Variable Assignment

• 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

    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

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:

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

The last line gets read as

    X := $(A)1 2

which gives "X" a value of "a1 b1 2".

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. 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. Substitution references are an abbreviation fot the "patsubst" function.

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

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.

Predefined Variables

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, depnding 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.

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_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.

Default: undefined

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.

Default: undefined

makepp_simple_concatenation

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

Default: undefined

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".

VPATH

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

Default: undefined

Variables and Perl

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' ) }