What kind of variable is cflags.debug?

I was looking for alternatives for compiling C-code using Makefile with conditional CFLAGS. For which I found this option. It worked perfectly.

Now, I want to understand what kind of variable is cflag.debug, etc. Or rather, what the dot notation means in this case. I look through the GNU Make Manual and couldn't find an answer. I'll appreciate some insight.

Solution

Dots in variable names have no syntactical significance to GNU Make. You may prefer to write cxxflags.debug rather than, say, cxxflags_debug. It makes no difference to make.

But be careful. Shell variables can't contain dots. So it will make a difference if you export a Make variable containing a dot to a sub-shell. See:

$ cat Makefile 
.PHONY: foo bar

export cxxflags_debug := -g -march=native
export cxxflags.release := -g -O3 -march=native -DNDEBUG

foo:
    @echo Make variable cxxflags_debug = $(cxxflags_debug)
    @echo Make variable cxxflags.release = $(cxxflags.release)

bar:
    @echo Exported shell variable cxxflags_debug = $$cxxflags_debug
    @echo Exported shell variable cxxflags.release = $$cxxflags.release
    
    
$ make foo
Make variable cxxflags_debug = -g -march=native
Make variable cxxflags.release = -g -O3 -march=native -DNDEBUG

$ make bar
Exported shell variable cxxflags_debug = -g -march=native
Exported shell variable cxxflags.release = .release