Question

I have a makefile, with two variables like this

OS = foo.o bar.o baz.o
WS = -DWITH_FOO -DWITH_BAR -DWITH_BAZ

And so on. Instead of writing this out manually I want to generate these two when the makefile is executed based on an environment variable called WITH containing something like foo bar baz. If this environment variable is not set, or is empty, the makefile should use some hard-coded fallback instead.

How would I do that? I’m not too good at makefiles, all I can think is some kind of ‘foreach` call but the specifics elude me.

Answer 1

Well, I would write something like this:

ifdef WITH
  OS := $(WITH:%=%.o)
  WS := $(WITH:%=-DWITH_%)
else
  # Fallback.
endif

This is the most straightforward solution I see, however, it is not 100% good because WS would be -DWITH_foo ... instead of -DWITH_FOO ....

If such behavior does not fit your needs, you can use tr command to convert WS to uppercase:

WS := $(shell echo '$(WS)' | tr 'a-z' 'A-Z')

Or, as more portable option, use tr function from GMSL:

WS := $(call tr,$([a-z]),$([A-Z]),$(WS))