Description
Reference: https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html
Currently, quite a few Arduino makefiles use CPPFLAGS (and other custom macros beginning with CPP) as the C++ counterpart for CFLAGS.
However, "CPP" in this context means "C preprocessor", not "C plus plus". The right name for C++ flags is CXXFLAGS (think of the X as a turned +).
From the Gnu make documentation:
the recipe used to compile a C source file actually says ‘$(CC) -c
$(CFLAGS) $ (CPPFLAGS)’
The affected files are:
javier@desktop:/tmp/Arduino-master$ grep ^CPP -lIR .
./hardware/arduino/sam/variants/arduino_due_x/build_gcc/gcc.mk
./hardware/arduino/sam/variants/arduino_due_x/build_gcc/libvariant_arduino_due_x.mk
./hardware/arduino/sam/firmwares/atmega16u2/arduino-usbserial/makefile
./hardware/arduino/avr/firmwares/atmegaxxu2/arduino-usbserial/makefile
./hardware/arduino/avr/firmwares/atmegaxxu2/arduino-usbdfu/makefile
./hardware/arduino/avr/bootloaders/caterina/Makefile
./hardware/arduino/avr/bootloaders/caterina-Arduino_Robot/Makefile
./hardware/arduino/avr/bootloaders/caterina-LilyPadUSB/Makefile
This is probably not causing any trouble since it seems that implicit recipes are not being used and these flags are explicitly passed to the respective compiler commands, but it might be misleading to developers and could cause some trouble in a future; I think it would be nice to fix it so that it makes sense.
Additionally, I've noticed some makefiles use LDFLAGS for linked libraries (such as -lm); these should go on LDLIBS instead, since LDFLAGS is (usually) passed BEFORE the source/object files (like most of the flags), but linked libraries must be passed AFTER them.
Alternatively, if all these are just being used as custom variables and you don't care about their "official" meaning, then do not use these special names and use custom ones (for example, cxxflags instead of CXXFLAGS), as recommended in https://www.gnu.org/software/make/manual/html_node/Using-Variables.html for "internal" variables.