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make: Reading Makefiles

 
 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, implicit and explicit rules, and
 constructs a dependency graph of all the targets and their
 prerequisites.  During the second phase, 'make' uses these internal
 structures to determine what targets will need to be rebuilt and to
 invoke the rules necessary to do so.
 
    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.  Here we will
 present a summary of the phases in which expansion happens for different
 constructs within the makefile.  We say that expansion is "immediate" if
 it happens during the first phase: in this case 'make' will expand any
 variables or functions in that section of a construct as the makefile is
 parsed.  We say that expansion is "deferred" if expansion is not
 performed immediately.  Expansion of a deferred construct is not
 performed until either the construct appears later in an immediate
 context, or until 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 += 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 +=
        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 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 becomes a simple
 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 construct the target is always
 deferred.  This general rule is true for explicit rules, pattern rules,
 suffix rules, static pattern rules, and simple prerequisite definitions.
 

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