make: Makefile Contents
3.1 What Makefiles Contain
==========================
Makefiles contain five kinds of things: "explicit rules", "implicit
rules", "variable definitions", "directives", and "comments". Rules,
variables, and directives are described at length in later chapters.
* An "explicit rule" says when and how to remake one or more files,
called the rule's "targets". It lists the other files that the
targets depend on, called the "prerequisites" of the target, and
may also give a recipe to use to create or update the targets.
Writing Rules Rules.
* An "implicit rule" says when and how to remake a class of files
based on their names. It describes how a target may depend on a
file with a name similar to the target and gives a recipe to create
or update such a target. Using Implicit Rules Implicit
Rules.
* A "variable definition" is a line that specifies a text string
value for a variable that can be substituted into the text later.
The simple makefile example shows a variable definition for
'objects' as a list of all object files (Variables Make
Makefiles Simpler Variables Simplify.).
* A "directive" is an instruction for 'make' to do something special
while reading the makefile. These include:
* Reading another makefile (Including Other Makefiles
Include.).
* Deciding (based on the values of variables) whether to use or
ignore a part of the makefile (Conditional Parts of
Makefiles Conditionals.).
* Defining a variable from a verbatim string containing multiple
lines (Defining Multi-Line Variables Multi-Line.).
* '#' in a line of a makefile starts a "comment". It and the rest of
the line are ignored, except that a trailing backslash not escaped
by another backslash will continue the comment across multiple
lines. A line containing just a comment (with perhaps spaces
before it) is effectively blank, and is ignored. If you want a
literal '#', escape it with a backslash (e.g., '\#'). Comments may
appear on any line in the makefile, although they are treated
specially in certain situations.
You cannot use comments within variable references or function
calls: any instance of '#' will be treated literally (rather than
as the start of a comment) inside a variable reference or function
call.
Comments within a recipe are passed to the shell, just as with any
other recipe text. The shell decides how to interpret it: whether
or not this is a comment is up to the shell.
Within a 'define' directive, comments are not ignored during the
definition of the variable, but rather kept intact in the value of
the variable. When the variable is expanded they will either be
treated as 'make' comments or as recipe text, depending on the
context in which the variable is evaluated.
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