重要章节1：makefile如何读取的

3.7 How make Reads a MakefileGNU 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 AssignmentVariable definitions are parsed as follows:     immediate = deferred     immediate ?= deferred     immediate := immediate     immediate += deferred or immediate          define immediate       deferred     endef          define immediate =       deferred     endef          define immediate ?=       deferred     endef          define immediate :=       immediate     endef          define immediate +=       deferred or immediate     endefFor the append operator, ‘+=’, the right-hand side is considered immediate if the variable was previously set as a simple variable (‘:=’), and deferred otherwise.Conditional DirectivesConditional 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 DefinitionA rule is always expanded the same way, regardless of the form:     immediate : immediate ; deferred     deferredThat 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.