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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 re-expanded 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 Functions for Transforming Text), just like any other reference. For example, using the subst function (see 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 The Two Flavors of Variables), though both are used together in complex ways when doing makefile programming.