Ubuntu Manpage: makepp_repositories -- How to use repositories for variant builds, for maintaining a central set of

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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.