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.