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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 "dependencies" of the
target, and "commands" 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.  (Note: 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').  Note: Arguments to
Specify the Goals.

* Rule Example
An example explained.
* Rule Syntax
General syntax explained.
* 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 commands or dependencies 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 dependencies according to the target name.
* Double-Colon
How to use a special kind of rule to allow several independent rules for one target.
* Automatic Dependencies
How to automatically generate rules giving dependencies from the source files themselves.

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