# Fac build system
Fac is a general-purpose build system inspired by make that utilizes
ptrace to ensure that all dependences are enumerated and that all
source files are added to a (git) repo. An important feature of fac
is that it automatically handles dependencies, rather than either
complaining about them or giving an incorrect build. Currently, fac
only runs on linux systems, but on those systems it is incredibly easy
to use!
<img src="kells-fac.svg" alt="Fac"/>
* Automatically tracks build dependencies in a way that is independent
of programming language. You are only required to specify the
minimum of dependencies for each rule (which is often an empty set),
and fac works out the rest for you. If you fail to specify
dependencies, fac should still build successfully if you use it
repeatedly.
* Parallel building.
* You are forced to write your configuration in a language of your own
choice. (Or conversely, you are not forced to use a language of
*my* choice, much less a custom-built language that I developed.)
* Integrates with git, to keep you from forgetting to `git add` a file
that is needed for the build.
## How does it work?
- Fac uses ptrace to track every system call your build command makes.
Thus we can see precisely which files are read, and which files are
modified.
- Fac has an extremely simple declarative
[file format](documentation.html). There are no variables, no
functions, no macros. Just data. This could be a problem for
larger projects if you were forced to write these files by hand.
But in most large projects you will just write a script to generate
these files.
- You write your "build" script as a program (in the language of your
choice) that creates a `.fac` file. This script is run (and
re-run) using the same dependency-tracking mechanism that is used to
for an ordinary build. Thus, you can get away with writing a simple
but inefficient script, since it will only seldom be run. (Unlike,
e.g. `scons` which has to rerun your `SConstruct` python file on
every build.)
- If your build rules depend on the operating system, or the system
environment, your "configure" script is the same program (in the
language of your choice) that creates a `.fac` file. Or perhaps
it provides input to the script that actually creates the `.fac`
file.
For a lengthier introduction and motivation for fac, see
[Introducing fac](introducing-fac.html).
