6.2.2 Simply Expanded Variable Assignment

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

Simply expanded variables are defined by lines using ‘:=’ or ‘::=’ (see Setting Variables). Both forms are equivalent in GNU make; however only the ‘::=’ form is described by the POSIX standard (support for ‘::=’ is added to the POSIX standard for POSIX Issue 8).

The value of a simply expanded variable is scanned once, expanding any references to other variables and functions, when the variable is defined. Once that expansion is complete the value of the variable is never expanded again: when the variable is used the value is copied verbatim as the expansion. If the value contained variable references the result of the expansion will contain their values as of the time this variable was defined. Therefore,

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

is equivalent to

y := foo bar
x := later

Here 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})
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’ recipe then looks like this:

${subdirs}:
        ${MAKE} -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 this:

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

Here 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 this:

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

Here 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!)