makeReads a Makefile
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 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.
All instances of conditional syntax are parsed immediately, in their
entirety; this includes the
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
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.