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GNU Info (make.info)Reading MakefilesHow `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. automatically generated by info2www version 1.2.2.9 |