8. The Makefile architecture

8.1. Debugging

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.

8.2. A small project

To get started, let us look at the Makefile for an imaginary small fptools project, small. Each project in fptools has its own directory in FPTOOLS_TOP, so the small project will have its own directory FPOOLS_TOP/small/. Inside the small/ directory there will be a Makefile, looking something like this:

#     Makefile for fptools project "small"

TOP = ..
include $(TOP)/mk/boilerplate.mk

SRCS = $(wildcard *.lhs) $(wildcard *.c)
HS_PROG = small

include $(TOP)/target.mk

this Makefile has three sections:

1. The first section includes [1] a file of “boilerplate” code from the level above (which in this case will be FPTOOLS_TOP/mk/boilerplate.mk). As its name suggests, boilerplate.mk consists of a large quantity of standard Makefile code. We discuss this boilerplate in more detail in Section 8.5.

   Before the include statement, you must define the make variable TOP to be the directory containing the mk directory in which the boilerplate.mk file is. It is not OK to simply say

   include ../mk/boilerplate.mk # NO NO NO

   Why? Because the boilerplate.mk 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 gmake is being run, not the directory in which the included sits.) In general, every file foo.mk assumes that $(TOP)/mk/foo.mk refers to itself. It is up to the Makefile doing the include to ensure this is the case.

   Files intended for inclusion in other Makefiles are written to have the following property: after foo.mk is included, it leaves TOP containing the same value as it had just before the include statement. In our example, this invariant guarantees that the include for target.mk will look in the same directory as that for boilerplate.mk.

2. The second section defines the following standard make variables: SRCS (the source files from which is to be built), and HS_PROG (the executable binary to be built). We will discuss in more detail what the “standard variables” are, and how they affect what happens, in Section 8.7.

   The definition for SRCS uses the useful GNU make construct $(wildcard $pat$), which expands to a list of all the files matching the pattern pat in the current directory. In this example, SRCS is set to the list of all the .lhs and .c files in the directory. (Let's suppose there is one of each, Foo.lhs and Baz.c.)

3. The last section includes a second file of standard code, called target.mk. It contains the rules that tell gmake how to make the standard targets (Section 7.8). Why, you ask, can't this standard code be part of boilerplate.mk? Good question. We discuss the reason later, in Section 8.4.

   You do not have to include the target.mk file. Instead, you can write rules of your own for all the standard targets. Usually, though, you will find quite a big payoff from using the canned rules in target.mk; the price tag is that you have to understand what canned rules get enabled, and what they do (Section 8.7).

In our example Makefile, most of the work is done by the two included files. When you say gmake all, the following things happen:

• gmake figures out that the object files are Foo.o and Baz.o.

• It uses a boilerplate pattern rule to compile Foo.lhs to Foo.o using a Haskell compiler. (Which one? That is set in the build configuration.)

• It uses another standard pattern rule to compile Baz.c to Baz.o, using a C compiler. (Ditto.)

• It links the resulting .o files together to make small, using the Haskell compiler to do the link step. (Why not use ld? Because the Haskell compiler knows what standard libraries to link in. How did gmake know to use the Haskell compiler to do the link, rather than the C compiler? Because we set the variable HS_PROG rather than C_PROG.)

All Makefiles should follow the above three-section format.

8.3. A larger project

Larger projects are usually structured into a number of sub-directories, each of which has its own Makefile. (In very large projects, this sub-structure might be iterated recursively, though that is rare.) To give you the idea, here's part of the directory structure for the (rather large) GHC project:

$(FPTOOLS_TOP)/ghc/
  Makefile
  mk/
    boilerplate.mk
    rules.mk
   docs/
    Makefile
    ...source files for documentation...
   driver/
    Makefile
    ...source files for driver...
   compiler/
    Makefile
    parser/...source files for parser...
    renamer/...source files for renamer...
    ...etc...

The sub-directories docs, driver, compiler, and so on, each contains a sub-component of GHC, and each has its own Makefile. There must also be a Makefile in $(FPTOOLS_TOP)/ghc. It does most of its work by recursively invoking gmake on the Makefiles in the sub-directories. We say that ghc/Makefile is a non-leaf Makefile, because it does little except organise its children, while the Makefiles in the sub-directories are all leaf Makefiles. (In principle the sub-directories might themselves contain a non-leaf Makefile and several sub-sub-directories, but that does not happen in GHC.)

The Makefile in ghc/compiler is considered a leaf Makefile even though the ghc/compiler has sub-directories, because these sub-directories do not themselves have Makefiles in them. They are just used to structure the collection of modules that make up GHC, but all are managed by the single Makefile in ghc/compiler.

You will notice that ghc/ also contains a directory ghc/mk/. It contains GHC-specific Makefile boilerplate code. More precisely:

• ghc/mk/boilerplate.mk is included at the top of ghc/Makefile, and of all the leaf Makefiles in the sub-directories. It in turn includes the main boilerplate file mk/boilerplate.mk.

• ghc/mk/target.mk is included at the bottom of ghc/Makefile, and of all the leaf Makefiles in the sub-directories. It in turn includes the file mk/target.mk.

So these two files are the place to look for GHC-wide customisation of the standard boilerplate.

8.4. Boilerplate architecture

Every Makefile includes a boilerplate.mk file at the top, and target.mk file at the bottom. In this section we discuss what is in these files, and why there have to be two of them. In general:

• boilerplate.mk 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 gmake how to construct one file from another.

  boilerplate.mk needs to be included at the top of each Makefile, so that the user can replace the boilerplate definitions or pattern rules by simply giving a new definition or pattern rule in the Makefile. gmake 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:

  SRC_HC_OPTS += -O

  thereby adding “-O” to the end of SRC_HC_OPTS.

• target.mk contains make rules for the standard targets described in Section 7.8. 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.

  target.mk must be included at the end (rather than being part of boilerplate.mk) for several tiresome reasons:

  • gmake commits target and dependency lists earlier than it should. For example, target.mk has a rule that looks like this:

    $(HS_PROG) : $(OBJS) $(HC) $(LD_OPTS) $< -o $@

    If this rule was in boilerplate.mk then $(HS_PROG) and $(OBJS) would not have their final values at the moment gmake encountered the rule. Alas, gmake 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 boilerplate.mk 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.

8.5. The main mk/boilerplate.mk file

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

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 boilerplate.mk.

8.6. Pattern rules and options

The file suffix.mk 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/config.mk—look in mk/config.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/opts.mk. Almost all of them are defined like this:

CC_OPTS = $(SRC_CC_OPTS) $(WAY$(_way)_CC_OPTS) $($*_CC_OPTS) $(EXTRA_CC_OPTS)

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

8.7. The main mk/target.mk file

target.mk contains canned rules for all the standard targets described in Section 7.8. 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, target.mk, 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.

All of these rules are “double-colon” rules, thus

install :: $(HS_PROG)
      ...how 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 gmake install both rules will be fired, and both the program and the library will be installed, just as you wanted.

8.8. Recursion

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, target.mk includes a rather neat rule for the standard targets (Section 7.8 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 gmake 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.

8.9. Way management

We sometimes want to build essentially the same system in several different “ways”. For example, we want to build GHC's Prelude libraries with and without profiling, 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 target.mk 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 gmake (usually in a recursive invocation of gmake by the system). It is never set inside a Makefile. So it is a global constant for any one invocation of gmake. 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 (Section 8.8). 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 gmake Foo.mp_o you should see a recursive invocation gmake 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 suffix.mk) 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.

8.10. 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.