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80 GNU make

return true for all deﬁnitions except those like foo =. To test for an empty

value, use ifeq ($(foo),). For example,

bar =

foo = $(bar)

ifdef foo

frobozz = yes

else

frobozz = no

endif

sets ‘frobozz’ to ‘yes’, while:

foo =

ifdef foo

frobozz = yes

else

frobozz = no

endif

sets ‘frobozz’ to ‘no’.

ifndef variable-name

If the variable variable-name has an empty value, the text-if-true is eﬀective;

otherwise, the text-if-false, if any, is eﬀective. The rules for expansion and

testing of variable-name are identical to the ifdef directive.

Extra spaces are allowed and ignored at the beginning of the conditional directive line,

but a tab is not allowed. (If the line begins with a tab, it will be considered part of a recipe

for a rule.) Aside from this, extra spaces or tabs may be inserted with no eﬀect anywhere

except within the directive name or within an argument. A comment starting with ‘#’ may

appear at the end of the line.

The other two directives that play a part in a conditional are else and endif. Each of

these directives is written as one word, with no arguments. Extra spaces are allowed and

ignored at the beginning of the line, and spaces or tabs at the end. A comment starting

with ‘#’ may appear at the end of the line.

Conditionals aﬀect which lines of the makeﬁle make uses. If the condition is true, make

reads the lines of the text-if-true as part of the makeﬁle; if the condition is false, make

ignores those lines completely. It follows that syntactic units of the makeﬁle, such as rules,

may safely be split across the beginning or the end of the conditional.

make evaluates conditionals when it reads a makeﬁle. Consequently, you cannot use

automatic variables in the tests of conditionals because they are not deﬁned until recipes

are run (see Section 10.5.3 [Automatic Variables], page 120).

To prevent intolerable confusion, it is not permitted to start a conditional in one makeﬁle

and end it in another. However, you may write an include directive within a conditional,

provided you do not attempt to terminate the conditional inside the included ﬁle.

7.3 Conditionals that Test Flags

You can write a conditional that tests make command ﬂags such as ‘-t’ by using the variable

MAKEFLAGS together with the findstring function (see Section 8.2 [Functions for String

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