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
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 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 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.
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.