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Chapter 7: Conditional Parts of Makeﬁles 77

7 Conditional Parts of Makeﬁles

A conditional directive causes part of a makeﬁle to be obeyed or ignored depending on the

values of variables. Conditionals can compare the value of one variable to another, or the

value of a variable to a constant string. Conditionals control what make actually “sees” in

the makeﬁle, so they cannot be used to control recipes at the time of execution.

7.1 Example of a Conditional

The following example of a conditional tells make to use one set of libraries if the CC variable

is ‘gcc’, and a diﬀerent set of libraries otherwise. It works by controlling which of two recipe

lines will be used for the rule. The result is that ‘CC=gcc’ as an argument to make changes

not only which compiler is used but also which libraries are linked.

libs_for_gcc = -lgnu

normal_libs =

foo: $(objects)

ifeq ($(CC),gcc)

$(CC) -o foo $(objects) $(libs_for_gcc)

else

$(CC) -o foo $(objects) $(normal_libs)

endif

This conditional uses three directives: one ifeq, one else and one endif.

The ifeq directive begins the conditional, and speciﬁes the condition. It contains two

arguments, separated by a comma and surrounded by parentheses. Variable substitution

is performed on both arguments and then they are compared. The lines of the makeﬁle

following the ifeq are obeyed if the two arguments match; otherwise they are ignored.

The else directive causes the following lines to be obeyed if the previous conditional

failed. In the example above, this means that the second alternative linking command

is used whenever the ﬁrst alternative is not used. It is optional to have an else in a

conditional.

The endif directive ends the conditional. Every conditional must end with an endif.

Unconditional makeﬁle text follows.

As this example illustrates, conditionals work at the textual level: the lines of the con-

ditional are treated as part of the makeﬁle, or ignored, according to the condition. This is

why the larger syntactic units of the makeﬁle, such as rules, may cross the beginning or the

end of the conditional.

When the variable CC has the value ‘gcc’, the above example has this eﬀect:

foo: $(objects)

$(CC) -o foo $(objects) $(libs_for_gcc)

When the variable CC has any other value, the eﬀect is this:

foo: $(objects)

$(CC) -o foo $(objects) $(normal_libs)

Equivalent results can be obtained in another way by conditionalizing a variable assign-

ment and then using the variable unconditionally:

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