Could not read .config parameters in Makefile - makefile

Question

I have a simple question about how can makefile read in a varibale is set in .config file.
for example i have CONFIG_a=y CONFIG_b and CONFIG_c as three variables in .config of my linux configuration . I have a Makefile which should define a variable depending upon which CONFIG_X is set
if CONFIG_A
DFLAGS=-DABC
if CONFIG_B
DFLAGS=-DBCD
if CONFIG_C
DFLAGS=-DCDE
How could i achieve this in make file.
I tried
ifeq ($(TARGET_A),y)
DFLAGS=-DABC
else
ifeq ($(TARGET_B),y)
DFLAGS=-DBCD
else
DFLAGS=-DCDE
endif
endif
THIS could not help each time -DCDE is set.

Answer 1

You seem to be using GNU Make.
Let's assume a that exactly one of CONFIG_a, CONFIG_b, CONFIG_c must be set
in foo.config, by a line of the form:
CONFIG_(a|b|c)\s*=\s*y
where \s* means 0 or more spaces.
Then you can parse the setting and conditionally set DFLAGS as shown:-
Makefile
CONFIG := $(filter CONFIG%=y,$(shell tr -d ' ' < foo.config))
ifneq ($(words $(CONFIG)),1)
$(error Exactly one CONFIG_? must be set)
endif
ifeq ($(CONFIG),CONFIG_a=y)
DFLAGS:=-DABC
endif
ifeq ($(CONFIG),CONFIG_b=y)
DFLAGS=-DBCD
endif
ifeq ($(CONFIG),CONFIG_c=y)
DFLAGS=-DCDE
endif
$(info CONFIG=$(CONFIG))
.PHONY: all
all:
#echo DFLAGS=$(DFLAGS)
With:
foo.config (1)
CONFIG_a = y
CONFIG_b
CONFIG_c
you'll get:
$ make
CONFIG=CONFIG_a=y
DFLAGS=-DABC
With:
foo.config (2)
CONFIG_a = n
CONFIG_b=y
CONFIG_c
you'll get:
$ make
CONFIG=CONFIG_b=y
DFLAGS=-DBCD
With:
foo.config (3)
CONFIG_a = y
CONFIG_b
CONFIG_c=y
you'll get:
$ make
Makefile:3: *** Exactly one CONFIG_? must be set. Stop.
And with:
foo.config (4)
CONFIG_a = n
CONFIG_b
CONFIG_c
again:
$ make
Makefile:3: *** Exactly one CONFIG_? must be set. Stop.
To understand how it works see:-
man tr
8.13 The shell Function in the GNU Make manual
8.2 Functions for String Substitution and Analysis in the GNU Make manual.