The Two Flavors of Variables

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The two flavors of variables

There are two ways that a variable in gnu make can have a value; we call them the two flavors of variables. The two flavors are distinguished in how they are defined and in what they do when expanded.

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 variables verbatim). 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 ex-panding some other string). When this happens, it is called recursive expansion. Consider the following example.

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

all:;echo $(foo)

This input 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 other versions of make. It has its advantages and its disadvantages. An advantage (most would say) is that the following statement will do what was intended: when CFLAGS is expanded in a command, it will expand to -Ifoo -Ibar -O.

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

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 referenced in the definition will be executed every time the variable is expanded(see Functions for transforming text).

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.

To avoid all the problems and inconveniences of recursively expanded variables, there is another flavor: simply expanded variables.

Simply expanded variables are defined by lines using := (see Setting variables). The value of a simply expanded variable is scanned once and for all, expanding any references to other variables and functions, when the variable is defined.

The actual value of the simply expanded variable is the result of expanding the text that you write.

It does not contain any references to other variables; it contains their values as of the time this variable was defined.

Therefore, consider the following statement.

x :=foo
y := $(x) bar
x := later

This is equivalent to the next statement.

y := foo bar
x := later

When a simply expanded variable is referenced, its value is substituted verbatim.

The following is a somewhat more complicated example, illustrating the use of := in conjunction with the shell function. See The shell function. This example also shows use of the variable, MAKELEVEL, which is changed when it is passed down from level to level. (See Communicating variables to a sub-make for information about MAKELEVEL.)

ifeq (0,${MAKELEVEL})
cur-dir := $(shell pwd)
whoami := $(shell whoami)
host-type := $(shell arch)
MAKE := ${MAKE} host-type=${host-type} whoami=${whoami}
endif

An advantage of this use of := is that a typical descend into a directory command then looks like this:

${subdirs}:
${MAKE} cur-dir=${cur-dir}/$@ -C $@ all

Simply expanded variables generally make complicated makefile programming more predictable because they work like variables in most programming languages. They allow you to redefine a variable using its own value (or its value processed in some way by one of the expansion functions) and to use the expansion functions much more efficiently (see Functions for transforming text).

You can also use them to introduce controlled leading whitespace into variable values. Leading whitespace characters are discarded from your input before substitution of variable references and function calls; this means you can include leading spaces in a variable value by protecting them with variable references like the following examples input.

nullstring :=
space := $(nullstring) # end of the line

With this statement, the value of the variable space is precisely one space.

The comment # end of the line is included here just for clarity. Since trailing space characters are not stripped from variable values, just a space at the end of the line would have the same effect (but be rather hard to read). If you put whitespace at the end of a variable value, it is a good idea to put a comment like that at the end of the line to make your intent clear.

Conversely, if you do not want any whitespace characters at the end of your variable value, you must remember not to put a random comment on the end of the line after some whitespace, such as the following.

dir := /foo/bar # directory to put the frobs in

With this statement, the value of the variable, dir, is /foo/bar (with four trailing spaces), which was probably not the intention. (Imagine something like $(dir)/file with this definition!)

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