Writing Rules

A rule appears in the makefile and says when and how to remake certain files, called the rule’s targets (most often only one per rule). It lists the other files that are the prerequisites of the target, and the recipe to use to create or update the target.

The order of rules is not significant, except for determining the default goal: the target for make to consider, if you do not otherwise specify one. The default goal is the target of the first rule in the first makefile. If the first rule has multiple targets, only the first target is taken as the default. There are two exceptions: a target starting with a period is not a default unless it contains one or more slashes, ‘/’, as well; and, a target that defines a pattern rule has no effect on the default goal. (See Defining and Redefining Pattern Rules.)

Therefore, we usually write the makefile so that the first rule is the one for compiling the entire program or all the programs described by the makefile (often with a target called ‘all’). See Arguments to Specify the Goals.

• Rule Example		An example explained.
• Rule Syntax		General syntax explained.
• Prerequisite Types		There are two types of prerequisites.
• Wildcards		Using wildcard characters such as ‘*’.
• Directory Search		Searching other directories for source files.
• Phony Targets		Using a target that is not a real file’s name.
• Force Targets		You can use a target without a recipe or prerequisites to mark other targets as phony.
• Empty Targets		When only the date matters and the files are empty.
• Special Targets		Targets with special built-in meanings.
• Multiple Targets		When to make use of several targets in a rule.
• Multiple Rules		How to use several rules with the same target.
• Static Pattern		Static pattern rules apply to multiple targets and can vary the prerequisites according to the target name.
• Double-Colon		How to use a special kind of rule to allow several independent rules for one target.
• Automatic Prerequisites		How to automatically generate rules giving prerequisites from source files themselves.