The Makefile architecture

make is great if everything works--you type make install and lo! the right things get compiled and installed in the right places. Our goal is to make this happen often, but somehow it often doesn't; instead some weird error message eventually emerges from the bowels of a directory you didn't know existed.

The purpose of this section is to give you a road-map to help you figure out what is going right and what is going wrong.


Debugging Makefiles is something of a black art, but here's a couple of tricks that we find particularly useful. The following command allows you to see the contents of any make variable in the context of the current Makefile:

$ make show VALUE=HS_SRCS

where you can replace HS_SRCS with the name of any variable you wish to see the value of.

GNU make has a -d option which generates a dump of the decision procedure used to arrive at a conclusion about which files should be recompiled. Sometimes useful for tracking down problems with superfluous or missing recompilations.

The output of `make -pq [TARGET]' command can be helpful as well. This command prints the database of rules and variables, running nothing.

A small example

To get started, let us look at the Makefile for an imaginary small program, small. Each program or library in the GHC source tree typically has its own directory, in this case we'll use $(TOP)/small. Inside the small/ directory there will be a Makefile, looking something like this:

TOP = ..


include $(TOP)/mk/
include $(TOP)/mk/

this Makefile has three sections:

  • We start by defining $(TOP) to point to the top of the tree.
  • Next there are some variables that we can set to control how small is built. The most common one is ENABLE_SHELL_WRAPPERS, which we can set to YES if we want to use Cabal's binary wrappers feature. This means that, on unix-like OSes, we install small.wrapper as a shell script wrapper around the real small binary.

    Less common are EXTRA_CLEAN and EXTRA_DISTCLEAN, which list extra files to be removed during make clean and make distclean.
  • Finally we include (One of the most important features of GNU make that we use is the ability for a Makefile to include another named file, very like cpp's #include directive.) the common build system components. First is which, as its name suggests, consists of a large quantity of standard Makefile code. We discuss this boilerplate in more detail in the mk/ file.

    Note that you must define the make variable TOP to be the top-level directory of the source tree, containing the mk directory in which the file is. It is not OK to simply say
    include ../mk/  # NO NO NO
    Why? Because the file needs to know where it is, so that it can, in turn, include other files. (Unfortunately, when an included file does an include, the filename is treated relative to the directory in which make is being run, not the directory in which the included sits.) In general, every file assumes that $(TOP)/mk/ refers to itself. It is up to the Makefile doing the include to ensure this is the case.

    Second is, which is the rules for how to build a Cabal package in the build system. It contains the rules that tell make how to make the standard targets. The reason that this standard code isn't part of is that some directories are built differently, either because they have not yet been migrated to build with Cabal, or because they have needs that don't fit well into a generic system.

Boilerplate architecture

Every Makefile includes a file at the top. In this section we discuss what is in this file. In general:

  • consists of:
    • Definitions of millions of make variables that collectively specify the build configuration. Examples: HC_OPTS, the options to feed to the Haskell compiler; NoFibSubDirs, the sub-directories to enable within the nofib project; GhcWithHc, the name of the Haskell compiler to use when compiling GHC in the ghc project.
    • Standard pattern rules that tell make how to construct one file from another. These rules are becoming more and more redundant as we move to building with Cabal instead. needs to be included at the top of each Makefile, so that the user can replace the boilerplate variable definitions by simply giving a new definition or pattern rule in the Makefile. make simply takes the last definition as the definitive one.

Instead of replacing boilerplate definitions, it is also quite common to augment them. For example, a Makefile might say:


thereby adding "-O" to the end of SRC_HC_OPTS.

  • is part of the old build system, but is still used to compile some parts of GHC. It contains make rules for the standard targets described in standard targets. These rules are selectively included, depending on the setting of certain make variables. These variables are usually set in the middle section of the Makefile between the two includes. must be included at the end (rather than being part of for several tiresome reasons:
    • make commits target and dependency lists earlier than it should. For example, has a rule that looks like this:
      $(HS_PROG) : $(OBJS)
      	 $(HC) $(LD_OPTS) $< -o $@
      If this rule was in then $(HS_PROG) and $(OBJS) would not have their final values at the moment make encountered the rule. Alas, make takes a snapshot of their current values, and wires that snapshot into the rule. (In contrast, the commands executed when the rule "fires" are only substituted at the moment of firing.) So, the rule must follow the definitions given in the Makefile itself.
    • Unlike pattern rules, ordinary rules cannot be overriden or replaced by subsequent rules for the same target (at least, not without an error message). Including ordinary rules in would prevent the user from writing rules for specific targets in specific cases.
    • There are a couple of other reasons I've forgotten, but it doesn't matter too much.

The mk/ file

If you look at $(GHC_TOP)/mk/ you will find that it consists of the following sections, each held in a separate file:

is the build configuration file we discussed at length in getting the build you want.

is part of the old build system, i.e. it is not used by packages that are built with Cabal. defines make variables for pathnames and file lists. This file contains code for automatically compiling lists of source files and deriving lists of object files from those. The results can be overriden in the Makefile, but in most cases the automatic setup should do the right thing.

The following variables may be set in the Makefile to affect how the automatic source file search is done:

  • ALL_DIRS Set to a list of directories to search in addition to the current directory for source files.
  • EXCLUDED_SRCS Set to a list of source files (relative to the current directory) to omit from the automatic search. The source searching machinery is clever enough to know that if you exclude a source file from which other sources are derived, then the derived sources should also be excluded. For example, if you set EXCLUDED_SRCS to include Foo.y, then Foo.hs will also be excluded.
  • EXTRA_SRCS Set to a list of extra source files (perhaps in directories not listed in ALL_DIRS) that should be considered.

The results of the automatic source file search are placed in the following make variables:

  • SRCS All source files found, sorted and without duplicates, including those which might not exist yet but will be derived from other existing sources. SRCS can be overriden if necessary, in which case the variables below will follow suit.
  • HS_SRCS all Haskell source files in the current directory, including those derived from other source files (eg. Happy sources also give rise to Haskell sources).
  • HS_OBJS Object files derived from HS_SRCS.
  • HS_IFACES Interface files (.hi files) derived from HS_SRCS.
  • C_SRCS All C source files found.
  • C_OBJS Object files derived from C_SRCS.
  • SCRIPT_SRCS All script source files found (.lprl files).
  • SCRIPT_OBJS "object" files derived from SCRIPT_SRCS (.prl files).
  • HSC_SRCS All hsc2hs source files (.hsc files).
  • HAPPY_SRCS All happy source files (.y or .hy files).
  • OBJS the concatenation of $(HS_OBJS), $(C_OBJS), and $(SCRIPT_OBJS).

Any or all of these definitions can easily be overriden by giving new definitions in your Makefile.

What, exactly, does consider a "source file" to be? It's based on the file's suffix (e.g. .hs, .lhs, .c, .hy, etc), but this is the kind of detail that changes, so rather than enumerate the source suffices here the best thing to do is to look in

defines make variables for option strings to pass to each program. For example, it defines HC_OPTS, the option strings to pass to the Haskell compiler. See Pattern rules and options.

is part of the old build system, i.e. it is not used by packages that are built with Cabal. defines standard pattern rules--see Pattern rules and options.

Any of the variables and pattern rules defined by the boilerplate file can easily be overridden in any particular Makefile, because the boilerplate include comes first. Definitions after this include directive simply override the default ones in

Platform settings

There are three platforms of interest when building GHC:

  • The build platform. The platform on which we are doing this build.
  • The host platform The platform on which these binaries will run.
  • The target platform The platform for which this compiler will generate code.

These platforms are set when running the configure script, using the --build, --host, and --target options. The mk/ file defines several symbols related to the platform settings (see mk/ for details).

We don't currently support build &amp; host being different, because the build process creates binaries that are both run during the build, and also installed.

If host and target are different, then we are building a cross-compiler. For GHC, this means a compiler which will generate intermediate .hc files to port to the target architecture for bootstrapping. The libraries and stage 2 compiler will be built as HC files for the target system (see Porting GHC for details).

More details on when to use BUILD, HOST or TARGET can be found in the comments in

Pattern rules and options

This section relates mainly to the old build system.

The file defines standard pattern rules that say how to build one kind of file from another, for example, how to build a .o file from a .c file. (GNU make's pattern rules are more powerful and easier to use than Unix make's suffix rules.)

Almost all the rules look something like this:

%.o : %.c
      $(RM) $@
      $(CC) $(CC_OPTS) -c $< -o $@

Here's how to understand the rule. It says that something.o (say Foo.o) can be built from something.c (Foo.c), by invoking the C compiler (path name held in $(CC)), passing to it the options $(CC_OPTS) and the rule's dependent file of the rule $< (Foo.c in this case), and putting the result in the rule's target $@ (Foo.o in this case).

Every program is held in a make variable defined in mk/ in mk/ for the complete list. One important one is the Haskell compiler, which is called $(HC).

Every program's options are are held in a make variables called <prog>_OPTS. the <prog>_OPTS variables are defined in mk/ Almost all of them are defined like this:


The four variables from which CC_OPTS is built have the following meaning:

  • SRC_CC_OPTS: options passed to all C compilations.
  • WAY_<way>_CC_OPTS: options passed to C compilations for way <way>. For example, WAY_mp_CC_OPTS gives options to pass to the C compiler when compiling way mp. The variable WAY_CC_OPTS holds options to pass to the C compiler when compiling the standard way. (Way management dicusses multi-way compilation.)
  • <module>_CC_OPTS: options to pass to the C compiler that are specific to module <module>. For example, SMap_CC_OPTS gives the specific options to pass to the C compiler when compiling SMap.c.
  • EXTRA_CC_OPTS: extra options to pass to all C compilations. This is intended for command line use, thus:

$ make libHS.a EXTRA_HC_OPTS="-v"

The main mk/ file

This section relates mainly to the old build system. contains canned rules for all the standard targets described in standard targets. It is complicated by the fact that you don't want all of these rules to be active in every Makefile. Rather than have a plethora of tiny files which you can include selectively, there is a single file,, which selectively includes rules based on whether you have defined certain variables in your Makefile. This section explains what rules you get, what variables control them, and what the rules do. Hopefully, you will also get enough of an idea of what is supposed to happen that you can read and understand any weird special cases yourself.

  • HS_PROG. If HS_PROG is defined, you get rules with the following targets:
    • HS_PROG itself. This rule links $(OBJS) with the Haskell runtime system to get an executable called $(HS_PROG).
    • install installs $(HS_PROG) in $(bindir).
  • C_PROG is similar to HS_PROG, except that the link step links $(C_OBJS) with the C runtime system.
  • LIBRARY is similar to HS_PROG, except that it links $(LIB_OBJS) to make the library archive $(LIBRARY), and install installs it in $(libdir).

Some rules are "double-colon" rules, thus

install :: $(HS_PROG) to install it...

GNU make treats double-colon rules as separate entities. If there are several double-colon rules for the same target it takes each in turn and fires it if its dependencies say to do so. This means that you can, for example, define both HS_PROG and LIBRARY, which will generate two rules for install. When you type make install both rules will be fired, and both the program and the library will be installed, just as you wanted.


In leaf Makefiles the variable SUBDIRS is undefined. In non-leaf Makefiles, SUBDIRS is set to the list of sub-directories that contain subordinate Makefiles. It is up to you to set SUBDIRS in the Makefile. There is no automation here--SUBDIRS is too important to automate.

When SUBDIRS is defined, includes a rather neat rule for the standard targets (standard targets that simply invokes make recursively in each of the sub-directories.

These recursive invocations are guaranteed to occur in the order in which the list of directories is specified in SUBDIRS. This guarantee can be important. For example, when you say make boot it can be important that the recursive invocation of make boot is done in one sub-directory (the include files, say) before another (the source files). Generally, put the most independent sub-directory first, and the most dependent last.

Way management in the old build system

We sometimes want to build essentially the same system in several different "ways". For example, we want to build GHC's runtime system libraries with and without thread support, so that there is an appropriately-built library archive to link with when the user compiles his program. It would be possible to have a completely separate build tree for each such "way", but it would be horribly bureaucratic, especially since often only parts of the build tree need to be constructed in multiple ways.

Instead, the contains some clever magic to allow you to build several versions of a system; and to control locally how many versions are built and how they differ. This section explains the magic.

The files for a particular way are distinguished by munging the suffix. The "normal way" is always built, and its files have the standard suffices .o, .hi, and so on. In addition, you can build one or more extra ways, each distinguished by a way tag. The object files and interface files for one of these extra ways are distinguished by their suffix. For example, way mp has files .mp_o and .mp_hi. Library archives have their way tag the other side of the dot, for boring reasons; thus, libHS_mp.a.

A make variable called way holds the current way tag. way is only ever set on the command line of make (usually in a recursive invocation of make by the system). It is never set inside a Makefile. So it is a global constant for any one invocation of make. Two other make variables, way_ and _way are immediately derived from $(way) and never altered. If way is not set, then neither are way_ and _way, and the invocation of make will build the "normal way". If way is set, then the other two variables are set in sympathy. For example, if $(way) is "mp", then way_ is set to "mp_" and _way is set to "_mp". These three variables are then used when constructing file names.

So how does make ever get recursively invoked with way set? There are two ways in which this happens:

  • For some (but not all) of the standard targets, when in a leaf sub-directory, make is recursively invoked for each way tag in $(WAYS). You set WAYS in the Makefile to the list of way tags you want these targets built for. The mechanism here is very much like the recursive invocation of make in sub-directories (Recursion). It is up to you to set WAYS in your Makefile; this is how you control what ways will get built.
  • For a useful collection of targets (such as libHS_mp.a, Foo.mp_o) there is a rule which recursively invokes make to make the specified target, setting the way variable. So if you say make Foo.mp_o you should see a recursive invocation make Foo.mp_o way=mp, and in this recursive invocation the pattern rule for compiling a Haskell file into a .o file will match. The key pattern rules (in look like this:
    %.$(way_)o : %.lhs
          $(HC) $(HC_OPTS) $< -o $@
    Neat, eh?
  • You can invoke make with a particular way setting yourself, in order to build files related to a particular way in the current directory. eg.
    $ make way=p
    will build files for the profiling way only in the current directory.

Way management in the new build system

When building with Cabal, way management is much simpler.

For libraries, we simply tell Cabal whether or not it should also build profiling versions of the library.

There are a couple of cases where we want something similar: For GHC itself, we want stage1, stage2 and stage3 builds; and for some utils, as discussed earlier, we want to build the util with both the bootstrapping compiler and the stage2 compiler.

When Cabal builds a package, by default it does its building in a dist directory. In order to build packages in different ways, we use a different directory for each way, using the --distpref flag to tell Cabal which one to use.

When the canned rule isn't right

Sometimes the canned rule just doesn't do the right thing. For example, in the nofib suite we want the link step to print out timing information. The thing to do here is not to define HS_PROG or C_PROG, and instead define a special purpose rule in your own Makefile. By using different variable names you will avoid the canned rules being included, and conflicting with yours.

Last modified 10 years ago Last modified on Feb 17, 2010 11:07:06 PM