GNU Info

(make.info)Reading Makefiles

---

Prev: Overriding Makefiles Up: Makefiles

---

Enter node , (file) or (file)node

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 Syntax
------------------

   All instances of conditional syntax are parsed immediately, in their
entirety; this includes the `ifdef', `ifeq', `ifndef', and `ifneq'
forms.

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.