 2 # a1 b1 2
GOOD := $(A)$( 1 2) # a1 a2 b1 b2

Substitution References

A substitution reference has the form "$(VAR:A=B)", where A is a pattern to match and B is
a pattern to replace it with. Substitution references are an abbreviation for the
"patsubst" function. For example:

source_files = a.c b.c c.c d.c
object_files = $(source_files:%.c=%.o)

will set "$(object_files)" to "a.o b.o c.o d.o". The "%" is a special character matches
any arbitrary string. If both A and B start with "%", a common case when replacing
suffixes, they can be omitted:

object_files = $(source_files:.c=.o)
object_files = $(source_files:c=o) # same, because . is not special

Whitespace in variables

If you need to control the whitespace in a variable, you must (currently) disable rc-style
substitution (by setting "makepp_simple_concatenation=1") and then use a syntax like this:

null =
T = -o $(null)

or, with an empty evaluation:

T = -o $()

When you do this, the variable "T" contains "-o" followed by a space.

This kind of a technique to handle whitespace is not recommended. If you need variables
in your makefile to contain spaces, you should think seriously about what you're doing.
If you need to handle spaces, it is usually much better to put Perl code into your
makefile to take care of it (using the "perl_begin" or "sub" statements), or to handle it
in shell statements in the actions of rules.

These cases typically come up when people attempt to use the same rules for different
architectures which do not use typical Unix command syntax. E.g., sometimes one sees
things like this in makefiles:

ifeq ($(ARCH),weirdarch)
O := /OUTPUT=
else
null :=
O := -o $(null)
endif

%.o : %.c
$(COMPILER) $(input) $(O)$(output)

You can do this with makepp if you really want to, but you will probably find that your
makefiles are substantially more readable if you have less complicated variable
substitution, e.g.,

ifeq ($(ARCH),weirdarch)
%.o : %.c
$(WEIRD_COMPILER) $(input) /OUTPUT=$(output)
else
%.o : %.c
$(CC) -c $(input) -o $(output)
endif

Whitespace is never allowed in variable names, only in their values. This is different
from some make implementations.

Automatic Variables
Automatic variables are variables that assume different values depending on which rule
they are evaluated in. Makepp supports most of the automatic variables that other
versions of make use. In addition, it has less cryptic, longer names for most of them
that you can use instead. (For legacy makefiles that happen to redefine these names, the
definition in the makefile overrides the default meaning. For example, if you say
"target = abc" in your makefile, then "$(target)" will always expand to "abc", and will no
longer be equivalent to $@.)

The following is a complete list of all the automatic variables that makepp supports:

output
target
$@ The target of the current rule. Actually, since makepp supports multiple targets for
any rule, this is the first target. For example, in the following rule

y.tab.c y.tab.h : parser.y
$(YACC) -o $(output) $(YFLAGS) $(input)

"$(output)" will contain the value y.tab.c. Since these magic variables are in fact
functions, you can also pass an index as argument. This counts from 1 or backwards
from -1. So "$(output 2)" or "$(output -1)" will contain the value y.tab.h.

While all three forms of this variable have the same value, there is a difference in
interpretation for multitarget rules. If you use the old-style cryptic name $@,
makepp will interpret that as an old-style rule set, rather than a modern rule that
produces all those targets in one go:

a b: # really: one rule each for a and b
touch $@

c d: # error: mpp complains that this didn't build d
touch $(output)

outputs
targets
All targets of the current rule. Same as "$(target)" unless there is more than one
target. In the above example, "$(outputs)" will be y.tab.c y.tab.h. You can pass an
index list, so "$(outputs 2 1)" will be y.tab.h y.tab.c.

dependency
input
$< The first explicit dependency of the rule. For example, in this rule

%.o : %.c
$(CC) $(CFLAGS) -c $(input) -o $(output)

"$(input)" will be the name of the .c file, regardless of what .h files makepp
discovers. If you specify more than one dependency, you can get them by passing an
index: "$(input $(INDEX))" is the INDEXth dependency.

dependencies
inputs
$^ All the explicit dependencies of the target, not including .h files discovered by
makepp_scanning for includes.

For example, in the rule

myprog.o : *.o
$(CC) $(CFLAGS) $(inputs) -o $(output)

"$(inputs)" will be all the .o files in the directory. You can pick only the ones you
want by passing an index list. If you explicitly specify different kinds of files,
you can pick them as in "$(inputs 2 3 4)" (but with a wildcard this is not too
promising).

sorted_dependencies
sorted_inputs
$+ All the dependencies of the target, in sorted order, with duplicates removed.
Equivalent to "$(sort $(inputs))".

changed_dependencies
changed_inputs
$? The dependencies of the target that have changed. This includes only explicit
dependencies (i.e., ones you list in the makefile), not implicitly discovered
dependencies from scanning (such as .h files).

This is commonly used in commands like this:

libmine.a : $(MODULES) : build_check ignore_action
$(AR) ru $@ $?

i.e., ar is told to replace only those modules that have changed. (Note the
"ignore_action" build check rule. If you don't specify this, makepp will force the
action to be executed whenever it changes. If no dependencies have changed, the
action string will be "ar ru libmine.a" which is probably different from what it was
last time you ran it, so without "ignore_action" makepp will execute it. In this
case, it's harmless, but with other commands, it could be a problem. See
makepp_build_check for details on "ignore_action".)

Building archives like this is not a good idea because it will make your builds less
reliable. The problem with this is that if you build the archive, then remove one of
the modules from the list of MODULES, the modules will still be in the archive and
makepp

stem
$* The stem in a pattern rule (i.e., whatever the '%' matched). Alternatively, if this
is not a pattern rule, returns the file name without the extension (i.e., it's
equivalent to "$(basename $(input))".

This is mostly for backward compatibility. For example, in old versions of make the
only way to tell it how to compile any .c file into the corresponding .o file was like
this:

.c.o:
$(CC) $(CFLAGS) -c $*.c -o $*.o

This is a lousy way to write the rule. It's much clearer to use GNU-make style
pattern rules, like this:

%.o : %.c
$(CC) $(CFLAGS) -c $(input) -o $(output)

foreach
The current filename from the "foreach" clause. "foreach" clauses are rarely used,
but they are the most general-purpose kind of pattern rule that makepp supports. For
example,

#
# Build .c files with some sort of a special preprocessor:
#
%.c : %.k
$(preprocessor) $(input) > $(output)

#
# Compile .c files into .o files:
#
%.o : %.c
$(CC) $(CFLAGS) -c $(input) -o $(output)

#
# Special alternate compilation flags for .c files which are derived
# from .k files:
#
$(foreach:%.k=%.o) : $(foreach:%.k=%.c) : foreach *.k
$(CC) $(SPECIAL_K_FLAGS) -c $(input) -o $(output)

See the documentation on the foreach clause in rules for more details and examples.

$/ This is essentially a constant, either "/", or on native Windows "\". You need it, if
you want to start a program portably, e.g. one you built in the current directory:

myoutput:
.$/myprog >$(output)

For filenames passed as arguments it is not so necessary as Windows can handle Unix
syntax there.

Option Variables
You can set these in all the ways explained above (except if stated otherwise) to modify
the bahaviour of makepp. By default they are all undefined.

MAKEPP_DEBUG
If you export this with a true value before calling makepp, you get more debugging
info. This is the "RULE_SOURCE" of files built while this is in effect under
makeppinfo, mppi and details about variable expansion and rule matching under
makepplog, mppl. This will also show you if makepp runs out of memory or hangs,
because it is endlessly expanding the same variables:

A = $A # deferred evaluation, need :=
B = $C
C = $B

MAKEPP_LN_CP
See the note under &ln.

makepp_percent_subdirs
Set this to some true value (like 1) to have "%" in targets or dependencies match
across more than one directory.

makepp_require_phony
Set this to some true value (like 1) to prevent implicit phonyness (i.e. if a rule
succeeds without producing its target). This variable should be true by default, to
prevent broken dependency chains, but that would break backward compatibility with
sloppy makefiles.

makepp_signature_C_flat
This is an option to signature C. Set this to some true value (like 1) to treat your
sources as flattened, i.e. all newlines (except around preprocessor statements) are
treated like whitespace and "#line" directives are ignored.

makepp_simple_concatenation
Set this to some true value (like 1) to prevent "rc-style substitution".

PERL
You can set this to the exact perl you are using to run the scripts in the makepp
suite, and export it. This can help to run sub-scripts with the same perl. This is
mostly useful for the test suite, the installation and for recursive builds.

VPATH
Setting this variable to some value implicitly calls "vpath % value".

Predefined Variables
Makepp predefines a few variables, which you can override:

AR Default: "ar".

ARFLAGS
Default: "rv".

AS Default: "as".

CC Default: The first found among "gcc", "egcc", "pgcc", "c89" or "cc", or on Windows
additionally "cl" or "bcc32".

CFLAGS
Default: If "$(CC)" is a GNU compiler "-g -Wall", if it is one of the two Windows
compilers nothing, else "-g".

CURDIR
The directory in which the current Makefile resides.

CXX Default: The first found among "g++", "c++", "pg++", "cxx", "C""C" or "aCC", or on
Windows additionally "cl" or "bcc32".

CXXFLAGS
Default: If "$(CXX)" is a GNU compiler "-g -Wall", if it is one of the two Windows
compilers nothing, else "-g".

F77 Default: The first found among "f77", "g77" or "fort77".

FC Default: "$(F77)".

LD Default: "ld".

LEX Default: The first found among "lex" or "flex".

LIBTOOL
Default: "libtool".

MAKE
This variable has two different values, depending on the presence or not of
"--traditional-recursive-make". Makepp recognizes when this variable is used and
turns off some features, even before the recursion actually takes place. This can be
undesirable while you are gradually eliminating it, so first convert the calls to a
makepp specific "$((MAKE))", which will not turn off features. You should not set it
because that breaks our smart recursion work around.

MAKECMDGOALS
This variable is set but not used by makepp. You can query it to do something only if
a certain target was requested.

Default: Whatever explicit targets the user (or a recursive invocation) provided.
Empty when implicitly building default target.

ifneq $(filter special-target, $(MAKECMDGOALS))
# special-target is one of the current explicit targets
else ifeq $(MAKECMDGOALS)
# no explicit targets
endif

MAKEFLAGS (exported)
The standard options with which makepp was called. Those that have a single letter
form are combined at the beginning without a leading "-" (heaven knows why gmake chose
to drop the "-").

MAKEINFO
Default: "makeinfo".

MAKEPPFLAGS (exported)
This is set to the same value as MAKEFLAGS, but only if this variable is present in
makepp's environment.

_MAKEPPFLAGS (exported)
The makepp specific options needed for POSIX/gmake compatibility, with which makepp
was called. These are in a separate variable so a legacy makefile can't break
compatibility by unsetting MAKEFLAGS. This is only set with
"--traditional-recursive-make".

MAKEPP_VERSION
The version of makepp you are running with. If it is a beta version, it will have a
hyphen followed by YYMMDD plus some more internal data. You can use this for "ifdef"
to hide makepp specific constructs from other makes.

Default: The same value displayed by "makepp --version"

PWD An alias for "CURDIR".

RM Default: "rm -f". This is meant for legacy Makefiles. For new ones preferably use
the builtin &rm command directly. If you want to write a phony clean rule, look at
the "makeppclean -r" command instead.

ROOT
The relative path to the root of your build system, i.e. the directory further up in
which you have a "RootMakeppfile(.mk)". If you don't have one, this variable is
empty.

SHELL
This variable is only respected if you "export" it. In that case it is the Shell
which is used to execute not-builtin actions with some special character in it (plain
ones being exec`ed directly). On Windows Strawberry or ActiveState Perl, if you have
a Unix-like Shell, you must instead set your SHELL variable to a value ending in "sh"
or "sh.exe" before calling makepp.

Default: The first found among "/usr/xpg4/bin/sh" (e.g. Solaris) or "/sbin/xpg4/sh"
(e.g. Reliant Unix) or "/bin/sh".

YACC
Default: The first found among "bison -y" or "yacc".

Variables and Perl
Variable values are stored as ordinary Perl scalars, so you can access them directly from
Perl code if you need to do any complicated manipulations with them; see makepp_extending
for details.

Accessing global variables from Perl is achieved by prefixing them with the "Mpp::global"
package. In fact any variable, not yet present in the current makefile, and which you
assign to in this package will from then on be global, as though you had just issued the
"global" statement for it.

This direct access is however error-prone! The user may have overridden these variables
on the command line or through the environment. Some other makefile loaded before this
one may have made the variable global or target specific. In these cases you would not
find the value of the variable, or when assigning it, might take away its property
(equivalent to an "override" modifier, except for target specifics.)

With direct access you also bypass the expansion of these variables, if they are of type
"=" or ";=". Special variables like "$(CC)" start out as functions, until they are
assigned to. So in many cases you won't see their value.

For these reasons it is better to let makepp determine the correct value. You can use the
"makeperl" variant, in which the variable has been evaluated before the Perl code gets
interpreted:

makeperl { $$current_value = '$(MAKEFILE_VAR)' }

If you need a variable in makefile perl blocks this is achieved via the Perl variable
$makefile as follows:

perl { $current_value = $makefile->expand_variable( 'MAKE_VAR' ) }

Functions always get the makefile object passed in as the second argument $_[1]:

sub f_f { $current_value = $_[1]->expand_variable( 'MAKE_VAR' ) }

Commands are supposed to be called within a rule action, where the makefile object is
accessible via "$Mpp::Subs::rule->{MAKEFILE}":

sub c_cmd { $current_value = $Mpp::Subs::rule->{MAKEFILE}->expand_variable( 'MAKE_VAR' ) }

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