6.2.1 Recursively Expanded Variable Assignment

The first flavor of variable is a recursively expanded variable. Variables of this sort are defined by lines using ‘=’ (see Setting Variables) or by the define directive (see Defining Multi-Line Variables). The value you specify is installed verbatim; if it contains references to other variables, these references are expanded whenever this variable is substituted (in the course of expanding some other string). When this happens, it is called recursive expansion.

For example,

foo = $(bar)
bar = $(ugh)
ugh = Huh?

all:;echo $(foo)

will echo ‘Huh?’: ‘$(foo)’ expands to ‘$(bar)’ which expands to ‘$(ugh)’ which finally expands to ‘Huh?’.

This flavor of variable is the only sort supported by most other versions of make. It has its advantages and its disadvantages. An advantage (most would say) is that:

CFLAGS = $(include_dirs) -O
include_dirs = -Ifoo -Ibar

will do what was intended: when ‘CFLAGS’ is expanded in a recipe, it will expand to ‘-Ifoo -Ibar -O’. A major disadvantage is that you cannot append something on the end of a variable, as in

CFLAGS = $(CFLAGS) -O

because it will cause an infinite loop in the variable expansion. (Actually make detects the infinite loop and reports an error.)

Another disadvantage is that any functions (see Functions for Transforming Text) referenced in the definition will be executed every time the variable is expanded. This makes make run slower; worse, it causes the wildcard and shell functions to give unpredictable results because you cannot easily control when they are called, or even how many times.