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Chapter 2: An Introduction to Makeﬁles 5

A recipe may follow each line that contains a target and prerequisites. These recipes say

how to update the target ﬁle. A tab character (or whatever character is speciﬁed by the

.RECIPEPREFIX variable; see Section 6.14 [Special Variables], page 73) must come at the

beginning of every line in the recipe to distinguish recipes from other lines in the makeﬁle.

(Bear in mind that make does not know anything about how the recipes work. It is up to

you to supply recipes that will update the target ﬁle properly. All make does is execute the

recipe you have speciﬁed when the target ﬁle needs to be updated.)

The target ‘clean’ is not a ﬁle, but merely the name of an action. Since you normally

do not want to carry out the actions in this rule, ‘clean’ is not a prerequisite of any other

rule. Consequently, make never does anything with it unless you tell it speciﬁcally. Note

that this rule not only is not a prerequisite, it also does not have any prerequisites, so the

only purpose of the rule is to run the speciﬁed recipe. Targets that do not refer to ﬁles

but are just actions are called phony targets. See Section 4.5 [Phony Targets], page 29, for

information about this kind of target. See Section 5.5 [Errors in Recipes], page 49, to see

how to cause make to ignore errors from rm or any other command.

2.3 How make Processes a Makeﬁle

By default, make starts with the ﬁrst target (not targets whose names start with ‘.’). This

is called the default goal. (Goals are the targets that make strives ultimately to update. You

can override this behavior using the command line (see Section 9.2 [Arguments to Specify

the Goals], page 99) or with the .DEFAULT_GOAL special variable (see Section 6.14 [Other

Special Variables], page 73).

In the simple example of the previous section, the default goal is to update the executable

program edit; therefore, we put that rule ﬁrst.

Thus, when you give the command:

make

make reads the makeﬁle in the current directory and begins by processing the ﬁrst rule. In

the example, this rule is for relinking edit; but before make can fully process this rule, it

must process the rules for the ﬁles that edit depends on, which in this case are the object

ﬁles. Each of these ﬁles is processed according to its own rule. These rules say to update

each ‘.o’ ﬁle by compiling its source ﬁle. The recompilation must be done if the source ﬁle,

or any of the header ﬁles named as prerequisites, is more recent than the object ﬁle, or if

the object ﬁle does not exist.

The other rules are processed because their targets appear as prerequisites of the goal.

If some other rule is not depended on by the goal (or anything it depends on, etc.), that

rule is not processed, unless you tell make to do so (with a command such as make clean).

Before recompiling an object ﬁle, make considers updating its prerequisites, the source

ﬁle and header ﬁles. This makeﬁle does not specify anything to be done for them—the ‘.c’

and ‘.h’ ﬁles are not the targets of any rules—so make does nothing for these ﬁles. But

make would update automatically generated C programs, such as those made by Bison or

Yacc, by their own rules at this time.

After recompiling whichever object ﬁles need it, make decides whether to relink edit.

This must be done if the ﬁle edit does not exist, or if any of the object ﬁles are newer than

it. If an object ﬁle was just recompiled, it is now newer than edit, so edit is relinked.

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