This document is a gentle introduction to PBS . It gives you some insight in why we designed it the way we did.
PBS is a build utility in the same spirit as make. It is not compatible with make and works completely differently!
PBS history starts with the complete frustration state we got in when trying to have gmake do anything complicated. To fix that problem, we introduced cons at our job and it worked fine but I was not completely happy with cons as were other people on the cons mailing list.
During Chrismas break, I always try to start a new project. The month was december, my frustration with make was at its top, Chrismas was one week away, PBS was born! After a week work I had a small working build system. At that time I had contact with Michel Pfeiffer who was working on, the now defunct, make.pl. Our goals where to write an advanced build system under 10 KB code (I can only smile when I see PBS is closer to 400 than 10 KB).
From make.pl home page:
Arcane make and its various derivatives (cook, GNU make, jam, makepp, ...) use a weird language mix of a variable and rule syntax plus, for actually doing something, embedded shell (along with sed, awk ...) The derivatives improve this language, but the improvements are not accessible with automake, and still don't make a really useable language.
Having a powerfull scripting language instead for a crippled one makes a huge difference for a build system maintainer. To the arcane make, I would add the ugly, unreadable XML based build systems.
I also wanted to try new ideas that haven't been used in other build systems.
META is a very overloaded word that computer engineers use when they want to make something sound more intelligent than it really is. PBS is not a build system, it's a library that makes it possible to write a build tool and eventually an instanciation of it, ie a build system.
Work must be invested in a build system, or more rightly in "the" build system you are using. This is true whatever tool you are using. We have such a tool, pbs. Note the lower case name.
PBS give you the possibility to do things, it's up to you to do them. If you are looking for some magical build system, there are a few out there that might do that for you. If you write build systems when you have free time or really, really when you need to do it, then PBS is not for you.
PBS is a three pass system, it build a dependency graph, checks the dependency graph to find out what is to be build, builds whatever node that needs it. This is very different from make which builds on the run. Both systems have advantages and disadvantages.
PBS lets you define rule in scripts written in perl. Using filters you could write them in whatever language you want. Once the rules are defined, PBS generates the dependency graph by applying the rules recursively on the top tager and it's dependencies. Having the whole dependency graph has these advantages:
It also has the following disadvantages:
To overcome the first disadvantage, PBS uses caching scheme we call warp.
PBS is a superset of Perl or an add-in to Perl (choose whichever you prefer). It introduces only a very few extra functions. Those functions are nothing more than plain Perl subs. The build scripts being perl, they are interpreted by perl within the frame of PBS.
I, Nadim Khemir, wrote PBS but the credit is not only mine. Anders Lindgren has been involved from the very begining with the architecture and he is also the one that can use PBS best. Ola Maartensson also deserves large credit for forcing us not to fix something for our needs but think about other users (mainly him :-). He set his mark in how things should look like and how verbose a build system should be. Ola and I maintained the build system at our work. It was based on make and that's why we tried to have it look a bit the same.
Here is a simple rule:
AddRule 's_objects' #name => [ '*/*.o' => '*.s' ] #depender => "%AS %ASFLAGS ... -o %FILE_TO_BUILD %DEPENDENCY_LIST" ; # builder
The rule could be defined in a rule library and that library could be included instead for definig the rule inline. This is exactely what we do in a file we call (wrongly as the rule above is for assembler files) C.pm. PBS has support for finding libraries and locally overidding libraries. The above (and much more) is replaced in our Pbsfiles by this single line:
We try to mimic Perl's use and we give our libraries the extension .pm. Pbsfiles should have extention .pl.
PBS will automatically push your script in a package. This is done to separate rules and configurations when doing a hierarchical build. all Pbsfiles run in strict mode.
Pbsfiles are perl scripts so all you can do in a perl scrip you can do in a Pbsfile, including using modules from CPAN.
You can add rules but you can also remove rules from the rules defined in the libraries you include.
PBS doesn't do much and does nothing by default. I don't like to guess what is going on so I find it empowering to have to tell the system what to do. Since PBS is a meta build system, you can write your own interface that makes it look easy or or use the less easy but straight forward native interface.
PBS has no built-in rules, period.
Rules have 6 components, 2 are mandatory.
A rule always has a name (mandatory). This is to simplify debugging (of _your_ build system).
A rule also bellongs to a rule namespace. More often than not, PBS handles this transparently and you don't even know it is there.
A rule can carry a node type, ie when a node matches a rule, the rule type are passed to the node.
To know if a rule matches a node, a (mandatory) depender is declared. Most often the depender is a list containing a regex and a dependency definition. A depender can also be a perl sub so it can get very powerfull and complicated, one such depender is the example C depender that comes with pbs.
PBS also needs to know how to build nodes. This is either done with a list of shell commands or, again, by using perl subs.
Finally (since version 0.28_3) PBS accepts what we call "node subs". Node subs are run when a node is added to the graph. These subs can do a wide range of operations that are described in the reference manual. you can also define you own "node subs".
All these elements are described in detail in the reference manual.
A meta rule is a rule made of multiple rules with some glue to say which of the base rules does match. An example is:
From which source code you build an object file when you hace a C file and and assembler file ?
You have three rules, one to build from a C file, one to build from an assembler file and one to arbitrate between the rules.
Dependers are perl subs, you can also define a sub that return a depender sub. These subs are run in order and the sum of the dependencies becomes the node under work dependencies. Note that this is different from make.
A builder is also a perl sub. To simplify things, for the user anyhow, AddRule also accepts a list of shell commands. PBS uses the builder from the last matching rule. It will also tell you (if you ask) if nodes had other builders.
PBS lets you define and query configuration variables. These are perl variables so they can contain live objects like mailer or anything you fancy. PBS will check if you try to override a configuration variable and show you what your variables contain if you ask.
PBS never changes directory and you should never change directory in a depender or a builder. Everything is based in '.', the current directory. The check step locates the files based in '.' and set their full path build names.
if you don't give a specific build directory to PBS, it will build int "out_" + your user name. So you never clutter your source directory unless you really want to.
PBS can locate your source files from multiple directories, these are also sometime named "repositories" in other build systems. Your repositories can also contain binary files.
PBS supports hirarchical builds, No extra process is started for a sub build (called sub pbs). This is a necessity as we must have the full dependency graph. Unlike cons, top build files and lower level build files are equivalent. This lets you use PBS for building projects or their sub components without the same set of Pbsfiles
There are two ways to start a sub pbs, by directly matching a rule that starts a subpbs or by defining a trigger. A trigger will start a sub pbs when a node "triggers" a rule (this is much easier to comprehend with an example). The nice thing is that triggers can be defined in the sub Pbsfile and imported from there. This allows for a better separation of the rules making up your build system. The user's manual has such an example.
Pbsfiles can be commented with POD and PBS can extract the documentation and even search it for you.
PBS has debugging hooks that let you run speciific perl code on certain events. The support works in the perl debugger too. (hmm what about latest perl version?)
Khemir Nadim ibn Hamouda. <email@example.com>and Ander Lindgren (ALI).
Thanks to Ola Maartensson for his input.
Parts of the development was funded by C-Technologies AB, Ideon Research Center, Lund, Sweden.
Artistic License 2.0