Computed variable names are a complicated concept needed only for sophisticated makefile programming. For most purposes you need not consider them, except to know that making a variable with a dollar sign in its name might have strange results. However, if you are the type that wants to understand everything, or you are actually interested in what they do, read on.
Variables may be referenced inside the name of a variable. This is called a computed variable name or a nested variable reference. For example,
x = y y = z a := $($(x))
defines a as `z': the `$(x)' inside `$($(x))' expands
to `y', so `$($(x))' expands to `$(y)' which in turn expands
to `z'. Here the name of the variable to reference is not stated
explicitly; it is computed by expansion of `$(x)'. The reference
`$(x)' here is nested within the outer variable reference.
The previous example shows two levels of nesting, but any number of levels is possible. For example, here are three levels:
x = y y = z z = u a := $($($(x)))
Here the innermost `$(x)' expands to `y', so `$($(x))' expands to `$(y)' which in turn expands to `z'; now we have `$(z)', which becomes `u'.
References to recursively-expanded variables within a variable name are reexpanded in the usual fashion. For example:
x = $(y) y = z z = Hello a := $($(x))
defines a as `Hello': `$($(x))' becomes `$($(y))'
which becomes `$(z)' which becomes `Hello'.
Nested variable references can also contain modified references and
function invocations (see section Functions for Transforming Text),
just like any other reference.
For example, using the subst function
(see section Functions for String Substitution and Analysis):
x = variable1 variable2 := Hello y = $(subst 1,2,$(x)) z = y a := $($($(z)))
eventually defines a as `Hello'. It is doubtful that anyone
would ever want to write a nested reference as convoluted as this one, but
it works: `$($($(z)))' expands to `$($(y))' which becomes
`$($(subst 1,2,$(x)))'. This gets the value `variable1' from
x and changes it by substitution to `variable2', so that the
entire string becomes `$(variable2)', a simple variable reference
whose value is `Hello'.
A computed variable name need not consist entirely of a single variable reference. It can contain several variable references, as well as some invariant text. For example,
a_dirs := dira dirb 1_dirs := dir1 dir2 a_files := filea fileb 1_files := file1 file2 ifeq "$(use_a)" "yes" a1 := a else a1 := 1 endif ifeq "$(use_dirs)" "yes" df := dirs else df := files endif dirs := $($(a1)_$(df))
will give dirs the same value as a_dirs, 1_dirs,
a_files or 1_files depending on the settings of use_a
and use_dirs.
Computed variable names can also be used in substitution references:
a_objects := a.o b.o c.o 1_objects := 1.o 2.o 3.o sources := $($(a1)_objects:.o=.c)
defines sources as either `a.c b.c c.c' or `1.c 2.c 3.c',
depending on the value of a1.
The only restriction on this sort of use of nested variable references is that they cannot specify part of the name of a function to be called. This is because the test for a recognized function name is done before the expansion of nested references. For example,
ifdef do_sort func := sort else func := strip endif bar := a d b g q c foo := $($(func) $(bar))
attempts to give `foo' the value of the variable `sort a d b g
q c' or `strip a d b g q c', rather than giving `a d b g q c'
as the argument to either the sort or the strip function.
This restriction could be removed in the future if that change is shown
to be a good idea.
You can also use computed variable names in the left-hand side of a
variable assignment, or in a define directive, as in:
dir = foo $(dir)_sources := $(wildcard $(dir)/*.c) define $(dir)_print lpr $($(dir)_sources) endef
This example defines the variables `dir', `foo_sources', and `foo_print'.
Note that nested variable references are quite different from recursively expanded variables (see section The Two Flavors of Variables), though both are used together in complex ways when doing makefile programming.