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       makepp_build_algorithm -- How makepp executes a makefile

       Makepp's	internals differ from the standard Unix	make in	fundamental
       ways.  This page	describes the different	philosophy in detail.

   Reverse vs. forward inference
       Makepp works in the opposite direction from the standard	Unix make.
       Traditional Unix	make is	given a	target to build, and then it finds a
       rule which matches the characters in the	target filename.  If the
       target is older than any	of its dependencies of the rule, then it is

       For example, consider this pattern rule:

	   %.o:	%.cxx
	       $(CXX) $(CXXFLAGS) -c $(input) -o $(output)

       When make realizes it needs to make a file called "xyz.o", it searches
       through its list	of pattern rules until it sees that "xyz.o" matches
       the pattern "%.o", and then it applies this rule.

       Makepp works in the opposite direction.	It first computes all files
       that it can possibly build by applying rules that match the characters
       in the dependency filenames.  Then when it needs	to build a file, it
       simply looks to see if it's one of the files that it knows how to
       build.  The list	of known files is stored based on the absolute

       When makepp encounters the above	pattern	rule, it searches for all
       files in	the directory matching the pattern "%.cxx" (i.e., "*.cxx").
       For each	of these files,	it then	remembers that it can produce the
       corresponding ".o" file.	 If subsequently makepp	discovers that it can
       make another ".cxx" file	that doesn't currently exist, this rule	will
       also be applied and the corresponding ".o" file will be marked.

       This might seem somewhat	inefficient, but it turns out not to be	that
       slow in most cases, and it is often true	that virtually all the files
       that can	be built are in	fact built.  And knowing the complete list of
       files that can be built has several advantages:

       o   Wildcards can match files which don't exist yet but can be built.

       o   Header files	which have been	detected by the	automatic dependency
	   scanner don't have to exist;	makepp knows where they	will be.
	   (Most other solutions to this problem assume	that any headers which
	   don't exist yet are in the current directory.)

       o   Repositories	are much simpler to implement since makepp knows
	   beforehand what files it can	make.  (See makepp_repositories	for

       o   It is possible to determine easily which files can be built (see
	   the "$(only_targets )" function.

       o   Makepp's "$(infer_objects)" function	is greatly simplified by
	   knowing what	objects	are available.

   Files vs. textual patterns
       Makepp associates build commands	with a target file, not	to a textual
       pattern for a filename.	It is therefore	not confused by	different
       names for the same file.	 Thus, for example, makepp will	know that
       "./xyz" and and "xyz" are the same file,	whereas	other make utilities
       may not.

       This is particularly important because (unlike the standard make)
       makepp loads makefiles from different directories.  In order for	the
       makefiles to be relatively independent, with no special position	given
       to a top-level makefile,	each makefile refers to	all files relative to
       its own directory. Thus if you load a makefile from the subdirectory
       "other_stuff", and that makefile	refers to "../xyz", makepp will	again
       realize that it's the same file referred	to above.  (It also won't be
       confused	by soft-linked directory names.)

   Stored build	information
       Makepp stores much more information about each file that	it builds
       beyond just the date stamp (which is all	that the standard make cares
       about).	This information includes:

       o   The signature of this file on the last build, so we know if the
	   file	itself has changed.

       o   The names of	each dependency	file, including	include	files and
	   other files inferred	automatically.	If this	list changes, then
	   makepp assumes it needs to rebuild.

       o   The signature of each dependency.  This way,	makepp knows to
	   rebuild not only when the dependencies are newer than the target,
	   but when they change	at all.	 This also makes it possible to	use
	   other kinds of signatures, such as cryptographic checksums, rather
	   than	the file date.

       o   The entire build command (and its cwd).  This way if	you change the
	   build command (e.g.,	change the compiler options), makepp knows to
	   rebuild even	if the files themselves	haven't	changed.

       o   The architecture.  If you have compiled your	program	on Linux and
	   then	switch to Solaris, makepp automatically	knows to recompile

       Makepp makes a subdirectory in every directory that it touches called
       ".makepp".  The build information for a file filename in	a directory is
       stored in .makepp/filename.  If you delete this subdirectory or alter
       the files, makepp will rebuild all affected files.

   Implicit loading
       If makepp is trying to build a target in	a directory and	doesn't	have a
       rule for	it yet,	or if it is looking for	files matching a wildcard in a
       directory, it will look in that directory to see	if a makefile is
       present.	 If so,	the makefile will be loaded automatically.

       This means that you usually don't have to tell makepp explicitly	where
       to find makefiles--all you have to do is	to reference a file in another
       directory, and makepp will automatically	figure out how to build	it.

       Implicit	loading	will occur only	if the directory is writable to	you.
       Thus if you want	to prevent makepp from trying to build a bunch of
       things that never change, simply	make the directory read-only.

       Implicit	loading	will not occur if you are in a tree under a
       RootMakeppfile(.mk) and the other makefile is outside that tree.	 If
       you do want this	once, you can give a "--do-build=/" option to makepp,
       to make everything outside the tree buildable.  If you always want
       this, you can put a "load_makefile" statement somewhere within the tree
       to explicitly connect it	to the tree.

       If implicit loading gets	in your	way (i.e., makepp loads	too many
       makefiles and it	wastes time, or	else you really	don't want it to try
       to rebuild all the stuff	described in the makefiles), you can turn it
       off for all directories using the "--noimplicit_load" command line
       option, or you can turn it off for selected directories using the
       "no_implicit_load" statement in your makefile.

perl v5.32.1			  2012-02-07	     MAKEPP_BUILD_ALGORITHM(1)


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