Question

I’d like to dump all Makefile variables that start with MYLIB_ and their values to a file called config.h as preprocessor definitions. This is what I have tried:

config:
    @echo "// AUTO-GENERATED - DO NOT EDIT!" > config.h
    $(foreach V, $(filter MYLIB_%, $(.VARIABLES)),
        $(shell echo "#define $(V) $($V)" >> config.h))

Unfortunately, $(filter) doesn’t return any values, even though $(.VARIABLES) contains items that begin with MYLIB_.

What am I doing wrong?

Answer 1

Your $(filter) looks correct.
Possibly the cause may be the evaluation order between @echo command
and $(foreach).
Does replacing @echo with $(shell echo ...) like the following
solve the problem?

config:
    $(shell echo "// AUTO-GENERATED - DO NOT EDIT!" > config.h) \
    $(foreach V, $(filter MYLIB_%, $(.VARIABLES)), \
        $(shell echo "#define $(V) $($V)" >> config.h))