Roland Ver. 4.5 Información técnica descargar pdf (Pagina 74)

64 GNU make

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,

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Roland Ver. 4.5 Información técnica Pagina 74