# The GAP build system

This file is meant to give an overview of how the GAP build system works. It
is targeted at people who need to work on the build system itself (to extend
it, fixes bugs in it, etc.). It should not be necessary to read this if all
you want to do is compile GAP and work on the GAP library and kernel.

Note that this really is just an overview; for details, please refer to the
comments inside the various parts of the build system.


## Prerequisites

In order to work on the build system, you need at least the following:

* GNU autoconf (we recommend 2.69 or later)
* GNU make

Note that we extensively use features provided by GNU make, so in general
another version of make, such as BSD make, is not suitable.


## Quick start: building GAP with no frills

If you are working with a fresh clone of the GAP repository, you need to
run the `autogen.sh` script first, which will generate the `configure`
script. Afterwards, or if you are using a release version of GAP, you
can follow the standard procedure:

```
./configure
make
```


== Overview of the files constituting the GAP build system

* `autogen.sh`: sets up the build system; typically the first thing to run in
  a fresh clone of the GAP repository. It runs `autoreconf` which in turn runs
  several tools include `aclocal`, `autoheader`, `autoconf`, `libtoolize`

* `configure`: generated by `autogen.sh` from `configure.ac`. 

* `configure.ac`: the GNU autoconf source of our configure script.

* `GNUmakefile`, `GNUmakefile.in`: The former file is generated from the
  latter by `configure`. It is the primary Makefile (GNU make prefers it
  over `Makefile`). It only contains variables and vpath settings, and
  includes `Makefile.rules` for the actual build rules.

* `Makefile`: This is a placeholder file, and serves two purposes:
   1. If the user runs `make` before `configure`, it prints a warning.
   2. If `configure` did run, but `make` is not GNU make, it produces
      a corresponding error message.

* `Makefile.rules`: This is the core of the build system. If you want
  to add or remove a kernel C source file, you need to add or remove
  its name here and only here.

* `bin/`: This directory is created for compatibility mode (see below).

* `cnf/`: All files in this directory are part of the build system.

* `extern/`: External libraries we bundle with GAP (such as GMP) are
  put in here.

* `gen/`: Generated code (such as `config.h` and `gap_version.c`) is put
  into this directory.

* `obj/`: All `*.o` files are placed into this directory.

- `.deps/` directories contain `*.d` files generated by the build system,
  and which are used to track dependencies, e.g. of C source files on header
  files.

- `.libs/` directories are created by libtool. Please refer to the libtool
  documentation for details.


## Out-of-tree builds

The old GAP build system had a concept of "configs" and "CONFIGNAME", which
allowed you to build GAP in different configurations from a single set of
sources. This is gone in the current build system. However, a similar goal can
be achieved by using so-called "out-of-tree builds".

In the following and also in the files that make up the build system, "srcdir"
refers to the directory containing the GAP files, i.e. it contains this
README, the src and lib directories and more.

To create a new out-of-tree build, create a new directory anywhere
in your filesystem. A typical setup places the out-of-tree dirs into
subdirectories of a "build" directory inside the srcdir. So you might
have directories

```
   srcdir/build/default
   srcdir/build/default32
   srcdir/build/hpcgap
   srcdir/build/hpcgap32
   ...   
```

We will refer to this directory from now on as the "builddir".

To initialize the out-of-tree build, change into the builddir and
execute the configure script from the srcdir, like this:

```
cd $builddir
$srcdir/configure
```

You can pass any additional options you like to configure, e.g. `ABI=32`
or `--enable-hpcgap`.

Once the configure script has completed, you can run `make` as usual,
and all the object files and the gap executable will be placed inside
builddir. Your srcdir will remain untouched.


## Compatibility mode

Compatibility mode emulates the build environment of the old GAP build system
in order to allow packages with kernel extensions to be used with the new
build system unmodified. As such, it mainly exists to ease the transition
between new and old build system, and the plan is to remove it once all
packages have been adapted to the new build system. However, that is still
far off.

Compatibility mode does the following things:

* create `sysinfo.gap` file in the build dir
* create a symlink `sysinfo.gap-default$ABI` pointing at `sysinfo.gap`
* create a `bin/$GAPARCH/config.h` symlink pointing at `gen/config.h`
* create a `bin/$GAPARCH/gac` symlink pointing at `gac`
* create a `bin/$GAPARCH/src` symlink pointing at either `$srcdir/src`
  or (for HPC-GAP build) at `$srcdir/hpcgap/src`
* for out-of-tree builds, it creates a `${builddir}/src/compiled.h` file
* ...

For now, using compatibility mode is required if one wants to build the
kernel extension for most packages which have one. In the future, we will
provide an alternative way to do this, and also will extend `gac` to
cleanly supported building kernel extensions.


## Dependency tracking

The build system tracks depdencies between files, such as between C source and
header files, via `*.d` files in `.deps` directories below `obj/` and `gen/`.
These files are mostly generated by the compiler; for this, the compiler needs
to support the relevant flags (gcc, clang, icc all do so).

For a detailed explanation of a very similar scheme, see here:
<http://make.mad-scientist.net/papers/advanced-auto-dependency-generation/>


## HPC-GAP integration

One of the main features of the new build system is that it optionally allows
to build HPC-GAP instead of plain GAP. HPC-GAP is an experimental fork of GAP
which implements concurrent programming, multi-threading, etc..

The HPC-GAP kernel and library were forked from the GAP kernel and library and
developed semi-independently for several years, with occasional merges between
the two. In order to recombine the two, we merged the HPC-GAP fork into a
subdirectory `hpcgap` of the GAP repository.  Then, all files inside `hpcgap`
which were identical to their counterparts in the GAP repository were deleted
(e.g. `hpcgap/src/ariths.c` was deleted as it was identical to `src/ariths.c`).

The new build system was then modified to optionally allow building HPC-GAP
(when the `--enable-hpcgap` flag is passed to the `configure` script). For
this work, and the resulting HPC-GAP binary to work, two tricks are used:

1. In order to get the library to work right, HPC-GAP mode employs multiple
GAP root paths. Specifically, the GAP kernel function `SySetGapRootPath` was
modified so that for every root directory `FOO` that gets added, we first add
`FOO/hpcgap` to the list of root directories. 

2. Ward (a tool for scanning our C sources and inserting guard statements into
it) is integrated as follows: Normally, `src/FOO.c` gets compiled to
`obj/FOO.o` (or rather `obj/FOO.lo`, as we use GNU libtool). With ward in the
mix, `src/FOO.c` is first turned into `gen/FOO.c` by ward, and then the
compiler turns the latter into `obj/FOO.lo`. For each conversion, dependencies
are tracked via `gen/.deps/FOO.d` resp. `obj/.deps/FOO.d` files. In
particular, if `src/FOO.c` is modified, then the dependency rules in
`gen/.deps/FOO.d` will ensure that `gen/FOO.c` is regenerated from
`src/FOO.c`; and the dependency rules in `obj/.deps/FOO.d` ensure that
`obj/FOO.lo` is regenerated from `gen/FOO.c`


## Open tasks

There are many things that still need to be done in the new build system. For
an overview, see
<https://github.com/gap-system/gap/issues?q=is:open+is:issue+label:"build system">

The main open task is to add support for `make install`. There are some
pitfalls to that, esp. when it comes to handling packages with kernel
extensions.
