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Writing Makefiles

The information that tells make how to recompile a system comes from reading a data base called the makefile.

What Makefiles Contain

Makefiles contain five kinds of things: explicit rules, implicit rules, variable definitions, directives, and comments. Rules, variables, and directives are described at length in later chapters.

What Name to Give Your Makefile

By default, when make looks for the makefile, it tries the following names, in order: `GNUmakefile', `makefile' and `Makefile'.

Normally you should call your makefile either `makefile' or `Makefile'. (We recommend `Makefile' because it appears prominently near the beginning of a directory listing, right near other important files such as `README'.) The first name checked, `GNUmakefile', is not recommended for most makefiles. You should use this name if you have a makefile that is specific to GNU make, and will not be understood by other versions of make. Other make programs look for `makefile' and `Makefile', but not `GNUmakefile'.

If make finds none of these names, it does not use any makefile. Then you must specify a goal with a command argument, and make will attempt to figure out how to remake it using only its built-in implicit rules. See section Using Implicit Rules.

If you want to use a nonstandard name for your makefile, you can specify the makefile name with the `-f' or `--file' option. The arguments `-f name' or `--file=name' tell make to read the file name as the makefile. If you use more than one `-f' or `--file' option, you can specify several makefiles. All the makefiles are effectively concatenated in the order specified. The default makefile names `GNUmakefile', `makefile' and `Makefile' are not checked automatically if you specify `-f' or `--file'.

Including Other Makefiles

The include directive tells make to suspend reading the current makefile and read one or more other makefiles before continuing. The directive is a line in the makefile that looks like this:

include filenames...

filenames can contain shell file name patterns.

Extra spaces are allowed and ignored at the beginning of the line, but a tab is not allowed. (If the line begins with a tab, it will be considered a command line.) Whitespace is required between include and the file names, and between file names; extra whitespace is ignored there and at the end of the directive. A comment starting with `#' is allowed at the end of the line. If the file names contain any variable or function references, they are expanded. See section How to Use Variables.

For example, if you have three `.mk' files, `a.mk', `b.mk', and `c.mk', and $(bar) expands to bish bash, then the following expression

include foo *.mk $(bar)

is equivalent to

include foo a.mk b.mk c.mk bish bash

When make processes an include directive, it suspends reading of the containing makefile and reads from each listed file in turn. When that is finished, make resumes reading the makefile in which the directive appears.

One occasion for using include directives is when several programs, handled by individual makefiles in various directories, need to use a common set of variable definitions (see section Setting Variables) or pattern rules (see section Defining and Redefining Pattern Rules).

Another such occasion is when you want to generate prerequisites from source files automatically; the prerequisites can be put in a file that is included by the main makefile. This practice is generally cleaner than that of somehow appending the prerequisites to the end of the main makefile as has been traditionally done with other versions of make. See section Generating Prerequisites Automatically.

If the specified name does not start with a slash, and the file is not found in the current directory, several other directories are searched. First, any directories you have specified with the `-I' or `--include-dir' option are searched (see section Summary of Options). Then the following directories (if they exist) are searched, in this order: `prefix/include' (normally `/usr/local/include' (1)) `/usr/gnu/include', `/usr/local/include', `/usr/include'.

If an included makefile cannot be found in any of these directories, a warning message is generated, but it is not an immediately fatal error; processing of the makefile containing the include continues. Once it has finished reading makefiles, make will try to remake any that are out of date or don't exist. See section How Makefiles Are Remade. Only after it has tried to find a way to remake a makefile and failed, will make diagnose the missing makefile as a fatal error.

If you want make to simply ignore a makefile which does not exist and cannot be remade, with no error message, use the -include directive instead of include, like this:

-include filenames...

This is acts like include in every way except that there is no error (not even a warning) if any of the filenames do not exist. For compatibility with some other make implementations, sinclude is another name for -include.

The Variable MAKEFILES

If the environment variable MAKEFILES is defined, make considers its value as a list of names (separated by whitespace) of additional makefiles to be read before the others. This works much like the include directive: various directories are searched for those files (see section Including Other Makefiles). In addition, the default goal is never taken from one of these makefiles and it is not an error if the files listed in MAKEFILES are not found.

The main use of MAKEFILES is in communication between recursive invocations of make (see section Recursive Use of make). It usually is not desirable to set the environment variable before a top-level invocation of make, because it is usually better not to mess with a makefile from outside. However, if you are running make without a specific makefile, a makefile in MAKEFILES can do useful things to help the built-in implicit rules work better, such as defining search paths (see section Searching Directories for Prerequisites).

Some users are tempted to set MAKEFILES in the environment automatically on login, and program makefiles to expect this to be done. This is a very bad idea, because such makefiles will fail to work if run by anyone else. It is much better to write explicit include directives in the makefiles. See section Including Other Makefiles.

How Makefiles Are Remade

Sometimes makefiles can be remade from other files, such as RCS or SCCS files. If a makefile can be remade from other files, you probably want make to get an up-to-date version of the makefile to read in.

To this end, after reading in all makefiles, make will consider each as a goal target and attempt to update it. If a makefile has a rule which says how to update it (found either in that very makefile or in another one) or if an implicit rule applies to it (see section Using Implicit Rules), it will be updated if necessary. After all makefiles have been checked, if any have actually been changed, make starts with a clean slate and reads all the makefiles over again. (It will also attempt to update each of them over again, but normally this will not change them again, since they are already up to date.)

If you know that one or more of your makefiles cannot be remade and you want to keep make from performing an implicit rule search on them, perhaps for efficiency reasons, you can use any normal method of preventing implicit rule lookup to do so. For example, you can write an explicit rule with the makefile as the target, and an empty command string (see section Using Empty Commands).

If the makefiles specify a double-colon rule to remake a file with commands but no prerequisites, that file will always be remade (see section Double-Colon Rules). In the case of makefiles, a makefile that has a double-colon rule with commands but no prerequisites will be remade every time make is run, and then again after make starts over and reads the makefiles in again. This would cause an infinite loop: make would constantly remake the makefile, and never do anything else. So, to avoid this, make will not attempt to remake makefiles which are specified as targets of a double-colon rule with commands but no prerequisites.

If you do not specify any makefiles to be read with `-f' or `--file' options, make will try the default makefile names; see section What Name to Give Your Makefile. Unlike makefiles explicitly requested with `-f' or `--file' options, make is not certain that these makefiles should exist. However, if a default makefile does not exist but can be created by running make rules, you probably want the rules to be run so that the makefile can be used.

Therefore, if none of the default makefiles exists, make will try to make each of them in the same order in which they are searched for (see section What Name to Give Your Makefile) until it succeeds in making one, or it runs out of names to try. Note that it is not an error if make cannot find or make any makefile; a makefile is not always necessary.

When you use the `-t' or `--touch' option (see section Instead of Executing the Commands), you would not want to use an out-of-date makefile to decide which targets to touch. So the `-t' option has no effect on updating makefiles; they are really updated even if `-t' is specified. Likewise, `-q' (or `--question') and `-n' (or `--just-print') do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, `make -f mfile -n foo' will update `mfile', read it in, and then print the commands to update `foo' and its prerequisites without running them. The commands printed for `foo' will be those specified in the updated contents of `mfile'.

However, on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options `-t' and so on do apply to them.

Thus, `make -f mfile -n mfile foo' would read the makefile `mfile', print the commands needed to update it without actually running them, and then print the commands needed to update `foo' without running them. The commands for `foo' will be those specified by the existing contents of `mfile'.

Overriding Part of Another Makefile

Sometimes it is useful to have a makefile that is mostly just like another makefile. You can often use the `include' directive to include one in the other, and add more targets or variable definitions. However, if the two makefiles give different commands for the same target, make will not let you just do this. But there is another way.

In the containing makefile (the one that wants to include the other), you can use a match-anything pattern rule to say that to remake any target that cannot be made from the information in the containing makefile, make should look in another makefile. See section Defining and Redefining Pattern Rules, for more information on pattern rules.

For example, if you have a makefile called `Makefile' that says how to make the target `foo' (and other targets), you can write a makefile called `GNUmakefile' that contains:

foo:
        frobnicate > foo

%: force
        @$(MAKE) -f Makefile $@
force: ;

If you say `make foo', make will find `GNUmakefile', read it, and see that to make `foo', it needs to run the command `frobnicate > foo'. If you say `make bar', make will find no way to make `bar' in `GNUmakefile', so it will use the commands from the pattern rule: `make -f Makefile bar'. If `Makefile' provides a rule for updating `bar', make will apply the rule. And likewise for any other target that `GNUmakefile' does not say how to make.

The way this works is that the pattern rule has a pattern of just `%', so it matches any target whatever. The rule specifies a prerequisite `force', to guarantee that the commands will be run even if the target file already exists. We give `force' target empty commands to prevent make from searching for an implicit rule to build it--otherwise it would apply the same match-anything rule to `force' itself and create a prerequisite loop!

How make Reads a Makefile

GNU make does its work in two distinct phases. During the first phase it reads all the makefiles, included makefiles, etc. and internalizes all the variables and their values, implicit and explicit rules, and constructs a dependency graph of all the targets and their prerequisites. During the second phase, make uses these internal structures to determine what targets will need to be rebuilt and to invoke the rules necessary to do so.

It's important to understand this two-phase approach because it has a direct impact on how variable and function expansion happens; this is often a source of some confusion when writing makefiles. Here we will present a summary of the phases in which expansion happens for different constructs within the makefile. We say that expansion is immediate if it happens during the first phase: in this case make will expand any variables or functions in that section of a construct as the makefile is parsed. We say that expansion is deferred if expansion is not performed immediately. Expansion of deferred construct is not performed until either the construct appears later in an immediate context, or until the second phase.

You may not be familiar with some of these constructs yet. You can reference this section as you become familiar with them, in later chapters.

Variable Assignment

Variable definitions are parsed as follows:

immediate = deferred
immediate ?= deferred
immediate := immediate
immediate += deferred or immediate

define immediate
  deferred
endef

For the append operator, `+=', the right-hand side is considered immediate if the variable was previously set as a simple variable (`:='), and deferred otherwise.

Conditional Syntax

All instances of conditional syntax are parsed immediately, in their entirety; this includes the ifdef, ifeq, ifndef, and ifneq forms.

Rule Definition

A rule is always expanded the same way, regardless of the form:

immediate : immediate ; deferred
	deferred

That is, the target and prerequisite sections are expanded immediately, and the commands used to construct the target are always deferred. This general rule is true for explicit rules, pattern rules, suffix rules, static pattern rules, and simple prerequisite definitions.


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