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.