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3.7 How 'make' Reads a Makefile
===============================

GNU 'make' does its work in two distinct phases.  During the first phase
it reads all the makefiles, included makefiles, etc.  and internalizes
all the variables and their values and implicit and explicit rules, and
builds a dependency graph of all the targets and their prerequisites.
During the second phase, 'make' uses this internalized data to determine
which targets need to be updated and run the recipes necessary to update
them.

   It's important to understand this two-phase approach because it has a
direct impact on how variable and function expansion happens; this is
often a source of some confusion when writing makefiles.  Below is a
summary of the different constructs that can be found in a makefile, and
the phase in which expansion happens for each part of the construct.

   We say that expansion is "immediate" if it happens during the first
phase: 'make' will expand that part of the construct as the makefile is
parsed.  We say that expansion is "deferred" if it is not immediate.
Expansion of a deferred construct part is delayed until the expansion is
used: either when it is referenced in an immediate context, or when it
is needed during the second phase.

   You may not be familiar with some of these constructs yet.  You can
reference this section as you become familiar with them, in later
chapters.

Variable Assignment
-------------------

Variable definitions are parsed as follows:

     IMMEDIATE = DEFERRED
     IMMEDIATE ?= DEFERRED
     IMMEDIATE := IMMEDIATE
     IMMEDIATE ::= IMMEDIATE
     IMMEDIATE :::= IMMEDIATE-WITH-ESCAPE
     IMMEDIATE += DEFERRED or IMMEDIATE
     IMMEDIATE != IMMEDIATE

     define IMMEDIATE
       DEFERRED
     endef

     define IMMEDIATE =
       DEFERRED
     endef

     define IMMEDIATE ?=
       DEFERRED
     endef

     define IMMEDIATE :=
       IMMEDIATE
     endef

     define IMMEDIATE ::=
       IMMEDIATE
     endef

     define IMMEDIATE :::=
       IMMEDIATE-WITH-ESCAPE
     endef

     define IMMEDIATE +=
       DEFERRED or IMMEDIATE
     endef

     define IMMEDIATE !=
       IMMEDIATE
     endef

   For the append operator '+=', the right-hand side is considered
immediate if the variable was previously set as a simple variable (':='
or '::='), and deferred otherwise.

   For the IMMEDIATE-WITH-ESCAPE operator ':::=', the value on the
right-hand side is immediately expanded but then escaped (that is, all
instances of '$' in the result of the expansion are replaced with '$$').

   For the shell assignment operator '!=', the right-hand side is
evaluated immediately and handed to the shell.  The result is stored in
the variable named on the left, and that variable is considered a
recursively expanded variable (and will thus be re-evaluated on each
reference).

Conditional Directives
----------------------

Conditional directives are parsed immediately.  This means, for example,
that automatic variables cannot be used in conditional directives, as
automatic variables are not set until the recipe for that rule is
invoked.  If you need to use automatic variables in a conditional
directive you _must_ move the condition into the recipe and use shell
conditional syntax instead.

Rule Definition
---------------

A rule is always expanded the same way, regardless of the form:

     IMMEDIATE : IMMEDIATE ; DEFERRED
             DEFERRED

   That is, the target and prerequisite sections are expanded
immediately, and the recipe used to build the target is always deferred.
This is true for explicit rules, pattern rules, suffix rules, static
pattern rules, and simple prerequisite definitions.