=head1 NAME

makepp_cookbook -- The best way to set up makefiles for various situations

=for vc $Id: makepp_cookbook.pod,v 1.26 2012/02/07 22:26:15 pfeiffer Exp $

=head1 DESCRIPTION

I discovered that practically no one ever reads a manual for a make tool,
because frankly no one really is interested in the make process itself--we are
only interested in results.  So this cookbook was put together in hopes that
people will be able to get what they need quickly from the examples without
wading through the manual.  This shows howto type questions, whereas stumbling
blocks will be found in the L<frequently asked questions|makepp_faq>.

=head2 Building libraries

=head3 Do you really need a library?

I have seen a number of large programs which consist of a large number
of modules, each of which lives in its own directory.  Commonly, each
directory is put into its own library, and then the final program links
with all of the libraries.

In many cases, I think rather than use a library, there is a better
approach.  Libraries are not really the right solution if each module
cannot or will not be reused in any other program, because then you get
all of the drawbacks of libraries and none of the advantages.  Libraries
are useful in the following cases:

=over 4

=item  1.

When you have a bunch of subroutines which have to be linked with
several different programs, and no program actually uses 100% of the
subroutines--each program uses a different subset.  In this case, it
probably is a good idea to use a static library (a F<.a> file, or an
archive file).

=item 2.

When you have a module which should be linked into several different
programs, and you want to load it dynamically so each program doesn't
have to have a separate copy of the library.  Dynamic libraries can save
executable file space and sometimes enhance system performance because
there is only one copy of the library loaded for all of the different
programs that use it.

=item 3.

When your link time is prohibitively long, using shared libraries for
large pieces of the program can significantly speed up the link.

=back

Using static libraries has one main disadvantage: on some systems (e.g.
Linux), the order in which you link the libraries is critically
important.  The linker processes libraries in the order specified on its
command line.  It grabs everything it thinks it needs from each library,
then moves on to the next library.  If some subsequent library refers to
a symbol which hasn't yet been incorporated from a previous library, the
linker does not know to go back and grab it from the previous library.
As a result, it can be necessary to list the library multiple times on
the linker command line.  (I worked on a project where we had to repeat
the whole list of libraries three times.  This project is what made me
prefer the alternative approach suggested below, that of incremental
linking.)

Using dynamic libraries has several disadvantages.  First, your program
can be slightly slower to start up if the library isn't already being
used by some other program, because it has to be found and loaded.
Second, it can be a real hassle to get all the dynamic libraries
installed in the correct locations; you can't just copy the program
executable, you also have to make sure that you copy all of its
libraries.  Third, on some systems, it is difficult to debug code inside
shared libraries because the debuggers do not support them well.

If your module will never be used in any other program, then there is
little reason to use a library: you get all of the disadvantages of
using libraries and none of the advantages.  The technique I prefer is
to use incremental linking, where it is available.

Here is how you can do this on Linux:

    my_module.o : $(filter_out my_module.o, $(wildcard *.o))
 	ld -r -o $(output) $(inputs)

What this will do is to create another F<.o> file called F<my_module.o>,
which will consist of all of the F<.o> files in this subdirectory.  The
linker will resolve as many of the references as it can, and will leave
the remaining references to be resolved in a subsequent stage of
linking.  At the top level, when you finally build your program, instead
of linking with F<libmy_module.a> or F<libmy_module.so>, you would
simply link with F<my_module.o>.  When you link F<.o> files, you don't
have problems with order-dependency in the linker command line.

=head3 Letting makepp figure out which library modules are needed

Even if you have a true library, where a given program needs only a few
files from it (rather than every single module), makepp might be able to
figure out which modules are needed from the library and include only
those in the build.  This can save compilation time if you are
developing the library along with a program, because you don't bother to
compile library modules that aren't needed for the particular program
you are working on.

If your library strictly observes the convention that all functions or classes
declared in a file F<xyz.h> are completely implemented in a source file that
compiles to F<xyz.o> (i.e., you don't split up the implementation into
F<xyz1.o> and F<xyz2.o>), then you can use the
L<C<$(infer_objects)>|makepp_functions/infer_objects_file1_file2_pattern>
function to tell makepp to pull out only the relevant modules from the
library.  This can work surprisingly well for libraries with even dozens of
include files.  Basically, C<$(infer_objects)> examines the list of F<.h>
files that are included, and looks for corresponding F<.o> files.  If you're
rapidly developing a library and a program together, this can save compilation
time, because you never bother to compile modules of the library that the
program doesn't use.

Here's an example of the way I use it:

    my_program: $(infer_objects *.o, $(LIB1)/*.o $(LIB2)/*.o)
 	$(CXX) $(inputs) -o $(output) $(SYSTEM_LIBRARIES)

The S<C<$(infer_objects )>> function returns its first argument (after doing
wildcard expansion on it), and also looks through the list of files in its
second argument, for files whose name is the same as the name of any F<.h>
files included by any file in its first argument.  If any such files are
found, these are added to the list.

=head3 Building a static library

If you are sure you actually need a library and incremental linking
isn't available or isn't what you want to do, there are a couple of ways
to do it.  First, here is an example where all the files are explicitly
listed:

    LIBRARY_FILES = a b c d e
 
    libmine.a: $(LIBRARY_FILES).o
 	&rm -f $(output)
 	$(AR) cr $(output) $(inputs)
 	ranlib $(output)     # May not be necessary, depending on your OS.

The C<&rm> is makepp's builtin C<rm> command.  If you're used to writing
makefiles, you may be a little suprised by this command; you may be used to
something more like this:

    libmine.a: $(LIBRARY_FILES).o
 	$(AR) ru $@ $?      # Not recommended!!!!!!!
 	ranlib $(output)

where C<$?> (also known as C<$(changed_inputs)>) is an automatic
variable that means any files which have changed since the last time the
library was built, and C<$@> is roughly the same as C<$(output)>.

This approach is not recommended for several reasons:

=over 4

=item *

Suppose you remove a source file from the current directory.  It's still
in the library, because you didn't rebuild the library from scratch.  As
a result, anything that links with this library will have the stale
F<.o> file, and that can screw up your builds.  (I once got thoroughly
confused by this when I was trying to remove dead code from a project: I
kept deleting files and it still linked, so I thought the code was dead.
had the modules I removed.  However, when someone else rebuilt the
project from scratch, it didn't link any more!  The problem was that the
old F<.o> files were still in the archive.)

Also, depending on your options to C<ar> and your implementation of
C<ar> (e.g., if you use the C<q> option instead of C<r>), you can wind
up having several versions of the same F<.o> inside the F<.a> file.  If
the different versions define different globals, the linker may try to
pull in both of them.  This is probably a bad thing.

This is why we first remove the library file, and create it from
scratch.  This will take slightly longer than just updating modules in a
library, but not much longer; on a modern computer, the amount of time
consumed by the F<ar> program is minuscule compared to what the C
compiler takes up in a typical build, so it's just not worth worrying
about.

=item *

One of the ways that makepp attempts to guarantee correct builds is that
it will automatically rebuild if the command line to build a given
target has changed.  But using the C<$?> variable can cause problems,
because each time the library is updated, the build command is
different.  (You can suppress this using
S<C<:build_check ignore_action>>; see L<makepp_build_check> for details.)

=item *

Updating the archive rather than rebuilding it will make it impossible
for makepp to put the file properly into a build cache (see
L<makepp_build_cache> for details).

=back

Sometimes you may find that listing all the files is a bit if a pain,
especially if a project is undergoing rapid development and the list of
files is constantly changing.  It may be easier to build the library
using wildcards, like this:

    libmine.a: $(only_targets *.o)
 	&rm $(output)
 	$(AR) cr $(output) $(inputs)

This puts all the F<.o> files in the current directory into the library.
The wildcard matches any F<.o> file which exists or can be built, so it
will work even if the files don't exist yet.

The C<only_targets> function is used to exclude F<.o> files which don't
have corresponding source files any more.  Suppose you had a file called
F<xyz.c> that you used to put into your library.  This means that
there's an F<xyz.o> file lying around.  Now you delete F<xyz.c> because
it's obsolete, but you forget to delete F<xyz.o>.  Without the
C<only_targets> function, F<xyz.o> would still be included in the list
of F<.o> files included in the library.

=head3 Building a dynamic library

The process of building dynamic libraries is entirely system dependent.
I would highly recommend using libtool to build a dynamic library (see
L<http://www.gnu.org/software/libtool/>), so you don't have to figure out
how to do it on your platform, and so that your makefile will continue
to work even when you switch to a different OS.  See the libtool
documentation for details.  Here's a sample Makefile:

    LIBTOOL := libtool
 
    libflick.la : $(only_targets *.lo)
 	$(LIBTOOL) --mode=link $(CC) $(inputs) -o $(output)
 
    %.lo : %.c
 	$(LIBTOOL) --mode=compile $(CC) $(CFLAGS) $(INCLUDES) -c $(input) -o $(output)

=head2 Building on several different machines or networks

One of the most annoying problems with makefiles is that they almost
never work when you switch to a different machine or a different
network.  If your makefiles have to work on every possible machine on
the planet, then you probably need some sort of a configure script.  But
if you only have to work on a few different machines, there are several
ways you can approach this problem:

=head3 Use a different include file in all the environments

At the beginning of each makefile, you can include a line like this:

    include system_defs.mk

The file F<system_defs.mk> would normally be located in a different
place for each environment.  If you want your build directories to be
identical on all machines, then put F<system_defs.mk> in a directory
above the build directories, or else supply an include path to makepp
using the C<-I> command line option.

This is usually kind of painful to do, but it works well if there are a
huge number of differences.

=head3 Use if statements

This is the ugliest way to do it, but it will usually work.

    ifsys i386
      CC := gcc
    else ifsys sun4u
      CC := cc
    else ifsys hpux11
      CC = c89
    endif

If all you need to do is to find a few programs or libraries or include
files in different places, there may be better ways (see below).

=head3 find_program, first_available, findfile

These functions can search various different directories in your system
to find the appropriate files.  This isn't as powerful as a configure
script, of course, but I find it useful.  For example, I do the
following:

    CXX ;= $(find_program g++ c++ pg++ cxx CC aCC)
 		    # Pick first C++ compiler which is available in PATH.
 		    # (Incidentally, if you don't define CXX at all, this
 		    # is the way it's defined.)
    TCL_INCLUDE ;= -I$(dir_noslash $(findfile tcl.h, \
 	   /usr/local/stow/tcl-8.4.5-nothread/include \
 	   /usr/include/tcl8.4 /usr/include/tcl \
 	   /net/na1/tcl8.4a3/include /net/na1/tcl8.4a3/include))
 		    # $(findfile ) looks for tcl.h in each of the indicated
 		    # directories and returns the full path.  This is then
 		    # converted into a compilation option by stripping the
 		    # filename (leaving the directory) and prefixing with -I.
    %.o : %.cpp
 	$(CXX) $(CXXFLAGS) $(TCL_INCLUDE) $(input) -o $(output)
 
    TCL_LIB ;= $((first_available
 	   /usr/local/stow/tcl-8.4.5-nothread/lib/libtcl8.4.so
 	   /usr/lib/libtcl8.4.so /usr/lib/libtcl.so
 	   /net/na1/tcl8.4a3/lib/libtcl8.4.a
 	   /net/na1/tcl8.4a3/lib/libtcl8.4.sl))
 		    # Find where the Tcl library is.  This is then explicitly
 		    # listed on the link command:
    my_program : *.o
 	$(CXX) $(CXXFLAGS) $(inputs) -o $(output) $(TCL_LIB)

=head3 Take advantage of Perl's config information

The above techniques may not be sufficient if you need some additional
information about your system, such as whether a long double exists, or
what the byte order is.  However, perl has already computed these
things, so you can just use its answers.

Perl's autoconfigure script makes all of its configuration information
available through the %Config hash.  There's no syntax to access a Perl
hash directly in makepp, but you can drop into Perl and set scalar
variables, which are directly accessible from makepp:

    perl_begin
    # Fetch values out of the config hash.
    use Config;
    $CC = $Config{'cc'};   # C compiler that perl used;
    $byteorder_flags = "-DBYTEORDER=$Config{'byteorder'}";
    $longdouble_defined = $Config{'d_longdbl'} eq 'define';
    $CFLAGS_for_shared_libs = $Config{'cccdlflags'};
    $LDFLAGS_for_shared_libs = $Config{'ccdlflags'};
    perl_end

Also, once you have done the 'use Config', you can use the S<C<$(perl )>>
statement, like this:

    SHARED_LIB_EXTENSION := $(perl $Config{'dlext'})

Type S<C<perldoc Config>> to see what information is available via the
C<%Config> hash.

Perl's config is a good place to get things like information about
integer types, byte order, and other things which usually require a
separate config script to locate.  Some of its information that relates
to the presence of things in the file system might not be valid.  For
example, C<$Config{'cc'}> refers to the C compiler that perl was built
with, which might not be the same C compiler you want to use.  In fact,
it might not even exist on your system, since you probably installed
Perl via a binary package.

=head2 Tips for using wildcards

=head3 Matching all files except a certain subset

Makepp's wildcards do not have any way at present of matching all files
I<except> a certain set, but you can do it with a combination of
functions.

For example, suppose you have a test program for each module in a
library, but you don't want to include the test programs in the
library.  If all the test programs begin with F<test>, then you can
exclude them like this:

    libproduction.a: $(filter_out test*, $(wildcard *.o))

The S<C<$(filter )>> and S<C<$(filter_out )>> functions are a very
powerful set of filters to do all kinds of set intersection and
difference operations.  For example,

    SUBDIRS ;= $(filter_out *test* *$(ARCH)*, $(shell find . -type d -print))
 			    # Returns all subdirectories that don't have
 			    # "test" or $(ARCH) in them.
 
    $(filter $(patsubst test_dir/test_%.o, %.o, $(wildcard test_dir/*.o)), \
 	     $(wildcard *.o))
 			    # Returns a list of .o files in the current
 			    # directory for which there is a corresponding
 			    # test_*.o file in the test_dir subdirectory.
    $(filter_out $(patsubst man/man3/%.3, %.o, $(wildcard man/man3/*.3)), \
 		 $(wildcard *.o))
 			    # Returns a list of .o files in the current
 			    # directory for which there is not a manual page
 			    # with the same filename in the man/man3 subdirectory.

=head3 Using the S<C<$(only_targets )>> function to eliminate stale F<.o> files

Suppose you are building a program or a library with a build command like
this:

    program: *.o
 	$(CC) $(inputs) -o $(output)

Suppose you now delete a source file.  If you forget to delete the
corresponding F<.o> file, it will still be linked in even though there
is no way to build it any more.  In the future, makepp will probably
recognize this situation automatically and exclude it from the wildcard
list, but at present, you have to tell it to exclude it manually:

    program: $(only_targets *.o)
 	$(CC) $(inputs) -o $(outputs)

Makepp does not know any way to build the stale F<.o> file any more
since its source file is gone, so the S<C<$(only_targets )>> function
will exclude it from the dependency list.

=head2 Tips for multiple directories

One of the main reasons for writing makepp was to simplify handling of
multiple directories.  Makepp is able to combine build commands from
multiple makefiles, so it can properly deal with a rule in one makefile
that depends on a file which is built by a different makefile.

=head3 What to do in place of recursive make

Makepp supports recursive make for backward compatibility, but it is highly
recommended that you B<not> use it.  If you don't know what it is, good.

See L<makepp/Better system for hierarchical builds> for details on why
you don't want to use recursive make, or else search on the web for
"recursive make considered harmful".

Instead of doing a recursive make to make the C<all> target in every
makefile, it is usually easier to let makepp figure out which targets
will actually need to be built.  Furthermore, if you put all of your
F<.o> and library files in the same directory as the makefiles, then
makepp will automatically figure out which makefiles are needed too--the
only thing that's needed is the have your top level make list the files
that are needed for the final linking step.  See the examples below.

=head3 One makefile for each directory: with implicit loading

The most common way to handle multiple directories is to put a makefile in
each directory which describes how to build everything in or from that
directory.  If you put F<.o> files in the same directory as the source
files, then implicit loading (see L<makepp_build_algorithm/Implicit
loading>) will automatically find all the makefiles.  If you put your
F<.o> files in a different directory (e.g., in an architecture-dependent
subdirectory), then you will probably have to load all the relevant makefiles
using the C<load_makefile> statement.

Here is a sample top-level makefile for a directory hierarchy that uses
implicit loading to build a program that consists of many shared
libraries (but see L<makepp_cookbook/Do you really need a library?>,
because making a program out of a bunch of shared libraries is not
necessarily a good idea):

    # Top level makefile:
    program : main.o **/*.la  # Link in shared libraries from all subdirectories.
 	$(LIBTOOL) --mode=link $(CC) $(CFLAGS) $(inputs) -o $(output) $(LIBS)

That's pretty much all you need in the top level makefile.  In each
subdirectory, you would probably do something like this:

    # Makefile in each subdirectory:
    include standard_defs.mk   # Searches ., .., ../.., etc. until it
 			      # finds the indicated include file.
    # override some variable definitions here
    SPECIAL_FLAGS := -do_something_different

Each makefile can probably be pretty much the same if the commands to
build the targets are quite similar.

Finally, you would put the following into the F<standard_defs.mk> file
(which should probably be located in the top-level directory):

    # Common variable settings and build rules for all directories.
    CFLAGS := -g -O2
    INCLUDE_DIR := $(find_upwards includes)
 			      # Searches ., .., ../.., etc. for a file or
 			      # directory called includes, so if you put
 			      # all your include files in there, this will
 			      # find them.
    INCLUDES := -I$(INCLUDE_DIR)
 
    %.lo : %.c
 	$(LIBTOOL) --mode=compile $(CC) $(CFLAGS) $(INCLUDES) -c $(input) -o $(output)
 
    lib$(relative_to ., ..).la: $(only_targets *.lo)
 	$(LIBTOOL) --mode=link $(CC) $(CFLAGS) -o $(output) $(inputs)
 		      # $(relative_to ., ..) returns the name of the current
 		      # subdirectory relative to the upper level
 		      # subdirectory.  So if this makefile is xyz/Makefile,
 		      # this rule will build xyz/libxyz.la.
 
    # Publish public include files into the top-level include directory:
    $(INCLUDE_DIR)/public_%.h : public_%.h
 	:build_check symlnk
 	&ln -fr $(input) $(output)

=head3 One makefile for each directory: explicit loading

If you want to put all of your F<.o> files into an
architecture-dependent subdirectory, then the above example should be
modified to be something like this:

    # Top level makefile:
    MAKEFILES := $(wildcard **/Makeppfile)  # List of all subdirectories to
 					   # get makefiles from.
 
    load_makefile $(MAKEFILES)       # Load them all in.
 
    include standard_defs.mk         # Get compile command for main.o.
 
    program : $(ARCH)/main.o */**/$(ARCH)/*.la
 	$(LIBTOOL) --mode=link $(CC) $(CFLAGS) $(inputs) -o $(output) $(LIBS)
 				    # */**/$(ARCH) excludes the subdirectory
 				    # $(ARCH), where we don't want to build
 				    # a shared library.

Each makefile would be exactly the same as before:

    # Makefile in each subdirectory:
    include standard_defs.mk
    # ... variable overrides here

And finally, F<standard_defs.mk> would contain something like the
following:

    # Common variable settings and build rules for all directories.
    ARCH	;= $(shell uname -s)-$(shell uname -m)-$(shell uname -r)
 			# 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.
    &mkdir -p $(ARCH)	# Make sure the output directory exists.
    CFLAGS := -g -O2
    INCLUDE_DIR := $(find_upwards includes)
 			# Searches ., .., ../.., etc. for a file or
 			# directory called includes, so if you put
 			# all your include files in there, this will
 			# find them.
    INCLUDES := -I$(INCLUDE_DIR)
 
    $(ARCH)/%.lo : %.c
 	$(LIBTOOL) --mode=compile $(CC) $(CFLAGS) $(INCLUDES) -c $(input) -o $(output)
 
    $(ARCH)/lib$(relative_to ., ..).la: $(only_targets *.lo)
 	$(LIBTOOL) --mode=link $(CC) $(CFLAGS) -o $(output) $(inputs)
 		      # $(relative_to ., ..) returns the name of the current
 		      # subdirectory relative to the upper level
 		      # subdirectory.  So if this makefile is xyz/Makefile,
 		      # this rule will build xyz/$(ARCH)/libxyz.la.
 
    # Copy public include files into the top-level include directory:
    $(INCLUDE_DIR)/public_%.h : public_%.h
 	&cp $(input) $(output)

=head3 Automatically making the makefiles

If your makefiles are all extremely similar (as in the above example),
you can tell Makepp to build them automatically if they don't exist.
Just add the following to your top-level makefile:

    SUBDIRS := $(filter_out unwanted_dir1 unwanted_dir2, $(wildcard */**))
    $(foreach)/Makeppfile: : foreach $(SUBDIRS)
 	&echo "include standard_defs.mk" -o $(output)
 	&echo "_include additional_defs.mk" -o >>$(output)
 			     # If the file additional_defs.mk exists, then
 			     # it will be included, but if it doesn't exist,
 			     # the _include statement will be ignored.

Now the makefiles themselves will be automatically built.

=head3 One makefile only at the top level

If all your makefiles are identical, you may ask: why should I have a
makefile at each level?  Why not put that all into the top-level
makefile?

Yes, this can be done.  The main disadvantage is that it becomes harder
to specify different build options for each subdirectory.  A second
disadvantage is that your makefile will probably become a bit harder to
read.

Here's an example of doing just that:

    # Top-level makefile for directory hierarchy.  Builds the program
    # out of a set of shared libraries as an example.  (See caveats above
    # for why you might want to use incremental linking or some other
    # approach rather than shared libraries.)
    makepp_percent_subdirs := 1     # Allow % to match multiple directories.
    SUBDIRS := $(filter_out *CVS* other-unwanted_dirs $(wildcard **))
    CFLAGS := -g -O2
    INCLUDES := -Iincludes
 
    %.lo: %.c
 	$(LIBTOOL) --mode=compile $(CC) $(INCLUDES) $(CFLAGS) -c $(input) -o $(output)
 
    $(foreach)/lib$(notdir $(foreach)).la: $(foreach)/*.lo : foreach $(SUBDIRS)
 	$(LIBTOOL) --mode=link $(CC) $(CFLAGS) -o $(output) $(inputs)
 			       # Rule to make all of the libraries.
 
    program : main.o **/*.la
 	$(LIBTOOL) --mode=link $(CC) $(CFLAGS) -o $(output) $(inputs)
 
    includes/$(notdir $(foreach)) : $(foreach) : foreach **/public_*.h
 	&cp $(input) $(output)
 			       # Sample rule for copying the publically
 			       # accessible .h files to the right place.

=head3 A clean target

Traditional makefiles contain a clean target, which allows removing everything
that was built.  There are three reasons why you should not do this with
makepp:

=over

=item 1

Makepp goes to great lengths to ensure a correct build.  So the desperate
"I don't know what's wrong", making you want to start from scratch is a thing of
the past.

=item 2

People will sometimes try to save time by doing two contradictory things at
once: "make clean all".  This can confuse makepp's smart wildcard system,
because it will first get the facts before doing anything.  Then comes the
clean action, which does not tell makepp what it does (indeed it can't,
because it undoes something -- the contrary of what a build tool is for).
Then comes "all", but the up to date files, which where there, are
mysteriously gone.

=item 3

There is the L<C<makeppclean>|makeppclean> command, which does the same thing,
and more efficiently.

=back

Nevertheless we retain this historical section, as it does tell you something
about the way makepp works:  A phony target called C<clean> is just a name for
a set of commands to remove all files that result from the make process.
Usually a clean target looks something like this:

    $(phony clean):
 	&rm -fm $(wildcard *.o .makepp_log)
 	# -m and .makepp_log gets rid of all of makepp's junk.

Instead of explicitly listing the files you want to delete, you can also
tell makepp to remove everything it knows how to build, like this:

    $(phony clean):
 	&rm -fm .makepp_log $(only_targets *)

This has the advantage that if any of your source files can be built
from other files, they will be deleted too; on the other hand, stale
F<.o> files (files which used to be buildable but whose source file has
since been removed) will not be deleted.

If you have a build that involves makefiles in several different
directories, your top-level makefile may reference the C<clean> target
(or any other phony target) in a different makefile:

    # Top-level makefile
    SUBDIRS := sub1 sub2
 
    # build rules here
 
    # Clean up after the build:
    $(phony clean): $(SUBDIRS)/clean
 	&rm -fm .makepp_log $(only_targets *)

Alternatively, you can put your C<clean> target only in the top-level
makefile, and have it process all of the directories, like this:

    $(phony clean):
 	&rm -fm $(only_targets **/*)

=head2 Using Qt's moc preprocessor

This example shows a makefile for a utility that uses Nokia's Qt GUI
library (see L<http://qt.nokia.com>).  The only thing that's slightly unusual
about this is that you must run a preprocessor called C<moc> on most C<.h>
files that contain widget definitions, but you don't want to run C<moc> on any
C<.h> files that don't use the C<Q_OBJECT> macro.

=head3 Automatically determining which files need moc files

You could, of course, just list all of the C<.h> files that need to have
C<moc> run on them.  If you're rapidly developing new widgets, however,
it may be something of a nuisance to keep updating the list in the
makefile.  You can get around the need to list the moc modules
explicitly with something like this:

    MOC	:= $(QTDIR)/bin/moc
    MODULES	:= whatever modules you happen to have in your program
    MOC_MODULES := $(patsubst %.h, moc_%, $(&grep -l /Q_OBJECT/ *.h))
 			# Scans all the .h files for the Q_OBJECT macro.
 
    my_program: $(MODULES).o $(MOC_MODULES).o
 	$(CXX) $(inputs) -o $(output)
 
    moc_%.cxx: %.h		# Makes the moc files from the .h files.
 	$(MOC) $(input) -o $(output)
 
    %.o: %.cxx
 	$(CXX) $(CXXFLAGS) -c $(input) -o $(output)

This approach scans each of your F<.h> files every time makepp is run,
looking for the C<Q_OBJECT> macro.  This sounds expensive, but it
probably won't take long at all.  (The F<.h> files will all have to be
loaded from disk anyway by the compilation process, so they will be
cached.)

=head3 #include the F<.moc> file

Another approach is to C<#include> the output from the C<moc>
preprocessor in your widget implementation file.  This means you have to
remember to write the C<#include>, but it has the advantage that there
are fewer modules to compile, and so compilation goes faster.  (For most
C++ compilation, the majority of the time is spent reading the header
files, and the output from the preprocessor needs to include almost as
many files as your widget anyway.)  For example:

    // my_widget.h
    class MyWidget : public QWidget {
      Q_OBJECT
    // ...
    }
 
    // my_widget.cpp
 
    #include "my_widget.h"
    #include "my_widget.moc"	// my_widget.moc is the output from the
 				// moc preprocessor.
    // Other implementation things here.
    MyWidget::MyWidget(QWidget * parent, const char * name) :
      QWidget(parent, name)
    {
     // ...
    }

Now you need to have a rule in your makefile to make all the C<.moc>
files, like this:

    MOC	:= $(QTDIR)/bin/moc
    # Rule to make .moc files:
    %.moc: %.h
 	$(MOC) $(input) -o $(output)


Makepp is smart enough to realize that it needs to make C<my_widget.moc>
if it doesn't already exist, or if it's out of date.

This second approach is the one that I usually use because it speeds up
compilation.

=head2 Replacements for deprecated make idioms

=head3 MAKECMDGOALS

Sometimes people have rules in their makefile depend on what target they
are building, using the special variable C<MAKECMDGOALS>.  For example,
one sometimes sees things like this:

    ifneq ($(filter production, $(MAKECMDGOALS)),)
      CFLAGS := -O2
    else
      CFLAGS := -g
    endif

This will work fine with makepp.  However, I recommend not to use
C<MAKECMDGOALS> for such cases (and so does the GNU make manual).  You
are better off putting your optimized and debug-compiled F<.o> files in
separate directories, or giving them different prefixes or suffixes, or
using repositories, to keep them separate.

Probably the only time when you might actually want to reference
C<MAKECMDGOALS> is if it takes a long time to load your makefiles, and you
don't need that for your C<clean> target (but you don't need a clean target).
For example,

    ifneq ($(MAKECMDGOALS),clean)
      load_makefile $(wildcard **/Makeppfile)
    else
      no_implicit_load . # Prevent automatic loading of any other makefiles.
    endif
 
    $(phony clean):
 	&rm -f $(wildcard **/*.o)

=head3 Recursive make to build in different directories

See L<makepp_cookbook/Tips for multiple directories>.

=head3 Recursive make to change value of a variable

Some makefiles reinvoke themselves with a different value of a variable,
e.g., the debug target in the following makefile fragment

    .PHONY: all debug
 
    optimized:
 	$(MAKE) program CFLAGS=-O2
 
    debug:
 	$(MAKE) program CFLAGS=-g
 
    program: a.o b.o
 	$(CC) $(CFLAGS) $^ -o $@
 
    %.o : %.c
 	$(CC) $(CFLAGS) -c $< -o $@

If the user types "make debug", it builds the program in default mode
with debug enabled instead of with optimization.

A better way to do it is to build two different programs, with two
different sets of object files, like this:

    CFLAGS := -O2
    DEBUG_FLAGS := -g
    MODULES := a b
 
    program: $(MODULES).o
 	 $(CC) $(CFLAGS) $(inputs) -o $(output)
 
    debug/program: debug/$(MODULES).o
 	 $(CC) $(DEBUG_FLAGS) $(inputs) -o $(output)
 
    %.o : %.c
 	 $(CC) $(CFLAGS) -c $(input) -o $(output)
 
    debug/%.o : %.c
 	 $(CC) $(DEBUG_FLAGS) -c $(input) -o $(output)
 
    $(phony debug): debug/program

The advantage of doing it this way is (a) you don't need to rebuild
everything when you switch from debug to optimized and back again; (b)

The above can be written somewhat more concisely using repositories.
The following makefile is exactly equivalent:

    repository debug=.     # Makes the debug subdirectory look like a copy of
 			   # the current subdirectory.
    load_makefile debug CFLAGS=-g
 			   # Override CFLAGS when invoked in debug subdirectory
    CFLAGS := -O2          # Value of CFLAGS when invoked in this subdirectory
 
    program: a.o b.o
 	$(CC) $(CFLAGS) $^ -o $@
 
    %.o : %.c
 	$(CC) $(CFLAGS) -c $< -o $@
 
    $(phony debug): debug/program
 			   # If user types "makepp debug", builds
 			   # debug/program instead of program.

=head2 Miscellaneous tips

=head3 How do I build one part differently just once?

Makepp makes this hard to do because the result is inconsistent with regard to
the rules.  But there are situations where you may need this, e.g. to compile
just one module with heavy debugging information.  You can achieve this in two
steps by first building the dependency separately, and then excluding it from
the link phase:

    makepp DEBUG=3 buggy.o	      # Build it with other option.
    makepp --dont-build=buggy.o buggy # Use it, despite "wrong" build option.

=head3 How do I make sure my output directories exist?

You can specify a rule to build the output directory, then make sure
that each file that goes in the output directory depends on it.  But
it's usually easier to do something like this:

    # The classical way
    dummy := $(shell test -d $(OUTPUT_DIRECTORY) || mkdir -p $(OUTPUT_DIRECTORY))
 		# This is usually easier than making all files depend on
 		# $(OUTPUT_DIRECTORY) and having a rule to make it.
 		# Note that you must use := instead of = to force it to
 		# execute immediately.
    # An alternative approach: using Perl code, OUTPUT_DIRECTORY local var
    perl_begin
      -d $OUTPUT_DIRECTORY or mkdir $OUTPUT_DIRECTORY;
    perl_end
    # The modern way, does nothing for existing directories
    &mkdir -p $(OUTPUT_DIRECTORY)

One of these statements should be near the top of your makefile, so they are
executed before anything that could possibly need the directory.

=head3 How do I force a command to execute on every build?

The easiest way is not to use the rule mechanism at all, but simply to
execute it, like this:

    dummy := $(shell date > last_build_timestamp)

Or put it in a perl block, like this:

    perl_begin
      system("command to execute");
    perl_end

This approach has the disadvantage that it will be executed even if an
unrelated target is being run.

A second approach is to declare the file as a phony target, even if it
is a real file.  This will force makepp to reexecute the command to
build it every time, but only if it appears in the dependency list of
some rule.

=head3 How do I shorten the displayed build commands?

Often there are so many options to compilation commands that what is
displayed on the screen is unreadable.  You can change what is
displayed by suppressing display of the entire command, and then
explicitly print out the interesting part of the command.  It's easy to
print out only the relevant part of the command by using
S<C<$(filter_out )>>, like this:

    ALL_CFLAGS = $(CFLAGS) $(INCLUDES) $(ADDL_CXX_FLAGS) $(DEBUG_FLAGS)
 
    %.o : %.c
 	@&echo $(notdir $(CC)) ... \
 	     $(filter_out -I* $(ADDL_CXX_FLAGS), $(ALL_CFLAGS)) \
 	     -c $(input)
 	@$(CC) $(ALL_CFLAGS) -c $(input) -o $(output)

(The C<@> in front of the command suppresses printing out the command.)

This will allow you to see most of the interesting options but won't display
all of the include directories (of which there are often very many!).  If the
part you are interested in is contiguous in your command, you can also use the
C<print> function (which adds a newline, so you don't want several of them):

    target:
 	@... $(print interesting part) ...
