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NAME


makepp_repositories -- How to use repositories for variant builds, for maintaining a
central set of sources, and other things

DESCRIPTION


A repository is a directory or directory hierarchy outside of the default directory that
contains files which the makefile needs in the current directory tree. Makepp can
automatically link files from the repository into the current directory tree if they are
needed. Repositories provide similar functionality to the "VPATH" variable, but (unlike
"VPATH" in other versions of make) you do not have to do anything special to your makefile
to get them to work.

Repositories are specified with the "-R" or "--repository" command line option or with the
"repository" statement in the makefile. Note that if you have a habit of calling makepp
in different subdirectories of your build tree, it is easy to accidentally reimport a
repository somewhere else. As a safeguard against this, if you use RootMakeppfile, makepp
will refuse to start if it finds one above or below where it would be imported.

This is somewhat comparable to operating system union filesystems (unionfs...) The
current directory is like the highest level writable layer. All repositories are like
lower read-only layers.

Repositories are useful in several different situations:

· When you want to place your object and executable files in a separate directory, but
the makefile is written to place them in the same directory as the sources.

· When you want to build the same program two different ways (e.g., with two different
sets of compilation options, or for two different architectures).

· When you don't have write access to all or part of the source tree.

· When several developers are working on the same project, and there is a common source
repository containing all the sources for the project. Each developer can modify only
the files he needs to change in his local directory without affecting the other
developers, and makepp will automatically fetch the unmodified files from the source
repository.

Makepp's implementation of repositories does not require rewriting of the build commands
at all, unlike (for example) repositories in cons. Makepp puts a symbolic link into the
directory where the command is expecting it. As long as the command does not refer to
absolute directories, the exact same shell command will work with files from a repository.
This means that it works not only for compilation commands, but any kind of command you
can think to put in your makefile.

Makepp has another kind of mechanism called a build cache which solves some of the same
sorts of problems as repositories in a different way. Depending on your problem, a build
cache may be more useful than a repository. See makepp_build_cache for information about
build caches and a comparison of build caches and repositories.

Examples
Repositories are best explained by several examples of what you can do.

Different compilation options

Suppose you have a simple program with a makefile that looks something like this:

CFLAGS = -O2
OBJECTS = a.o b.o c.o
my_program: $(OBJECTS)
cc $(inputs) -o $(output)

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

This makefile places the files "a.o", "b.o", "c.o", and "my_program" in the same directory
as the source files.

Sometimes you want to place the binary files into a separate directory. For example, you
might build your program on several different architectures, and you don't want the binary
files on one architecture to be replaced with the binary files on the other. Or you might
want to make a temporary change and recompile without wiping out the previous compilation
results. Without repositories, you would have to modify your makefile to place the
objects elsewhere.

With a repository, however, you don't have to touch your makefile at all. Consider the
following sequence of commands:

% cd my_program_source
% makepp # Builds using the above makefile, and
# object files go into the directory
# my_program_source.
% cd ..
% mkdir binary-debug # Make a clean directory for building the
% cd binary-debug # same program with different options.
% makepp -R ../my_program_source CFLAGS=-g
# Now objects go into binary-debug.

The first makepp command compiles the source files with optimization and puts the objects
into the directory "my_program_source", because that's what the makefile is supposed to
do. Now we want to rebuild the program, but we want to change the value of "CFLAGS" to
compile for debug. We specify the new value of "CFLAGS" on the command line, and we also
tell makepp that the "my_program_source" directory is a repository using the "-R" option.

Every time makepp realizes that it needs a file that it doesn't already have in current
directory, it looks in the repository. In this case, it first looks for the makefile,
which doesn't exist in the "binary-debug" subdirectory. So it creates a symbolic link to
it from the makefile in "my_program_source", and then reads in the makefile. Then it
notices that it needs the file "a.c" in order to build "a.o", and so it links in "a.c"
from the repository. If "a.c" includes any files contained in "my_program_source", then
these will be automatically linked in as well. Note: Those links are useful for things
like debugging, but if you don't like them, "makeppclean -R" can remove them.

Running the build command in "binary-debug" won't touch any of the files in
"my_program_source". Thus from the same set of source files, you now have two different
copies of the program, one compiled with optimization and one compiled for debug. And
this happened without touching the makefile at all.

The advantage of using repositories instead of simply recompiling and overwriting the
original binaries is that now if we fix our bugs and want to go back to the optimized
version, we don't have to recompile everything. Since the original object files are still
around, and most of them are still valid, we can save a lot of time on recompilation.
This does not make a big difference when only three source files are involved, but for a
larger build that takes minutes or hours to complete, the savings in programmer time and
frustration can be significant.

Rebuilding one file with a minor modification to the compilation commands

Makepp doesn't fetch only source files from the repository. If the object files in the
repository don't need rebuilding, it will use them. For example, consider a slight
modification to the above makefile:

CFLAGS := -O2
A_CFLAGS := -O6 -funroll-loops

OBJECTS := a.o b.o c.o

my_program: $(OBJECTS)
cc $(inputs) -o $(output)

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

a.o: a.c
cc $(A_CFLAGS) -c $(input) -o $(output)

The idea is that "a.o" contains the time-critical code, so it is compiled with higher
optimization than the rest of the objects. Now suppose we want to test just how different
the timing is with different compile options. A repository can help with this, too:

% cd my_program_source
% makepp # Builds using the above makefile, and
# object files go into the directory
# my_program_source.
% cd ..
% mkdir no-unrolling # Make a clean directory for building the
% cd no-unrolling # same program with different options.
% makepp -R ../my_program_source A_CFLAGS=-O2
% cd ..
% time no-unrolling/my_program # Benchmark the two versions of the program.
% time my_program_source/my_program

Makepp proceeds as before, linking in a copy of the makefile and then examining the object
files. Now only the "a.o" module needs recompiling, since the options for "b.o" and "c.o"
haven't changed. Makepp notices that it can use "b.o" and "c.o" from the repository, so
it just links those in. However, it will recompile "a.o" in the "no-unrolling" directory.
Once the compilation is finished, the two different versions of the program can be
benchmarked.

Rebuilding with a minor modification to the source

Now suppose we want to make a change to "a.c" and benchmark the program before and after
the change. Repositories can help again. Consider this sequence of commands:

% mkdir modified-a
% cp my_program_source/a.c modified-a
% cd modified-a
% emacs a.c # Make some modifications just to this module.
% makepp -R ../my_program_source

Here we have created a new directory that just contains the single source file we want to
modify. Makepp now takes "a.c" from the "modified-a" subdirectory, but uses the copies of
"b" and "c" from the "my_program_source" directory. Without changing any of the binary
files in "my_program_source", we have created a separate copy of the program that
incorporates our changes to "a.c". If there are other developers using the sources in
"my_program_source", they will be unaffected by our changes.

Repositories can thus be used as a quick way to build variants of a program, without
adding complicated conditions to the makefile. None of the files in the original
directory are modified; they are used as needed.

Using a directory hierarchy

A repository is actually not just a single directory, it's a whole directory hierarchy.
Suppose you use /our/library as a repository. Now /our/library may well contain many
subdirectories, e.g., /our/library/gui and /our/library/network. Consider this command:

% makepp -R /our/library

Any commands in the makefile that refer to files in the directory ./network will actually
get files from /our/library/network, and similarly for ./gui. Makepp automatically
creates any directories that exist in the repository but not in the current directory.

Linking to any place in the file system

All of the above examples show files from a repository being linked into the current
directory or its subdirectories, but you can actually have makepp link them into any place
in the file system that you have write access to. This is done by specifying
"-R new-location=old-location".

For example, sometimes it's a little tedious to type the following:

mkdir alternate-build
cd alternate-build
makepp -R ..

You can do it all with one command, like this:

makepp -R alternate-build=. -F alternate-build

"-F" or "-makeppfile" changes to that directory before loading the makefile. You must
specify "-R" before "-F". Note that this example puts the new build tree inside the
repository. That will not work if you use a RootMakeppfile because makepp safeguards
against nested trees. It's also not a good idea if you use **, because if you ever build
in the repository it will also find edited and generated files in this subtree.

Assigning a different location in the file system may be also useful for more complicated
builds, where there are several library subdirectories. For example, here's a command I
have used to build variants of one of my programs:

% makepp -R test-build/seescape=/src/seescape \
-R test-build/HLib=/src/HLib \
-R test-build/H5pp=/src/H5pp \
-R qwt=/src/external_libraries/qwt \
-F test-build/seescape

This command loads in files from four different repositories, and then cds to the
./test-build/seescape directory and executes the makefile there. Files contained in the
directory tree beginning with /src/seescape are linked into ./test-build/seescape. In
other words, makepp will temporarily link the file /src/seescape/gui/image_canvas.cxx to
./test-build/seescape/gui/image_canvas.cxx when it is needed. This command will work even
if the "test-build" directory doesn't exist yet; makepp will create it for you. (But you
must specify the "-R" options before the "-F" option on the command line.)

Multiple equivalent repositories

Say your project is maintained by several fairly autonomous groups. You could have one
complete repository with all the sources as they are in production or at least
successfully tested. Every group can have a mostly empty repository with (part of) the
same structure, containing the files group members have finished developing.

Developers' current directories will have the files they are still working on. The group
repository will be the first one given and the production repository the last one, so that
it furnishes the files not found in the group repository:

$ makepp -R/path/to/group/repository -R/path/to/production/repository

Since this is probably fairly static for that directory, you may want to put a file
.makepprc at its root with the following content:

-R/path/to/group/repository -R/path/to/production/repository

Or, presuming that it has a fixed path, you could write into your makefile:

repository /path/to/production/repository

and, because options are seen before makefiles are read, you can then call just

$ makepp -R/path/to/group/repository

Repositories as fixed part of your build system

If you know you always use some repository you can use the "repository" or "vpath"
statements in your makefile.

Caveats with repositories
When the links get in the way

For finding your way around your file hierarchy and for allowing the debugger to find the
sources it is useful to have the links used while building. But when you want to edit a
file or resync it with your version control, the links can get in the way. That is
because the system traverses the link and writes to the file in the repository. Unless
it's your personal repository used just for keeping things apart, that may not be what you
want.

As a safeguard against inadvertent overwriting of public files it is suggested to make the
sources in the repository unwritable. It might even not be enough to remove the write
bit, because a version control system which insists on your locking the files for editing
might also do that, but temporarily make the file writable while resyncing it. If that is
the case for you, the repository should actually belong to a different user.

There are a few tactics to surmount this:

· Keep the sources you edit in a repository, separate from your build tree. Whenever
you put a file, which was previously fetched from another repository, into this
editing repository, makepp will notice and fetch it from there, provided it is the
first repository you specify.

· Remember to delete any file, before you create a copy for writing. If you follow the
safeguard suggestion above, forgetting to do this will give an error message when
writing. To help you, the following function "delink" will replace one link by a copy
of the linked file. The first variant is for all kinds of Bournish Shells, the second
one for csh (or at least tcsh):

$ delink() { { rm $1 && cat >$1; } <$1; }
% alias delink '( rm \!:1 && cat >\!:1; ) <\!:1'

· If you feel you don't need them, you can delete them all, whenever you want, e.g.
after every makepp run, possibly backgrounded (either short or long form):

makeppclean --recurse --only-repository-links
mppc -rR

Don't build in a repository during use

A repository is meant to be read-only while it is being used as a repository. Makepp will
not work properly if you change files in your repository during the course of a build.
Nightly builds may be ok for you, if no one else uses the repository at that time. Before
it starts the build, makepp gets a list of all the files that exist in the repository, and
never updates its list, except for files it expects to appear.

If you need a repository that's changing as you build, you might want to consider makepp's
build cache mechanism (see makepp_build_cache). Alternatively, you can use a "poor man's
repository": you can put explicit rules into your makefile to create the soft links, like
this:

%.c : $(directory_I_wish_was_a_repository)/%.c
&ln -fs $(input) $(output)

This works only for source files; you can't easily use this to link a file if it is
already built in the repository, but build it here if it's not already built, since there
is only allowed to be one way to build a file.

Use only relative filenames

Repositories work completely transparently if the makefiles use only relative filenames.
In the above example, it's ok if the makefile in /src/seescape refers to ../HLib, but the
above command will not work as expected if it refers to /src/HLib. If you need to use
absolute file names, you can put them into make variables and then override them on the
command line, like this:

% makepp -R test-build/seescape=/src/seescape SEESCAPE=/home/holt/test-build/seescape \
-R test-build/HLib=/src/HLib HLIB=/home/holt/test-build/HLib \
-R test-build/H5pp=/src/H5pp H5pp=/home/holt/test-build/H5pp \
-R qwt=/src/external_libraries/qwt QWT=/home/holt/test-build/qwt \
-F test-build/seescape

The above will work as long as the "HLib" directory is referred to as "$(HLIB)" in all the
makefiles. Note that you have to specify absolute paths for the directories, because
makepp cd's to "test-build/seescape" before reading the makefile. This leads to long and
complicated make commands; use relative paths when possible.

Makepp must know about all dependencies

Repositories will not work if there are hidden dependencies that makepp doesn't know
about. (In fact, doing a build using repositories, is one way of checking for forgotten
dependencies. But, just for this check, don't combine it with a build cache, since
fetching something there, instead of building it, might hide a forgotten dependency.)
Sometimes these dependencies can be fairly subtle. For example, the libtool command will
not only create ".lo" and ".la" files as listed on the command line, but it also may
create a subdirectory called ".libs" which contains the actual object files. To prevent
build mistakes, makepp refuses to link in a ".la" file from a repository. Hopefully in
the future libtool will be better supported.

Many hidden dependencies related to compilation are caught by the command line scanner.
If your compiler uses the common Unix compilation flags (e.g., "-I", "-D", etc.), then
makepp will usually figure out where all your include files are. You may have to be
careful if you have any homegrown scripts that create files that makepp doesn't know
about. For correct builds, it is vitally important to list all targets and dependencies
(or determine them automatically by scanning).

Putting absolute filenames into programs

Repositories will also not work if any of the files built contain absolute file names in
them (e.g., if any of your build commands write out an absolute filename). For example,
it turns out that the ".la" files produced by libtool have this property. (If you look at
the contents of the ".la" file you'll see that the dependency list contains absolute
filenames.) In order to solve this particular problem, makepp will not link ".la" files
from a repository; it will insist on rebuilding them.

Avoid linking in unnecessary directories

Repositories can be slow on startup and use a lot of memory if there are a lot of
unnecessary files in the repository. For example, if you use an automatic HTML
documentation generator which makes thousands of ".html" files from your source code, you
may not want to put them in a subdirectory of a directory that's used as a repository.
It's better to put them in a different directory tree entirely, so the repository
mechanism won't load in their names.

Too Many Files

The disadvantage of repositories is that symbolic links, which the repository mechanism
uses, are individual files (though they use almost no disk space). This is unlike real
links, but those can't cross file system boundaries. In extreme cases the presence of
very many symbolic links can lead to exhaustion of the number of foreseen files (so called
inodes), even though there is plenty of space left. In this case the sysadmin will need
to tune the file system.

Overriding repository copies
If you make any modifications to a file locally, makepp will ordinarily realize this and
recompile the file using the local copy rather than the repository copy.

If you're using a repository to maintain a central code base, and you have developers
working on local copies which contain only the files they have modified, one problem that
comes up is: what if a developer wants to remove a file from his local build but the
repository still contains it? If the developer removes the local copy, makepp will
happily put in the copy from the repository, and the build will proceed as if the file
existed.

One technique (alas not for user root) for this problem is to make the file that you want
not to include in the build process unreadable, like this:

chmod a-rw file-to-be-excluded

This will prevent makepp from incorporating it from the repository. Makepp also includes
special code so that unreadable files do not match wildcards or pattern rules.

Similarly, to prevent makepp from incorporating an entire subdirectory, make a local
directory that has the same name but is unwritable. If you want makepp to ignore the
directory entirely, then make it unreadable too. (Read-only directories are searched but
targets in them are usually not built.)

The other way to do this is calling makepp with one or more exclusion options:

mpp -R /path/to/rep --dont-read=/path/to/rep/file-to-be-excluded

Don't use repositories for files which can change!
Don't try to use a repository for a file which is part of your build. For example, you
might be tempted to try to use repositories to put all of your public .h files in the same
directory, like this:

# top level makefile
repository include=module1/include
repository include=module2/include
repository include=module3/include
repository include=module4/include

This is probably not a good idea if any of the .h files are themselves outputs of a
program (e.g., yacc or some other program that spits out C source code), because makepp
assumes that files in repositories never change. If the build needs include/xyz.h, and
module2/include/xyz.h actually needs to be produced by some program, makepp will not know
to run the program. It's better to use a technique like this to put all of your .h files
into a common include directory:

# module1/Makeppfile
../include/%.h : include/%.h
&cp $(input) $(output)
# You could also (more efficiently but problematic on Windows) do the following:
# &ln -r $(input) $(output)

Makepp might still try to build files that happen to be in a repository if something asks
for them directly, but it won't build them on behalf of the local directory. The result
of this can be quite confusing, because it can lead to a repository symbolic link being
used while its repository target is out-of-date, but that target might get updated later
in the build. You can prevent this from happening either by making sure that the
repository is referred to only through the repository path, or by making sure that there
is also a local rule for all the generated repository files.

Another way to avoid recompiling identical files in different directories is to use a
build cache (see makepp_build_cache for details). A build cache does not have the
restriction that the file may not change.

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