=head1 NAME

makepp_tutorial -- Tutorial on writing makefiles

=for vc $Id: makepp_tutorial.pod,v 1.15 2012/02/07 22:26:15 pfeiffer Exp $

=head1 DESCRIPTION

A I<makefile> is the set of instructions that you use to tell makepp how to
build your program.  Makepp can accept most makefiles written for the standard
Unix make, but if you're starting from scratch, it is often much simpler to
use some of makepp's advanced features.  This is an introduction for writing
makefiles that are specific to makepp.

If you already know a lot about writing makefiles, you might want to at
least peruse the later sections of this file because they show the
preferred way to do things with makepp, which is often different from
the traditional way to do it with make.  Another source of examples and
advice on writing makefiles for makepp is L<makepp_cookbook>.

Building a program from its source files can be a complicated and
time-consuming operation.  The commands are too long to be typed in
manually every time.  However, a straightforward shell script is seldom
used for compiling a program, because it's too time-consuming to
recompile all modules when only one of them has changed.

However, it's too error-prone to allow a human to tell the computer
which files need to be recompiled.  Forgetting to recompile a file can
mean hours of frustrating debugging.  A reliable automatic tool is
necessary for determining exactly which modules need recompilation.

Makepp (short for Make-plus-plus, or B<make++>) is a tool for solving
exactly this problem.  It is an improvement on the B<make> program, a
standard tool that has been around for many years.  It relies either on
its own builtin knowledge (in very simple cases), or on a file called a
F<Makefile> that contains a detailed recipe for building the program.

Usually, the input files are program source code, and the output files
are executables, but makepp doesn't care what they are.  You can use a
makefile to control any kind of procedure where you need to selectively
execute certain commands depending on which files have changed.  You
could, for example, use makepp to do data analysis, where your input
files are raw data and analysis programs, and your output files are
processed data or graphs or whatever.  Makepp will figure out which of
the processed data files need to be updated whenever some of the data
files or analysis programs change.  The examples in this introduction
will assume you are building an executable program from source code, but
you can do a lot more with makepp than just that if you use your
imagination.

If your program consists of a single module, you probably don't need
makepp, because you know that any change that you make requires
recompiling that module.  However, if your program consists of even just
two modules, then you will definitely want to use a program like makepp.

=head2 Do I need a makefile?

If your program is relatively simple and doesn't require anything particularly
special, makepp may already know how to build it without your explicitly
giving instructions.  For example, suppose you have a program in a single
source file, called C<test.c>.  You can just type S<C<makepp test>> and your
program will build like this:

    % makepp test
    makepp: Entering directory `/somewhere/or/other'
    gcc -g -Wall -c test.c -o test.o
    gcc -g -Wall test.o     -o test
    Warning: on Unix, to run a program called 'test', you must type
       ./test
    rather than just 'test'.


These are the basic commands needed to compile a program on Unix.  If
these commands don't make any sense to you, see
L<makepp_tutorial_compilation>.

Makepp contains builtin rules for C, C++, and Fortran.

Makepp can sometimes figure out how to compile programs that are
contained in more than one source file, or programs that must be linked
with various system libraries.  It does this by guessing which source
files and libraries you need based on the files that you include.  The
actual algorithm is too complicated to discuss here in a tutorial (but
see L<makepp_builtin>); you can try it, and if it doesn't work
automatically for you, you need to write your own makefile.

By default, for C and C++, makepp compiles the program with debug
information and without optimization.  If you want to turn on
optimization so that your program runs faster, change the command line
to:

    makepp CFLAGS=-O2 test

If you're compiling C++ instead of C, use C<CXXFLAGS=-O2> instead of
C<CFLAGS=-O2>.  For a complete list of other options you can configure without
writing a makefile, see L<makepp_builtin>.

Makepp's builtin rules are somewhat more powerful than the
standard Unix make, but if you write programs of any complexity,
it's likely that you'll need a makefile eventually to tell
makepp what to do.

If you are not familiar with Unix compilation commands, it may be helpful at
this point to read L<makepp_tutorial_compilation> for a description of what
these cryptic Unix compilation commands do.

=head2 A simple makefile

Suppose you are writing a C++ program which has two source
modules, C<processing.cxx> and C<gui.cxx>, along
with numerous include files.  If you were to build your program from
scratch, you would need to execute something like these
commands:

    c++ -c processing.cxx -o processing.o
    c++ -c gui.cxx -o gui.o
    c++ processing.o gui.o -o my_program

The first two commands are compilation commands, and the third invokes
the linker to combine the two object files into a single executable.  If
you make changes to C<gui.cxx> but not to C<processing.cxx>, then you
don't need to reexecute the first command, but you do need to execute
the last two commands.  makepp can figure this out for you
automatically.

(If you've never worked with make before, you may be thinking that you
could combine the above three commands into a single command, like this:

    c++ processing.cxx gui.cxx -o my_program

When you omit the C<-c> option to the compiler, it combines the
compilation and linking step.  This is often quite convenient when you
are not writing a makefile.  However, it's not a good idea to do this in
a makefile, because it always recompiles both modules even if one of
them hasn't changed, and this can take a significant amount of extra
time.)

In order to use makepp to control the build process, you'll need to write a
I<makefile>.  The makefile is a text file that contains the recipe for
building your program.  It usually resides in the same directory as the
sources, and it is usually called C<Makeppfile>, C<Makefile> or, only at the
root of your build tree C<RootMakeppfile>.

Each one of these commands should be a separate I<rule> in a makefile.
A rule is an instruction for building one or more output files from one
or more input files.  Makepp determines which rules need to be
reexecuted by determining whether any of the input files for a rule have
changed since the last time the rule was executed.

A rule has a syntax like this:

    output_filenames : input_filenames
 	actions

The first line of the rule contains a space-separated list of output
files, followed by a colon, followed by a space-separated list of input
files.  The output files are also called I<targets>, and the input files
are also called I<dependencies>; we say that the target file depends on
the dependencies, because if any of the dependencies change, the target
must be rebuilt.

The remaining lines of the rule (the I<actions>) are shell commands to
be executed.  Each action must be indented with at least one space
(traditional make requires a tab character).  Usually, there's just one
action line, but there can be as many as you want; each line is executed
sequentially, and if any one of them fails, the remainder are not
executed.  The rule ends at the first line which is not indented.

Some or all of the I<actions> may also be Perl statements.  These may be
on a single line and surrounded by C<perl { ... }>.  Or they may span
several lines, in which case the braces must be doubled, C<perl {{ ...
}}>.

You can place the rules in any order in the makefile, but it is
traditional to write the rule that links the program first, followed by
the compilation rules.  One reason for this is that if you simply type
"C<makepp>", then makepp attempts to build the first target in the file,
which means that it will build your whole program and not just a piece
of it.  (If you want to build something other than the first target, you
have to specify the name of the target on the command line, e.g.,
"S<C<makepp processing.o>>".)

The above compilation commands should be written as three separate
rules.  A makefile for building this program could look like this:

    # Link command:
    my_program: processing.o gui.o
 	c++ processing.o gui.o -o my_program
 	perl {{
 	  print "my_program is built\n";
 	}}
 
    # Compilation commands:
    processing.o: processing.cxx
 	c++ -c processing.cxx -o processing.o
 
    gui.o: gui.cxx
 	c++ -c gui.cxx -o gui.o


(Characters on a line following a C<#> are ignored; they are just
comments.  You do not need the "S<C<# Link command:>>" comment in the
makefile at all.)

To use this makefile, simply cd to the directory and type "C<makepp>".
Makepp will attempt to build the first target in the makefile, which is
C<my_program>.  (If you don't want it to build the first target, then
you have to supply a the name of the target you actually want to build
on the command line.)

When makepp attempts to build C<my_program>, it realizes that it first
must build C<processing.o> and C<gui.o> before it can execute the link
command.  So it looks at the other rules in the makefile to determine
how to build these.

In order to build C<processing.o>, makepp uses the second rule.  Since
C<processing.o> depends on C<processing.cxx>, makepp will also try to
make C<processing.cxx>.  There is no rule to make C<processing.cxx>; it
must already exist.

Makepp checks whether C<processing.cxx> has changed since the last time
C<processing.o> was built.  By default, it determines this by looking at
the dates on the file.  Makepp remembers what the date of
C<processing.cxx> was the last time C<processing.o> was made by storing
it in a separate file (in a subdirectory called C<.makepp>).  Makepp
will execute the actions to build the target if any of the following is
true:

=over 4

=item *

The target does not exist.

=item *

The target exists, but makepp does not have any information about the last
build.

=item *

The date on any input file has changed since the last build, and there is a
significant difference in the input file.  For C or C++ files this means a
change other than whitespace or within comments.

=item *

The date on any target has changed since the last build, and there is a
difference in the target file.

=item *

The actions have changed since the last build.

=item *

The last build occurred on a different architecture (different CPU type or
operating system type).

=item *

Any environment variable listed on the rule has changed in value.

=back

It might seem a little funny that makepp executes the action if either
the output file or the input files have changed since the last build.
Makepp is designed to guarantee that your build is correct, according to
the commands in the makefile.  If you go and modify the file yourself,
then makepp can't guarantee that the modified file is actually correct,
so it insists on rebuilding.  (For more information on how makepp
decides whether to rebuild, and how you can control this, see
L<makepp_signatures> and L<makepp_command>.)

Now C<processing.o> might not depend only on C<processing.cxx>; if
C<processing.cxx> includes any C<.h> files, then it needs to be
recompiled if any of those C<.h> files has changed, even if
C<processing.cxx> itself has not changed.  You could modify the rule
like this:

    # Unnecessary listing of .h files
    processing.o: processing.cxx processing.h simple_vector.h list.h
 	c++ -c processing.cxx -o processing.o

However, it is a real nuisance to modify the makefile every time you change
the list of files that are included, and it is also extremely error prone.
You would not only have to list the files that C<processing.cxx> includes, but
also all the files that those files include, etc.  B<You don't have to do
this.> Makepp is smart enough to check for include files automatically.  Any
time it sees a command that looks like a C or C++ compilation (by looking at
the first word of the action), it reads in the source files looking for
C<#include> directives.  It knows where to look for include files by parsing
your compiler command line for C<-I> options .  Any files which are included
are automatically added to the dependency list, and any files which those
include.  If any of them has changed, the file will be recompiled.

Once makepp knows that C<processing.o> is up to date, it then determines
whether C<gui.o> needs to be rebuilt by applying the same procedure to
the third rule.  When both C<processing.o> and C<gui.o> are known to be
built correctly, then makepp applies the same procedure to see if the
link command needs to be reexecuted.

The above makefile will work, but even for this simple problem, an
experienced user is not likely to write his makefile this way.  Several
improvements are discussed in the next sections.

=head3 Using variables

So far, our makefile for compiling our program of two modules
looks like this:

    # Link command:
    my_program: processing.o gui.o
 	c++ processing.o gui.o -o my_program
 
    # Compilation commands:
    processing.o: processing.cxx
 	c++ -c processing.cxx -o processing.o
 
    gui.o: gui.cxx
 	c++ -c gui.cxx -o gui.o


This works wonderfully, but suppose now we want to change some compilation
options.  Or maybe we want to use a different compiler.  We'd have to change
all three compilation lines.

Similarly, suppose we want to change the list of modules to compile.  We'd
have to change it in two places.

Duplication of information like this is a recipe for disaster.  If you
go and change your makefile, it's pretty much guaranteed that at some
point, you or someone else will forget to change one of the places.
Depending on what the change is (especially if it affects preprocessor
definitions), this can lead to subtle and hard-to-debug problems in your
program.

The way to avoid duplication of information is to specify the
information only once and store it in a variable, which can be accessed
each time the information is needed.

    # Define the symbols we might want to change:
    CXX	:= c++
    CXXFLAGS := -g
 
    OBJECTS	:= processing.o gui.o
 
    my_program: $(OBJECTS)
 	$(CXX) $(OBJECTS) -o my_program
 
    processing.o: processing.cxx
 	$(CXX) $(INCLUDES) $(CXXFLAGS) -c processing.cxx -o processing.o
 
    gui.o: gui.cxx
 	$(CXX) $(CXXFLAGS) -c gui.cxx -o gui.o


Here C<$(CXX)> expands to be the value of the variable C<CXX>, and
similarly for C<$(CXXFLAGS)> and C<$(OBJECTS)>.  Now we can just change
one line in the makefile, and all relevant compilation commands are
affected.

In fact, we don't even need to change the makefile to change compilation
options.  Assignments specified on the command line override assignments
in the makefile.  For example, we could type this to the shell:

    makepp CXXFLAGS="-g -O2"


which overrides the setting of C<CXXFLAGS> in the makefile.  It is as if
the makefile contained the line

    CXXFLAGS := -g -O2

instead of the definition it does contain.

It might not at all be useful to be able to override these things for
your own development, but if you distribute your sources to other
people, they might appreciate it.

Variable names are case sensitive (e.g., C<OBJECTS> is different from
C<objects>).  Usually people write most variables in upper case only,
but you don't have to.

If you need to put a literal dollar sign into a rule action, write it
with a double dollar sign, like this:

    test:
 	for testfile in *.test; do run_test $$testfile; done

Conventionally, there are a few variables which you might want to set.
These are just conventions, but you will see them in a lot of makefiles.

    CC	    := cc		# The C compiler.
    CFLAGS  := -g		# C compilation options which relate to
 				# optimization or debugging (usually
 				# just -g or -O).  Usually this wouldn't
 				# include -I options to specify the
 				# include directories, because then you
 				# couldn't override it on the command line
 				# easily as in the above example.
    CXX	     := c++             # The C++ compiler.  (Sometimes "CPP" instead
 				# of CXX.)
    CXXFLAGS := -g		# C++ compilation options related to
 				# optimization or debugging (-O or -g).
    F77	     := f77		# The Fortran compiler.
    FFLAGS   :=	        	# Optimization flags for Fortran.


Makepp will guess appropriate values for some of these variables if you
don't specify them (see L<makepp_builtin>), but it is usually best to
set them explicitly--it makes it easier on anyone reading your makefile.

There are a lot more extremely powerful things you can do with
variables, but first we need to explain some more things about
makefiles.

=head3 Pattern rules

Having one rule for each compilation command is fine when there are only
a few files, but what if your program consists of dozens of source
files?  Most of them have to be compiled with very similar commands.  It
is tedious to type in a separate rule for each source file, and then if
you decide to change the rules, you have to change the makefile in a
dozen places.  A better solution to this problem is to use a I<pattern
rule>.

A pattern rule is a concise way of specifying a rule for many files at
once.  The rule will depend on the file names, but usually it depends on
them in a simple way.  You specify a pattern by using the C<%> wildcard.
When present in the dependency list, C<%> matches any string of any
length; when present in the list of targets, C<%> stands for the string
that C<%> in the dependency list matched.

The following pattern rule will take any C<.c> file and compile it into
a C<.o> file:

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


(This assumes that you have the variables C<CC>, C<CFLAGS>, and
C<INCLUDES> defined to be something suitable.  Makepp will guess a value
for C<CC> and C<CFLAGS>.)

The first line of the rule says that it applies to every possible input
file that matches the pattern C<%.c>.  These C<.c> files can be
transformed into the corresponding C<.o> file using the specified
actions.

The action of rule is quite similar to the other actions we've seen
previously, except that it uses I<automatic variables>.  An automatic variable
is a variable whose value is automatically set by makepp depending on the rule
that it appears in.  The most useful automatic variables are:

=over 4

=item C<$(input)>

The name of the first input file.  In this rule, this would be the file
that matches the C<%.c> pattern.  C<$(dependency)> is a synonym for
C<$(input)>.  In older makefiles, you will also see the cryptic symbol
C<$E<lt>> used as well.

=item C<$(output)>

The name of the first output file.  In this rule, this would be the file
that matches the C<%.o> pattern.  C<$(target)> and C<$@> are synonyms.

=item C<$(inputs)>

The name of all explicitly listed input files.  In this case, since
there is only one, C<$(inputs)> is equivalent to C<$(input)>.
C<$(dependencies)> and C<$^> are synonyms.

=item C<$(outputs)>

The name of all explicitly listed targets.  In this case, since there is
only one, C<$(outputs)> is equivalent to C<$(output)>.  C<$(targets)> is
a synonym for C<$(outputs)>.

=back

Note that these variables are lower case.

You can use these automatic variables even for non-pattern
rules.  This avoids repeating target filenames.

You can actually do considerably more complicated things with
pattern rules.  For example,

    # Put the object files into a separate directory:
    objects/%.o: %.cpp
 	$(CXX) $(CXXFLAGS) -c $(input) -o $(output)
 
    # Run a preprocessor to make source files:
    moc_%.cxx: %.h
 	$(MOC) $(input) -o $(output)

Using pattern rules and automatic variables, we'd probably
rewrite our makefile for our simple program like this:

    CXX	:= c++
    CXXFLAGS := -g
    INCLUDES := -I.		# This would contain any -I options to the
 				# compiler, if there are any.
    LIBS     := -L/usr/X11R6/lib -lX11 # Contains libraries we need to link in.
    OBJECTS  := processing.o gui.o
 
    my_program: $(OBJECTS)
 	$(CXX) $(inputs) -o $(output) $(LIBS)
 
    %.o: %.cxx
 	$(CXX) $(INCLUDES) $(CXXFLAGS) -c $(input) -o $(output)

Now we don't have to have an explicit rule for each object file we need
to produce.  If we want to add another module to our program, we only
have to change the one line that defines the C<OBJECTS> variable.  Note
that this makefile is now much more concise than our original makefile.
Each piece of information occurs only once so there is no possibility of
making a mistake by changing information in one place and forgetting to
change it in others.

When you use pattern rules, it's not uncommon for there to be two
different rules that can produce the same file.  If both rules are
pattern rules, then the one that occurs later in the makefile is
actually used.  If one rule is a pattern rule, and the other is an
explicit rule (one that actually names the target file explicitly), then
the explicit rule is used.  This is often helpful if you want to compile
most modules with the same command, but there is one module that needs
slightly different compilation options, as shown in this makefile
fragment:

    CXXFLAGS := -g -O2
    FAST_CXXFLAGS := -DNO_DEBUG -O6 -malign-double -funroll-all-loops
 
    %.o: %.cpp
 	$(CXX) $(CXXFLAGS) -c $(input) -o $(output)
 
    time_critical_subs.o: time_critical_subs.cpp
 	$(CXX) $(FAST_CXXFLAGS) -c $(input) -o $(output)

There is also another syntax that can be more convenient for affecting
compilation options for just one or a few targets.  It is possible to
tell makepp that a variable should have a different value for certain
specific targets.  In this example, it would look like this:

    CXXFLAGS := -g -O2
    FAST_CXXFLAGS := -DNO_DEBUG -O6 -malign-double -funroll-all-loops
 
    %.o: %.cpp
 	$(CXX) $(CXXFLAGS) -c $(input) -o $(output)
 
    time_critical_subs.o: CXXFLAGS := $(FAST_CXXFLAGS)

In general, if you specify a variable name after a list of targets, then
it takes a different value when the build command for those targets is
being determined.

If you find yourself wanting to do something with patterns that isn't
expressed easily using the C<%> wildcard, makepp has another syntax
which is somewhat harder to read, but considerably more powerful.  See
L<the foreach clause in rules|makepp_rules/Foreach rules> for more
details.

Makepp actually has builtin rules for compiling C or C++ or Fortran
code, which are available if you don't override them with your own
rules.  The builtin rules are almost identical to the examples above.
Most makefiles contain pattern rules for compilation, but you can depend
on the builtin rules if you want.

=head2 Phony targets

Often it is convenient to put commands into the makefile that don't
actually build a file, but are somehow logically associated with the
build process.  For example, a very common procedure in makefiles is
something like this:

    prefix=/usr/local
 
    install: our_program
 	&install -m 0755 our_program $(prefix)/bin
 	&install -m 0644 $(wildcard *.png) $(prefix)/share/our_program/icons
 
    .PHONY: install

When someone types S<C<makepp install>>, then makepp first builds
C<our_program>, then runs the commands associated with the install
target.  The C<install> command simply copies its arguments to the
specified directory, and sets the file's protection to the indicated
value.  So it copies F<our_program> into F</usr/local/bin>, and some
associated data files into F</usr/local/share/our_program/icons>.  But
this doesn't create a file called C<install> in the current directory.

The command itself, C<&install>, is preceded by an ampersand.  This means that
it is not a Shell command, but a similar builtin command.  (You see the
difference in that you need the C<$(wildcard)> function, because the Shell is
not expanding it for you.)  Makepp has quite a few of these, both for
portability reasons -- C<install>, when it is at all present, differs wildly,
and even trivial commands like C<echo> do -- and performance.  See the
L<builtin commands|makepp_builtins> for more details.

The C<install> target here is called a I<phony target> because makepp
treats it as if it were a real file, but it is not actually a file, it's
just a trick for forcing makepp to build its dependencies and then run
some commands.

That's what the line

    .PHONY: install

is for.  It tells makepp that it really shouldn't expect the file
C<./install> to exist after the commands have executed.  If you forget
the phony declaration, then makepp will expect the file C<install> to
exist after executing the commands, and it will complain loudly if it
does not.


You can also write the phony declaration like this:
    $(phony install): our_program
 	...

and then omit the S<C<.PHONY: install>> line.  This means that you can
declare the target as phony on the same line as you define it, which may
make your makefiles more readable.

Phony targets are extremely common in makefiles.  In almost all
makefiles, the first target is the phony target C<all>, like this:

    $(phony all): program1 program2 program3

If no target is specified on the command line, makepp attempts to build
the first target in the file.  If your makefile makes more than just one
program, you most likely want to build all of the programs by default.
In this example, if the programmer just types C<makepp> without any
arguments, makepp attempts to build C<all>, which forces it to build all
three programs from this directory.

Here is a sample makefile fragment that illustrates some commonly used
phony targets:

    PROGRAMS	:= combobulator discombobulator
 
    $(phony all): $(PROGRAMS)	# All is the first target, so it's the default.
 
    combobulator: $(COMBOBULATOR_OBJS)
 	$(CXX) $(inputs) -o $(output)
 
    discombobulator: $(DISCOMBOBULATOR_OBJS)
 	$(CXX) $(inputs) -o $(output)
 
    #
    # This target makes sure everything is compiled, and then puts the
    # programs into a place where everyone can access them.  We make the
    # directories if they don't exist yet.  We don't use &mkdir, because
    # &install keeps track of everything it does, so &uninstall can later
    # undo this.
    #
    prefix	:= /usr/local
 
    $(phony install): all
 	&install -d $(prefix)/bin $(prefix)/share/combobulate
 	&install -m 0755 $(PROGRAMS) $(prefix)/bin
 	&install -m 0644 $(wildcard *.xbm) $(prefix)/share/combobulate
 
    #
    # This target makes a source distribution for shipping out to someone.
    #
    VERSION := 3.14
 
    $(phony distribution):
 	rm -rf combobulate-$(VERSION)	# Get rid of previous junk, if any.
 	&mkdir combobulate-$(VERSION)
 	&cp -l $(wildcard *.c *.h) Makefile README INSTALL combobulate-$(VERSION)
 	tar cf - combobulate-$(VERSION) | gzip -9c > combobulate-$(VERSION).tar.gz
 	rm -rf combobulate-$(VERSION)
 
    #
    # This target runs regression tests to make sure the program(s) are
    # doing what they are supposed to do.
    #
    $(phony test): $(PROGRAMS)
 	noecho for testfile in *.test; do \
 	   ./combobulate $$testfile | ./discombobulate - > junk_output; \
 	  if cmp -s junk_output $$testfile; then \
 	    echo passed $$testfile; \
 	  else \
 	    echo failed $$testfile; \
 	  fi; \
 	done
    #
    # If "noecho" is the first word of the action, the action itself is not
    # printed before it is executed.  In this case, printing the action
    # would merely clutter up the screen so it is very common to suppress
    # printing for such long commands.
    #
    # Note the use of the double dollar sign to pass a single dollar sign to
    # the shell.  Note also the backslashes at the end of a line to indicate
    # that a shell command continues to the next line.
    #

=head2 Working with several directories

If your program grows to a substantial size, or if it uses libraries
that need to be built but should be kept separate, it is quite likely
that you have split up your sources into several directories.  One of
the main motivations for writing makepp was to make dealing with several
directories much easier than with the standard make utility.  If you're
familiar with the standard Unix make, you'll notice that with makepp,
you don't have to mess around with ugly complexities like recursive
invocations of make.

With makepp, you simply put a separate makefile in each directory that
builds the relevant files in that directory.  When a makefile refers to
files whose build commands are in different makefiles, makepp
automatically finds the appropriate build rules in the other makefiles.
All actions in each makefile are executed with the current directory set
to be the directory containing the makefile, so each makefile can be
written independently of all the others.  No makefile has to know
anything about the other makefiles; it does not even have to tell makepp
to load the rules from those other makefiles.

When you've written your makefiles, cd to the directory that contains
your main program, and type C<makepp> just like you usually would.
Makepp will load in the makefile from that directory.  It will notice
that this makefile refers to files in other directories, and it will
examine those other directories to see if there is a makefile in them.
In this way, all relevant makefiles will be loaded.

As a simple example, suppose your top level directory contains the following
makefile (suggested name is C<RootMakeppfile>, but C<Makeppfile> will also
work):

    # Top level makefile:
 
    CXX	:= c++
    CXXFLAGS := -O2
    my_program: main.o goodies/libgoodies.so
 	$(CXX) $(inputs) -o $(output)
 
    %.o: %.cxx
 	$(CXX) $(CXXFLAGS) -c $(input) -o $(output)

You would need to write a makefile in the directory
C<goodies> which builds C<libgoodies.so>, like
this:

    # goodies/Makefile
 
    CXX	:= c++
    CXXFLAGS := -O2
 
    MODULES = candy.o chips.o licorice.o cookies.o popcorn.o spinach.o
 
    libgoodies.so: $(MODULES)
 	$(CXX) -shared $(inputs) -o $(output)
 			# Note that the command is written assuming that
 			# the current directory is the subdirectory
 			# "goodies", not the top level subdirectory.
 			# Makepp cds into this directory before executing
 			# any commands from this makefile.
 
    %.o: %.cxx
 	$(CXX) $(CXXFLAGS) -fpic -c $(input) -o $(output)


And that's all you need to do.

Any variables which you specify on the command line override the
definition of the variable in I<all> makefiles.  Thus, for example, if
you type S<C<makepp CXXFLAGS="-g">>, all modules will be recompiled for
debug because the definition of C<CXXFLAGS> in both makefiles is
overridden.

The directories containing other sources need not be subdirectories of
the top-level directory (as they are in this example).  They can be
anywhere in the file system; makepp will automatically load a makefile
from any directory that contains a file which is a dependency of some
target it is trying to build.  It will also load a makefile from any
directory that is scanned by a wildcard.

Automatic loading works if files built by your makefile all reside in
the same directory as the makefile itself.  If you write your makefile
so that its rules produce files in a different directory than the
makefile itself, then you might have to tell makepp where to look for
the makefiles, since it doesn't have any way of guessing.  You can do
this using the C<load_makefile> statement in your makefile.  For more
information about this and other issues related to multi-directory
builds, see L<makepp_cookbook/Tips for multiple directories>.

One caveat: if you reference the variable C<$(MAKE)> in your makefile,
makepp automatically goes into backward compatibility mode and turns off
automatic loading.

=head3 Template or boilerplate files

Makepp has several other features which make life slightly easier for
programmers who have to maintain a program spanning several directories.
In the above examples, you'll notice that the definitions of the
variables C<CXX> and C<CXXFLAGS> have to be repeated in each makefile.
It can be a nuisance to reenter the same information into every
makefile, and it could be a problem if you ever decide to change it--you
may have to modify dozens of different makefiles.


What you can do instead is to put all of the information that's common
to each makefile into a separate file, located perhaps at the top of the
directory tree.  Common information usually includes variable
definitions, and sometimes also pattern rules.  (In the above example,
however, the pattern rules are not the same in both makefiles.)  Let's
suppose you've called this file C<standard_defs.mk>.  Then each makefile
simply needs to contain a statement like this:

    include standard_defs.mk

When makepp sees this statement, it inserts the contents of the file
into the makefile at that point.  The C<include> statement first looks
for the file in the current directory, then in the parent of the current
directory, and so on up to the top level of the file system, so you
don't actually need to specify C<../standard_defs.mk> or
C<../../../../standard_defs.mk>.


So we could rewrite the above makefiles to look like this.
C<standard_defs.mk> would exist in the top level directory, and it might
contain the following definitions:

    # standard_defs.mk
    CXX	:= c++
    CXXFLAGS := -O2
 
    #
    # We've also included a pattern rule that might be useful in one or more
    # subdirectories.  This pattern rule is for C compilation for putting
    # things into a shared library (that's what the -fpic option is for).
    #
    %.o: %.cxx
 	$(CXX) $(CXXFLAGS) -fpic -c $(input) -o $(output)


Note that since the included file is actually inserted into each
makefile, rules in the included file are applied with the default
directory set to the directory containing the makefile that included the
file, not the directory containing the include file.

The top level C<Makefile> might look like this:

    # Top level makefile
    include standard_defs.mk
 
    my_program: main.o goodies/libgoodies.so
 	$(CXX) $(inputs) -o $(output)
 
    #
    # Note that this pattern rule overrides the one found in standard_defs.mk,
    # because makepp sees it later.  This pattern rule is for compilation for
    # a module that doesn't belong in a shared library.
    #
    %.o: %.cxx
 	$(CXX) $(CXXFLAGS) $(input) -o $(output)


And the subdirectory's makefile might look like this:

    # goodies/Makefile
    include standard_defs.mk
 
    MODULES = candy.o chips.o licorice.o cookies.o popcorn.o spinach.o
 
    libgoodies.so: $(MODULES)
 	$(CXX) -shared $(inputs) -o $(output)
 
    # We don't need the pattern rule for compilation of .cxx to .o files,
    # because it's contained in standard_defs.mk.

=head3 The -F compilation option

If you run makepp from within an editor such as emacs, and you are
editing sources from several different directories, you may find that
the default directory for makepp differs depending on which file you
were most recently editing.  As a result, makepp may not load the
correct makefile.

What you can do to ensure that makepp always loads the correct
makefile(s), no matter what directory happens to be your current
directory, is to use the C<-F> command line option, like this:

    makepp -F ~/src/my_program

Makepp will first cd to the directory C<~/src/my_program> before it
attempts to load a makefile.

=head2 Using Wildcards

Up until this point, we've had to explicitly list all of the modules
that go into a program or a library.  The previous makefile, for
example, contained this line:

    MODULES = candy.o chips.o licorice.o cookies.o popcorn.o spinach.o
 
    libgoodies.so: $(MODULES)
 	$(CXX) -shared $(inputs) -o $(output)

In this case, listing all of the modules that go into C<libgoodies.so>
is not such a big deal since there aren't very many of them.  But
sometimes it can be a real nuisance to list all of the object files,
especially if this list is changing rapidly during development.
Frequently, you want every single module in the whole directory to be
compiled into your program or library.  It would be a lot easier if you
could just tell makepp to do that without listing them all.

Well, you can.  The above lines could be rewritten as:

    libgoodies.so: *.o
 	$(CXX) -shared $(inputs) -o $(output)

The C<*.o> wildcard matches any existing C<.o> files, or any C<.o> files
which do not yet exist but can be made by any of the rules that makepp
knows about from any makefiles that it has read.  So the wildcard will
return the same list of files, no matter whether you haven't compiled
anything yet, or whether all the modules have been compiled before.

Of course, if you contaminate your directories with extra files that
shouldn't be compiled directly into your library, (e.g., if you write
little test programs and leave them in same directory as the library
source files), then these modules will be incorrectly included into your
library.  If you choose to use wildcards, it's up to you to keep the
directory clean enough.

Makepp supports the usual Unix wildcards and one additional one:

=over 4

=item C< *>

Matches any string of 0 or more characters.  It will not match the C</>
character.  For example, C<a*c> matches C<ac>, C<abc>, and C<aaaaabc>,
but not C<aa/bc>.

=item C<?>

Matches exactly one character (not including C</>).  For example,
C<???.o> matches all filenames that have 3 characters before the C<.o>
extension.


=item [chars]

Matches any of a list of characters at that position.  For example,
C<[abc].o> matches C<a.o>, C<b.o>, C<c.o>, but not C<abc.o> or C<d.o>.
You can also specify a range of characters, e.g., C<data_[0-9]> will
match C<data_0>, C<data_1>, etc.

=item C< **>

This is a special wildcard, found only in makepp (and the zsh shell, from
which I stole the idea, as bash has done since).  It matches any number of
intervening directories.  For example, C<**/*.o> matches C<xyz.o>,
C<test_programs/abc.o>, and C<a/deeply/nested/subdirectory/def.o>.

If your sources are contained in several subdirectories, and
you want to link all the object modules together, you could
write it like this:

    liboodles.so: **/*.o
 	$(CXX) -shared $(inputs) -o $(output)

=back

=head2 Functions and Advanced Variable Usage

Makepp has a number of extremely powerful ways of manipulating
text. This tutorial shows a few of the more useful ways, but you might
want to glance through L<makepp_variables> and L<makepp_functions> for a
more complete list.

=head3 Lists of corresponding files

A common problem in makefiles is the maintenance of two lists of files
which correspond.  Consider the following two variables:

    SOURCES := a.cpp bc.cpp def.cpp
    OBJS := a.o bc.o def.o

We might want to have a list of sources if the makefile can build source
distributions, and we might need a list of objects for the link command.
It's tedious to change both lines whenever a new module is added, and
it's not unlikely that a programmer will change one line and forget to
change the other.  Here we will show four different ways to avoid the
duplication.

=over 4

=item The patsubst function

The first is to use makepp's functions to convert one list into another.
A function invocation looks a little like a variable, except that a
function can take arguments:

    $(function arg1 arg2 arg3 ...)

Makepp supplies many powerful functions, but probably the most useful of
them is the C<patsubst> function.  You could write the above lines like
this:

    SOURCES = a.cpp bc.cpp def.cpp
    OBJS = $(patsubst %.cpp, %.o, $(SOURCES))

The C<patsubst> function applies a pattern to every word in a list of
words, and performs a simple textual substitution.  Any words in the
list that match the pattern in the first argument are put into the
output after making the substitution indicated by the second argument.
The C<%> wildcard matches any string of 0 or more characters.  In this
example, the pattern C<%.cpp> is applied to every word in C<$(SOURCES)>.
The first word, C<a.cpp> matches the pattern, and the C<%> wildcard
matches the string C<a>.  The C<%> in the second argument is then
replaced by C<a>, and the result is C<a.o>.  For the second argument,
C<%> matches C<bc>, so the result is C<bc.o>.

Makepp's functions can strip directory names, remove extensions, filter
out matching words, return the output from shell commands, and other
useful tricks.  In addition, you can also write your own functions in
perl that can be called from other parts of the makefile.  See
L<makepp_extending> for details.

=item Substitution references

Since the C<patsubst> function is so common, there is an abbreviated
syntax for it called a I<substitution reference>.  We could have written
the above lines like this:

    SOURCES = a.cpp bc.cpp def.cpp
    OBJS = $(SOURCES:%.cpp=%.o)
    OBJS = $(SOURCES:.cpp=.o)	# Shorthand, when both begin with %.

=item rc-style substitution

Sometimes invocations of C<patsubst> or the equivalent substitution
references can be somewhat cryptic.  Makepp provides another option
which is sometimes more convenient: I<rc-style substitution> (so called
because it was pioneered by the rc shell).

    MODULES := a bc def
    SOURCES := $(MODULES).cpp
    OBJS := $(MODULES).o

What happened here is that when it evaluated C<$(MODULES).cpp>, makepp
appended C<.cpp> to every word in C<$(MODULES)>, and similarly for
C<$(MODULES).o>.  In general, any characters preceding the
C<$(variable)> (up to a word delimiter) are placed before each word in
C<$(variable)>, and any characters following C<$(variable)> are placed
after each word in C<$(variable)>.  Thus the result of evaluating
C<x$(MODULES)y> would be S<C<xay xbcy xdefy>>.

=item Inline Perl code

If you know Perl, you can insert Perl code to perform arbitrary
manipulations on variables into your makefile.  This is best illustrated
by an example:

    SOURCES := a.cpp bc.cpp def.cpp
    perl_begin
    ($OBJS = $SOURCES) =~ s/\.cpp/.o/g;
    perl_end

Any text between the C<perl_begin> statement and the C<perl_end> statement is
passed off to the perl interpreter.  You can also use an equivalent more
perlish syntax of C<perl { ... }>.  All variables in the makefile (except
automatic variables) are accessible as Perl scalars.  Any variables you set
with Perl code will be accessible in the makefile.

So what the above example does is to copy the text from C<$SOURCES> to
C<$OBJS>, then substitute each occurrence of C<.cpp> with C<.o>.

In this example, using inline Perl code is probably unnecessary since
there are easier and clearer ways of doing the same manipulation.  But
the full power of the perl interpreter is available if you need it.

=back

=head3 Source/Object Separation and Variant Builds

Up to this point all of the makefiles we have seen put the object files
in the same directory as the source files.  This is usually the way
makefiles are written, and it's certainly the simplest way to do things.
However, suppose you have to compile your program on both a Linux
machine and a Solaris machine.  The binaries from the two machines are
incompatible, of course.  Unlike the traditional make, makepp is smart
enough to know that if the last compilation was on Linux, and the
current compilation is on Solaris, a recompilation of everything is
necessary.

But this still leaves a problem: when you recompile on Solaris, you wipe
out your Linux binaries.  Then when you switch back to Linux, you have
to recompile everything again, even though the source files that haven't
changed.

A related problem is if you build your program with several different
options.  Suppose for example that you usually compile your program with
optimization:

    CFLAGS	:= -O2
 
    %.o: %.c
 	$(CC) $(CFLAGS) -c $(input) -o $(output)
 
    my_program: *.o
 	$(CC) $(inputs) -o $(output)

However, you discover a bug, and you want to enable debugging on all
files, so you do change C<CFLAGS>:

    CFLAGS	:= -g -DMALLOC_DEBUG

Makepp realizes that the build commands have changed, and it needs to
recompile everything.  But again, recompiling with debugging enabled
wipes out your old binaries, so if you want to turn optimization back
on, everything must be recompiled again, even the files that haven't
changed.

The obvious solution to these problems is to put the
architecture-dependent or build-variant-dependent files in a separate
subdirectory.  There are two basic techniques for doing this: explicitly
specifying an alternate directory, or using repositories.

=head3 Explicit specifications of alternate directories

You could rewrite the rules in your makefile to dump the objects into a
different directory, like this:

    ARCH	;= $(shell uname -s)-$(shell uname -m)-$(shell uname -r)
 			# ARCH becomes the output from the uname commands.
			# Sometimes people use only $(shell uname -m), but
			# this will be the same for FreeBSD and Linux on an
			# x86.  The -r is not really useful on Linux, but is
			# important for other OSes: binaries for SunOS 5.8
			# typically won't run on SunOS 5.7.  Note the ;= which
			# means to evaluate this at most once, when first
			# needed.
    CFLAGS	:= -O2
    OBJDIR	;= $(ARCH)-optim
 
    $(OBJDIR)/%.o: %.c
 	$(CC) $(CFLAGS) -c $(input) -o $(output)
 
    $(OBJDIR)/my_program: $(OBJDIR)/*.o
 	$(CC) $(inputs) -o $(output)

Now when you run makepp, C<ARCH> is automatically set to something
different for each architecture, and all of the objects are placed in a
different directory for each architecture, so they don't overwrite each
other.  If you want to recompile turning on debugging, then you would
have to change both C<CFLAGS> and C<OBJDIR>.

One problem with this approach is that implicit loading will no longer work.
The only place that makepp knows to look for a makefile when it needs to build
something is in the directory of the file it's trying to build.  If this is a
problem for you, then you can explicitly tell makepp where to look using the
C<load_makefile> statement.

=head3 Repositories

Repositories are a magical way of using a makefile that is written to
put objects in the same directory, but having makepp automatically put
the objects in a different directory.  Suppose we start with the
original makefile above (before we modified it to put the objects in a
different directory), and we've been working on Linux so our source
directory is filled with Linux binaries.  When we want to recompile our
code on Solaris instead of Linux, we use the following command instead
of just typing C<makepp>:

    % mkdir solaris
    % cd solaris
    % makepp -R ..

What the C<-R> option to makepp does in this case is to declare the
directory C<..> (which is the original source directory) as a
repository.  A repository is just a way of getting makepp to trick all
of the actions into believing that all files in one directory tree are
actually located in a different directory tree in the file system.  In
the above example, makepp pretends that all the files in C<..> (and all
subdirectories of C<..>) are actually in the current directory (and
corresponding subdirectories).

More precisely, a repository is a place where makepp looks if it needs
to find a file that doesn't exist in the current directory tree.  If the
file exists in the current directory tree, it is used; if it doesn't
exist, but a file exists in the repository, makepp makes a temporary
symbolic link from the file in the repository to the current directory.
(A symbolic link is an alias for the original file.  It's like a copy,
except that trying to access the link actually accesses the original
file.)  The rule actions then act on the file in the current directory,
but actually reference the files in the repository.

In this example, initially we start off with a blank new directory
F<solaris>.  (It doesn't have to be blank, of course, and it won't be
the second time you run makepp.)  Makepp is run in this directory, and
it sees that there is no makefile there.  However, there is a makefile
in the repository, so it links in the one from the repository, and reads
it.  The pattern rule in the makefile that converts C<.c> files into
C<.o> files causes makepp to link all the C<.c> files that it needs from
the repository, and run the compilation command from the F<solaris>
subdirectory.  Therefore the C<.o> files are now placed into the
F<solaris> subdirectory, not in the top level directory.  When the build
command is finished, any files linked from the repository are deleted,
so the F<solaris> subdirectory will contain only the binary files for
Solaris.  Any C<.o> files that exist in the repository are unmodified,
so when you go back to your Linux machine and rerun makepp, most of your
program will not have to be recompiled.

Sometimes it might be more convenient to use a different form of the
repository command.  The above three shell commands could be entirely
replaced by the following one command:

    % makepp -R solaris=. -F solaris

What this does is to say that the files in the current directory are to
be linked into the F<solaris> subdirectory as necessary.  (The
F<solaris> subdirectory will be created automatically if it does not
exist.)  Then the C<-F> option causes makepp to cd to the F<solaris>
directory and execute the makefile there (which will be linked from the
repository).

Using a repository does not have the same drawbacks as explicitly
specifying an object directory; makefiles will be implicitly loaded as
expected, since as far as makepp is concerned, the makefile actually is
in the same directory as the target files.  However, if your build
involves not just one but several directory trees, using repositories
can become quite complicated.

Repositories are just a way of pretending that things located at one
place in the file system are actually in a different place for the
duration of the build.  This is a very powerful technique that can be
used for more than just separating your sources and binaries.  For more
details, see L<makepp_repositories>.

=head2 Debugging Makefiles

=head3 Log File

If you have a complicated build procedure, you find that makepp is rebuilding
things more often than you think they need to be rebuilt.  Or you may find
that it is not rebuilding things when it should.  You don't have to keep
staring at your makefiles until you see the problem.  On every build, makepp
produces a log file that explains which rule it thought it was supposed to use
to build each target, what files it thought each target depended on, and (if
it did decide to rebuild) why it thought a rebuild was necessary.  This binary
file is viewed L<makepplog, mppl|makepplog> utility.

The output format is more or less self-explanatory.  Indentation conveys depth
in makepp's inference tree.  Suppose the target is C<all>, and C<all> depends
on C<my_program>, and C<my_program> depends on C<*.o>, which depend on the
corresponding C<.c> files.  Log messages related to C<all> will not be
indented, log messages related to building the target C<my_program> will be
indented two spaces, log messages related to building any of the object files
will be indented 4 spaces, and log messages related to building any of the
source files will be indented 6 spaces.

If you're doing a parallel make (using the C<-j> command line option),
the order of the messages in the log file will not make nearly as much
sense since messages from different targets will be interspersed.  You
might try debugging a serial make first.

=head3 Common errors in makefiles

=over 4

=item Not specifying all dependencies

Makepp is designed to be extremely clever about finding dependencies,
and if you just use a standard Unix C or C++ compiler command, it is
actually somewhat difficult to get makepp to miss something.  (Please
send me examples if you find that it missed something, so I can make
makepp smarter.)  However, if you are running commands other than
compilation, or dealing with languages other than C or C++, it is much
easier to run into problems.

If you don't tell makepp all of the dependencies of a file, and it can't infer
them by parsing the command line or scanning the files for includes, then it
may not rebuild a file when it should.  You can make this kind of error less
likely by using only automatic variables in your actions, rather than
repeating the dependency lists.  For example,

    combined_file: a b c
 	 do_something a b c d > combined_file

has an error because F<d> is mentioned in the action but not in the
dependency list.  If the command had been written using automatic
variables like this:

    combined_file: a b c d
 	do_something $(inputs) > combined_file

then it would have been impossible to make this mistake.

Another way that a missing dependency can occur is if a program actually
uses a file but doesn't take the file's name on the command line.  For
example, if you're compiling Fortran code, makepp at the moment doesn't
know how to scan for included files.  Thus you must explicitly list any
files that are included.

One thing that is sometimes helpful for testing is to start with a
completely clean directory--just the bare minimum you think should be
necessary--and rebuild absolute everything from scratch.  This can be
most conveniently done by using repositories, like this:

    % rm -rf test-build-dir
    % makepp -R test-build-dir=. -F test-build-dir

If the build fails because some file is not present, it means that
makepp didn't realize some file was a dependency, because it only links
files from the repository that it thought were needed.  Performing this
test occasionally may save hours of debugging later.  I have worked on
projects where this was never done for months because recompilation took
so long.  As a result, many little problems crept in.  There were some
object files that didn't have source files any more, some source files
that were never properly rebuilt by a preprocessing command, etc.

Of course, this won't catch all missing dependencies, but it will catch
some of them.

=item Not specifying all targets

You must specify B<all> files that a given command modifies as targets,
or else makepp may not have realized they have changed.  You can specify
more than one target.  For example,

    y.tab.h y.tab.c: parse.y
 	yacc -d parse.y

If you had forgotten to specify F<y.tab.h> as a target, then makepp
would not know to rebuild F<y.tab.h> using this command, and files that
depend on F<y.tab.h> might not be recompiled after the yacc command is
run.

=back

Please suggest things that you have found confusing or dangerous, and
I'll either note them or try to fix makepp so they aren't a danger any
more.
