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60 GNU make

could be used to compile a C program prog.c. Since spaces before the variable value are

ignored in variable assignments, the value of foo is precisely ‘c’. (Don’t actually write your

makeﬁles this way!)

A dollar sign followed by a character other than a dollar sign, open-parenthesis or open-

brace treats that single character as the variable name. Thus, you could reference the

variable x with ‘$x’. However, this practice is strongly discouraged, except in the case of

the automatic variables (see Section 10.5.3 [Automatic Variables], page 120).

6.2 The Two Flavors of Variables

There are two ways that a variable in GNU make can have a value; we call them the two

ﬂavors of variables. The two ﬂavors are distinguished in how they are deﬁned and in what

they do when expanded.

The ﬁrst ﬂavor of variable is a recursively expanded variable. Variables of this sort are

deﬁned by lines using ‘=’ (see Section 6.5 [Setting Variables], page 65) or by the define

directive (see Section 6.8 [Deﬁning Multi-Line Variables], page 69). The value you specify is

installed verbatim; if it contains references to other variables, these references are expanded

whenever this variable is substituted (in the course of expanding some other string). When

this happens, it is called recursive expansion.

For example,

foo = $(bar)

bar = $(ugh)

ugh = Huh?

all:;echo $(foo)

will echo ‘Huh?’: ‘$(foo)’ expands to ‘$(bar)’ which expands to ‘$(ugh)’ which ﬁnally

expands to ‘Huh?’.

This ﬂavor of variable is the only sort supported by most other versions of make. It has

its advantages and its disadvantages. An advantage (most would say) is that:

CFLAGS = $(include_dirs) -O

include_dirs = -Ifoo -Ibar

will do what was intended: when ‘CFLAGS’ is expanded in a recipe, it will expand to ‘-Ifoo

-Ibar -O’. A major disadvantage is that you cannot append something on the end of a

variable, as in

CFLAGS = $(CFLAGS) -O

because it will cause an inﬁnite loop in the variable expansion. (Actually make detects the

inﬁnite loop and reports an error.)

Another disadvantage is that any functions (see Chapter 8 [Functions for Transforming

Text], page 83) referenced in the deﬁnition will be executed every time the variable is

expanded. This makes make run slower; worse, it causes the wildcard and shell functions

to give unpredictable results because you cannot easily control when they are called, or

even how many times.

To avoid all the problems and inconveniences of recursively expanded variables, there is

another ﬂavor: simply expanded variables.

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Roland Ver. 4.5 Technical Information Seite 70