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# (make.info.gz) Computed Names

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6.3.2 Computed Variable Names
-----------------------------

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 ( Functions for Transforming Text
Functions.), just like any other reference.  For example, using the
`subst' function ( Functions for String Substitution and Analysis
Text Functions.):

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" ( The Two Flavors of Variables
Flavors.), though both are used together in complex ways when doing
makefile programming.

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