This rest of this guide is intended for duffers like me, who aren't really interested in Makefiles and systems configurations, but who need a mental model of the interlocking pieces so that they can make them work, extend them consistently when adding new software, and lay hands on them gently when they don't work.
First, a historical note. The GHC build system used to be called "fptools": a generic build system used to build multiple projects (GHC, Happy, GreenCard, H/Direct, etc.). It had a concept of the generic project-independent parts, and project-specific parts that resided in a project subdirectory.
Nowadays, most of these other projects are using Cabal, or have faded away, and GHC is the only regular user of the fptools build system. We decided therefore to simplify the situation for developers, and specialise the build system for GHC. This resulted in a simpler organisation of the source tree and the build system, which hopefully makes the whole thing easier to understand.
You might find old comments that refer to "projects" or "fptools" in the documentation and/or source; please let us know if you do.
If you just want to build the software once on a single platform, then your source tree can also be your build tree, and you can skip the rest of this section.
We often want to build multiple versions of our software for different architectures, or with different options (e.g. profiling). It's very desirable to share a single copy of the source code among all these builds.
So for every source tree we have zero or more
build trees. Each build tree is initially
an exact copy of the source tree, except that each file is a
symbolic link to the source file, rather than being a copy of
the source file. There are “standard” Unix
utilities that make such copies, so standard that they go by
are two (If you don't have either, the source distribution
includes sources for the X11
utils/lndir). See Section 6.4, “The story so far” for a typical invocation.
The build tree does not need to be anywhere near the source tree in the file system. Indeed, one advantage of separating the build tree from the source is that the build tree can be placed in a non-backed-up partition, saving your systems support people from backing up untold megabytes of easily-regenerated, and rapidly-changing, gubbins. The golden rule is that (with a single exception—Section 6.3, “Getting the build you want”) absolutely everything in the build tree is either a symbolic link to the source tree, or else is mechanically generated. It should be perfectly OK for your build tree to vanish overnight; an hour or two compiling and you're on the road again.
You need to be a bit careful, though, that any new files you create (if you do any development work) are in the source tree, not a build tree!
Remember, that the source files in the build tree are
symbolic links to the files in the source
tree. (The build tree soon accumulates lots of built files like
Foo.o, as well.) You can
delete a source file from the build tree
without affecting the source tree (though it's an odd thing to
do). On the other hand, if you edit a
source file from the build tree, you'll edit the source-tree
file directly. (You can set up Emacs so that if you edit a
source file from the build tree, Emacs will silently create an
edited copy of the source file in the build tree, leaving the
source file unchanged; but the danger is that you think you've
edited the source file whereas actually all you've done is edit
the build-tree copy. More commonly you do want to edit the
Like the source tree, the top level of your build tree
must be (a linked copy of) the root directory of the GHC source
tree.. Inside Makefiles, the root of your build tree is called
In the rest of this document path names are relative to
otherwise stated. For example, the file
mk/target.mk is actually
When you build GHC you will be compiling code on a particular host platform, to run on a particular target platform (usually the same as the host platform). The difficulty is that there are minor differences between different platforms; minor, but enough that the code needs to be a bit different for each. There are some big differences too: for a different architecture we need to build GHC with a different native-code generator.
There are also knobs you can turn to control how the software is built. For example, you might want to build GHC optimised (so that it runs fast) or unoptimised (so that you can compile it fast after you've modified it. Or, you might want to compile it with debugging on (so that extra consistency-checking code gets included) or off. And so on.
All of this stuff is called the configuration of your build. You set the configuration using a three-step process.
NOTE: if you're starting from a source distribution, rather than darcs sources, you can skip this step.
Change directory to
issue the command
(with no arguments). This GNU program (recursively) converts
to a shell script called
If autoreconf bleats that it can't write the file
then delete the latter and try again. Note that you must use autoreconf,
and not the old autoconf! If you erroneously use the latter, you'll get
a message like "No rule to make target 'mk/config.h.in'".
Some parts of the source tree, particularly
libraries, have their own configure script.
autoreconf takes care of that, too, so all you have
to do is calling autoreconf in the top-level directory
These steps are completely platform-independent; they just mean
that the human-written files (
aclocal.m4) can be short, although the resulting
files (the configure shell scripts and the C header
mk/config.h.in) are long.
Runs the newly-created configure script, thus:
$ ./configure [
configure's mission is to scurry
round your computer working out what architecture it has,
what operating system, whether it has the
vfork system call, where
tar is kept, whether
gcc is available, where various obscure
#include files are, whether it's a
leap year, and what the systems manager had for lunch. It
communicates these snippets of information in two
substituting for things between
@” brackets. So,
@HaveGcc@” will be
replaced by “
NO” depending on what
mk/config.mk is included by every
Makefile (directly or indirectly), so the
configuration information is thereby communicated to
configure takes some optional
./configure --help to
get a list of the available arguments. Here are some of
the ones you might need:
Specifies the path to an installed GHC which
you would like to use. This compiler will be used
for compiling GHC-specific code (eg. GHC itself).
This option cannot be specified
build.mk (see later),
because configure needs to
auto-detect the version of GHC you're using. The
default is to look for a compiler named
ghc in your path.
Specifies the path to any installed Haskell
compiler. This compiler will be used for compiling
generic Haskell code. The default is to use
ghc. (NOTE: I'm not sure it
actually works to specify a compiler other than GHC
here; unless you really know what you're doing I
suggest not using this option at all.)
Specifies the path to the installed GCC. This
compiler will be used to compile all C files,
except any generated by the
installed Haskell compiler, which will have its own
idea of which C compiler (if any) to use. The
default is to use
Next, you say how this build of
GHC is to differ from the standard
defaults by creating a new file
in the build tree. This file is the
one and only file you edit in the build tree, precisely
because it says how this build differs from the source.
(Just in case your build tree does die, you might want to
keep a private directory of
files, and use a symbolic link in each build tree to point
to the appropriate one.) So
mk/build.mk never exists in the
source tree—you create one in each build tree from
the template. We'll discuss what to put in it
And that's it for configuration. Simple, eh?
What do you put in your build-specific configuration file
mk/build.mk? For almost all
purposes all you will do is put make variable definitions that
override those in
mk/config.mk.in. The whole point of
mk/config.mk.in—and its derived
mk/config.mk—is to define
the build configuration. It is heavily commented, as you will
see if you look at it. So generally, what you do is look at
mk/config.mk.in, and add definitions in
mk/build.mk that override any of the
config.mk definitions that you want to
change. (The override occurs because the main boilerplate file,
For your convenience, there's a file called
build.mk.sample that can serve as a starting
point for your
The accompanying comment explains that this is the list of
flags passed to GHC when building GHC itself. For doing
development, it is wise to add
enable debugging code. So you would add the following to
GhcHcOpts += -DDEBUG
GNU make allows existing definitions to
have new text appended using the “
operator, which is quite a convenient feature.
Haskell compilations by default have
turned on, by virtue of this setting from
SRC_HC_OPTS += -H16m -O
SRC_HC_OPTS means "options for HC from
the source tree", where HC stands for Haskell Compiler.
SRC_HC_OPTS are added to every Haskell
compilation. To turn off optimisation, you could add this to
SRC_HC_OPTS = -H16m -O0
Or you could just add
GhcHcOpts to turn off optimisation for the
compiler. See Section 5, “Quick start for GHC developers” for some more
that anything between “@...@” signs is going to be substituted
by configure later. You
can override the resulting definition if
you want, but you need to be a bit surer what you are doing.
For example, there's a line that says:
TAR = @TarCmd@
This defines the Make variables
to the pathname for a tar that
configure finds somewhere. If you have your
own pet tar you want to use instead, that's
fine. Just add this line to
TAR = mytar
You do not have to have a
mk/build.mk file at all; if you don't,
you'll get all the default settings from
You can also use
build.mk to override
anything that configure got wrong. One place
where this happens often is with the definition of
variable is supposed to be the canonical path to the top of your
source tree, but if your system uses an automounter then the
correct directory is hard to find automatically. If you find
that configure has got it wrong, just put the
correct definition in
Let's summarise the steps you need to carry to get yourself a fully-configured build tree from scratch.
Get your source tree from somewhere (darcs repository
or source distribution). Say you call the root directory
myghc (it does not have to be
(Optional) Use lndir or mkshadowdir to create a build tree.
$ cd myghc $ mkshadowdir . /scratch/joe-bloggs/myghc-x86
(N.B. mkshadowdir's first argument is taken relative to its second.) You probably want to give the build tree a name that suggests its main defining characteristic (in your mind at least), in case you later add others.
Change directory to the build tree. Everything is going to happen there now.
$ cd /scratch/joe-bloggs/myghc-x86
Prepare for system configuration:
(You can skip this step if you are starting from a
source distribution, and you already have
Do system configuration:
Don't forget to check whether you need to add any
configure; for example, a
common requirement is to specify which GHC to use with
Create the file
adding definitions for your desired configuration
You can make subsequent changes to
mk/build.mk as often as you like. You do
not have to run any further configuration programs to make these
changes take effect. In theory you should, however, say
make clean; make, because configuration
option changes could affect anything—but in practice you
are likely to know what's affected.
At this point you have made yourself a fully-configured build tree, so you are ready to start building real things.
The first thing you need to know is that you must use GNU make. On some systems (eg. FreeBSD) this is called gmake, whereas on others it is the standard make command. In this document we will always refer to it as make; please substitute with gmake if your system requires it. If you use a the wrong make you will get all sorts of error messages (but no damage) because the GHC Makefiles use GNU make's facilities extensively.
To just build the whole thing, cd to the top of your build tree and type make. This will prepare the tree and build the various parts in the correct order, resulting in a complete build of GHC that can even be used directly from the tree, without being installed first.
GHC requires a 2-stage bootstrap in order to provide full functionality, including GHCi. By a 2-stage bootstrap, we mean that the compiler is built once using the installed GHC, and then again using the compiler built in the first stage. You can also build a stage 3 compiler, but this normally isn't necessary except to verify that the stage 2 compiler is working properly.
Note that when doing a bootstrap, the stage 1 compiler
must be built, followed by the runtime system and libraries, and
then the stage 2 compiler. The correct ordering is implemented
by the top-level
Makefile, so if you want
everything to work automatically it's best to start
make from the top of the tree. The top-level
Makefile is set up to do a 2-stage
bootstrap by default (when you say make).
Some other targets it supports are:
Build everything as normal, including the stage 1 compiler.
Build the stage 2 compiler only.
Build the stage 3 compiler only.
Build stage 1 followed by stage 2.
Build stages 1, 2 and 3.
Install everything, including the compiler built in
stage 2. To override the stage, say
n is the stage to install.
make a binary distribution. This is the target we use to build the binary distributions of GHC.
make a source distribution. Note that this target does “make distclean” as part of its work; don't use it if you want to keep what you've built.
Makefile also arranges
to do the appropriate
make boot steps (see
below) before actually building anything.
stage3 targets also work in the
compiler directory, but don't forget that
each stage requires its own
make boot step:
for example, you must do
$ make boot stage=2
make stage2 in
In any directory you should be able to make the following:
does the one-off preparation required to get ready for the real work. Notably, it does make depend in all directories that contain programs. It also builds the necessary tools for compilation to proceed.
explicitly is not normally necessary. From the top-level
make boot to be invoked in various
subdirectories first, in the right order. Unless you
really know what you are doing, it is best to always say
make from the top level first.
If you're working in a subdirectory somewhere and
need to update the dependencies,
boot is a good way to do it.
makes all the final target(s) for this Makefile. Depending on which directory you are in a “final target” may be an executable program, a library archive, a shell script, or a Postscript file. Typing make alone is generally the same as typing make all.
installs the things built by
(except for the documentation). Where does it install
them? That is specified by
mk/config.mk.in; you can override it
mk/build.mk, or by running
configure with command-line arguments
./configure --help for the full
installs the documentation. Otherwise behaves just
reverses the effect of
install (WARNING: probably doesn't work).
Delete all files from the current directory that are normally created by building the program. Don't delete the files that record the configuration, or files generated by make boot. Also preserve files that could be made by building, but normally aren't because the distribution comes with them.
Delete all files from the current directory that are
created by configuring or building the program. If you
have unpacked the source and built the program without
creating any other files,
distclean should leave only the files that were
in the distribution.
clean, but may refrain from
deleting a few files that people normally don't want to
Delete everything from the current directory that
can be reconstructed with this Makefile. This typically
includes everything deleted by
distclean, plus more: C source files
produced by Bison, tags tables, Info files, and so
One exception, however:
maintainer-clean should not delete
configure even if
configure can be remade using a rule
Makefile. More generally,
make maintainer-clean should not delete
anything that needs to exist in order to run
configure and then begin to build the
configure will be necessary before
All of these standard targets automatically recurse into sub-directories. Certain other standard targets do not:
.depend file in each
directory that needs it. This
file contains mechanically-generated dependency
information; for example, suppose a directory contains a
Haskell source module
imports another module
Baz. Then the
.depend file will contain
Foo.o : Baz.hi
which says that the object file
Foo.o depends on the interface file
Baz.hi generated by compiling module
file is automatically included by every Makefile.
Makefiles have targets other
than these. You can discover them by looking in the
If you want to build GHC and just use it direct from the
build tree without doing
make install first,
you can run the in-place driver script. To run the stage 1
stage 2 is
Do NOT use
compiler/stage1/ghc-6.xx, as these are the
scripts intended for installation, and contain hard-wired paths
to the installed libraries, rather than the libraries in the
Sometimes the dependencies get in the way: if you've made a small change to one file, and you're absolutely sure that it won't affect anything else, but you know that make is going to rebuild everything anyway, the following hack may be useful:
$ make FAST=YES
This tells the make system to ignore dependencies and just
build what you tell it to. In other words, it's equivalent to
temporarily removing the
.depend file in
the current directory (where mkdependHS and
friends store their dependency information).
A bit of history: GHC used to come with a fastmake script that did the above job, but GNU make provides the features we need to do it without resorting to a script. Also, we've found that fastmaking is less useful since the advent of GHC's recompilation checker (see the User's Guide section on "Separate Compilation").