Monday, July 09, 2007

Gnu-make: define virables

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 section
Setting Variables) or by the define directive (see section 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 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 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 command, it
will expand to `-Ifoo -Ibar -O'. A major disadvantage is that you cannot
append something on the end of a variable, as in


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 section 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

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
section 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.

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

is equivalent to

y := foo bar
x := later

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

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