CPPFLAGS
is supposed to be for flags for the C PreProcessor; CXXFLAGS
is for flags for the C++ compiler.
The default rules in make (on my machine, at any rate) pass CPPFLAGS
to just about everything, CFLAGS
is only passed when compiling and linking C, and CXXFLAGS
is only passed when compiling and linking C++.
By default, CPPFLAGS
will be given to the C preprocessor, while CXXFLAGS
will be given to the C++ compiler.
The GNU Make Manual is a good resource for questions like this (see Implicit Variables).
By default, they're set to something.
In practice, you need to know what every single project does. Virtually no one uses those defaults built into make, and if you rely on, for example, CPPFLAGS meaning "flags to the C preprocessor" you'll find that the project you care about has used it to mean "flags to the C++ compiler" instead. And does the CFLAGS flag get passed to C++ compile lines? Sometimes. Not always. Etc, etc, etc.
Success story sharing
x
is a+
turned on it's side becauseC++FLAGS
would blow up the compiler. ... I may have arrived to the party late, but that's still better than arriving on time to the wrong party.CPPFLAGS
is NOT for C Plus Plus butCXXFLAGS
is.