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6.14 Other Special Variables

GNU make supports some variables that have special properties.

MAKEFILE_LIST

Contains the name of each makefile that is parsed by make, in the order in which it was parsed. The name is appended just before make begins to parse the makefile. Thus, if the first thing a makefile does is examine the last word in this variable, it will be the name of the current makefile. Once the current makefile has used include, however, the last word will be the just-included makefile.

If a makefile named Makefile has this content:

name1 := $(lastword $(MAKEFILE_LIST))

include inc.mk

name2 := $(lastword $(MAKEFILE_LIST))

all:
        @echo name1 = $(name1)
        @echo name2 = $(name2)

then you would expect to see this output:

name1 = Makefile
name2 = inc.mk
.DEFAULT_GOAL

Sets the default goal to be used if no targets were specified on the command line (see Arguments to Specify the Goals). The .DEFAULT_GOAL variable allows you to discover the current default goal, restart the default goal selection algorithm by clearing its value, or to explicitly set the default goal. The following example illustrates these cases:

# Query the default goal.
ifeq ($(.DEFAULT_GOAL),)
  $(warning no default goal is set)
endif

.PHONY: foo
foo: ; @echo $@

$(warning default goal is $(.DEFAULT_GOAL))

# Reset the default goal.
.DEFAULT_GOAL :=

.PHONY: bar
bar: ; @echo $@

$(warning default goal is $(.DEFAULT_GOAL))

# Set our own.
.DEFAULT_GOAL := foo

This makefile prints:

no default goal is set
default goal is foo
default goal is bar
foo

Note that assigning more than one target name to .DEFAULT_GOAL is invalid and will result in an error.

MAKE_RESTARTS

This variable is set only if this instance of make has restarted (see How Makefiles Are Remade): it will contain the number of times this instance has restarted. Note this is not the same as recursion (counted by the MAKELEVEL variable). You should not set, modify, or export this variable.

MAKE_TERMOUT
MAKE_TERMERR

When make starts it will check whether stdout and stderr will show their output on a terminal. If so, it will set MAKE_TERMOUT and MAKE_TERMERR, respectively, to the name of the terminal device (or true if this cannot be determined). If set these variables will be marked for export. These variables will not be changed by make and they will not be modified if already set.

These values can be used (particularly in combination with output synchronization (see Output During Parallel Execution) to determine whether make itself is writing to a terminal; they can be tested to decide whether to force recipe commands to generate colorized output for example.

If you invoke a sub-make and redirect its stdout or stderr it is your responsibility to reset or unexport these variables as well, if your makefiles rely on them.

.RECIPEPREFIX

The first character of the value of this variable is used as the character make assumes is introducing a recipe line. If the variable is empty (as it is by default) that character is the standard tab character. For example, this is a valid makefile:

.RECIPEPREFIX = >
all:
> @echo Hello, world

The value of .RECIPEPREFIX can be changed multiple times; once set it stays in effect for all rules parsed until it is modified.

.VARIABLES

Expands to a list of the names of all global variables defined so far. This includes variables which have empty values, as well as built-in variables (see Variables Used by Implicit Rules), but does not include any variables which are only defined in a target-specific context. Note that any value you assign to this variable will be ignored; it will always return its special value.

.FEATURES

Expands to a list of special features supported by this version of make. Possible values include, but are not limited to:

archives

Supports ar (archive) files using special file name syntax. See Using make to Update Archive Files.

check-symlink

Supports the -L (--check-symlink-times) flag. See Summary of Options.

else-if

Supports “else if” non-nested conditionals. See Syntax of Conditionals.

jobserver

Supports “job server” enhanced parallel builds. See Parallel Execution.

oneshell

Supports the .ONESHELL special target. See Using One Shell.

order-only

Supports order-only prerequisites. See Types of Prerequisites.

second-expansion

Supports secondary expansion of prerequisite lists.

shortest-stem

Uses the “shortest stem” method of choosing which pattern, of multiple applicable options, will be used. See How Patterns Match.

target-specific

Supports target-specific and pattern-specific variable assignments. See Target-specific Variable Values.

undefine

Supports the undefine directive. See Undefine Directive.

guile

Has GNU Guile available as an embedded extension language. See GNU Guile Integration.

load

Supports dynamically loadable objects for creating custom extensions. See Loading Dynamic Objects.

.INCLUDE_DIRS

Expands to a list of directories that make searches for included makefiles (see Including Other Makefiles).

.EXTRA_PREREQS

Each word in this variable is a new prerequisite which is added to targets for which it is set. These prerequisites differ from normal prerequisites in that they do not appear in any of the automatic variables (see Automatic Variables). This allows prerequisites to be defined which do not impact the recipe.

Consider a rule to link a program:

myprog: myprog.o file1.o file2.o
       $(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LDLIBS)

Now suppose you want to enhance this makefile to ensure that updates to the compiler cause the program to be re-linked. You can add the compiler as a prerequisite, but you must ensure that it’s not passed as an argument to link command. You’ll need something like this:

myprog: myprog.o file1.o file2.o $(CC)
       $(CC) $(CFLAGS) $(LDFLAGS) -o $@ $(filter-out $(CC),$^) $(LDLIBS)

Then consider having multiple extra prerequisites: they would all have to be filtered out. Using .EXTRA_PREREQS and target-specific variables provides a simpler solution:

myprog: myprog.o file1.o file2.o
       $(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LDLIBS)
myprog: .EXTRA_PREREQS = $(CC)

This feature can also be useful if you want to add prerequisites to a makefile you cannot easily modify: you can create a new file such as extra.mk:

myprog: .EXTRA_PREREQS = $(CC)

then invoke make -f extra.mk -f Makefile.

Setting .EXTRA_PREREQS globally will cause those prerequisites to be added to all targets (which did not themselves override it with a target-specific value). Note make is smart enough not to add a prerequisite listed in .EXTRA_PREREQS as a prerequisite to itself.


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