(make.info.gz) Reading Makefiles

Info Catalog

(make.info.gz) Overriding Makefiles

(make.info.gz) Makefiles

(make.info.gz) Secondary Expansion

 
 3.9 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 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 += DEFERRED or IMMEDIATE
 
      define IMMEDIATE
        DEFERRED
      endef
 
    For the append operator, `+=', the right-hand side is considered
 immediate if the variable was previously set as a simple variable
 (`:='), and deferred otherwise.
 
 Conditional Statements
 ----------------------
 
 All instances of conditional syntax are parsed immediately, in their
 entirety; this includes the `ifdef', `ifeq', `ifndef', and `ifneq'
 forms.  Of course this means that automatic variables cannot be used in
 conditional statements, as automatic variables are not set until the
 command script for that rule is invoked.  If you need to use automatic
 variables in a conditional you _must_ use shell conditional syntax, in
 your command script proper, for these tests, not `make' conditionals.
 
 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 commands used to construct the target are always
 deferred.  This general rule is true for explicit rules, pattern rules,
 suffix rules, static pattern rules, and simple prerequisite definitions.
 

Info Catalog

(make.info.gz) Overriding Makefiles

(make.info.gz) Makefiles

(make.info.gz) Secondary Expansion