[gcc] CFLAGS, CCFLAGS, CXXFLAGS - what exactly do these variables control?

I am using GNU make to compile my C++ code, and i would like to understand how to make my compilations customizable.

I read in different places that CFLAGS, CCFLAGS and CXXFLAGS are used for this purpose. So how should i use them? If i have additional command-line arguments to the compiler, should i append them to CFLAGS or prepend them? Is there a common practice?

Why the three different variables? I suppose the C compiler should get CFLAGS and CCFLAGS, while the C++ compiler should get CFLAGS and CXXFLAGS - did i get it right?

Is the human user supposed to set these variables at all? Do any automatic tools (automake, autoconf, etc) set them? The linux system that i am supposed to use doesn't define any of these variables - is this typical?

Currently my Makefile looks like this, and i feel it's a bit dirty:

ifdef code_coverage
    GCOV_FLAG := -fprofile-arcs -ftest-coverage
else
    GCOV_FLAG :=
endif

WFLAGS := -Wall

INC_FLAGS := -Istuff -Imore_stuff -Ietc

CCFLAGSINT := -O3 $(WFLAGS) $(INC_FLAGS) $(CCFLAGS)

... (somewhere in the makefile, the command-line for compilation looks like this)
    $(CC) $(CCFLAGSINT) -c $< -o $@

... (somewhere in the makefile, the command-line for linking looks like this)
    $(CC) $(GCOV_FLAG) $(CCFLAGSINT) $(OBJLIST) $(LDFLAGS) -o $@

I am pretty sure there are no bugs here; the Makefile works very well. But is there anything that goes against conventions (like CCFLAGSINT - should i just overwrite CCFLAGS instead? Or CXXFLAGS? FUD!)

Sorry for so many questions; you will obviously not answer them all but i hope the answers will help me understand the general idea behind these settings.

According to the GNU make manual:

CFLAGS: Extra flags to give to the C compiler.
CXXFLAGS: Extra flags to give to the C++ compiler.
CPPFLAGS: Extra flags to give to the C preprocessor and programs that use it (the C and Fortran compilers).

src: https://www.gnu.org/software/make/manual/make.html#index-CFLAGS
note: PP stands for PreProcessor (and not Plus Plus), i.e.

CPP: Program for running the C preprocessor, with results to standard output; default ‘$(CC) -E’.

These variables are used by the implicit rules of make

Compiling C programs
n.o is made automatically from n.c with a recipe of the form
‘$(CC) $(CPPFLAGS) $(CFLAGS) -c’.

Compiling C++ programs
n.o is made automatically from n.cc, n.cpp, or n.C with a recipe of the form
‘$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c’.
We encourage you to use the suffix ‘.cc’ for C++ source files instead of ‘.C’.

src: https://www.gnu.org/software/make/manual/make.html#Catalogue-of-Rules

Minimal example

And just to make what Mizux said as a minimal example:

main_c.c

#include <stdio.h>

int main(void) {
    puts("hello");
}

main_cpp.cpp

#include <iostream>

int main(void) {
    std::cout << "hello" << std::endl;
}

Then, without any Makefile:

make CFLAGS='-g -O3' \
     CXXFLAGS='-ggdb3 -O0' \
     CPPFLAGS='-DX=1 -DY=2' \
     CCFLAGS='--asdf' \
     main_c \
     main_cpp

runs:

cc -g -O3 -DX=1 -DY=2   main_c.c   -o main_c
g++ -ggdb3 -O0 -DX=1 -DY=2   main_cpp.cpp   -o main_cpp

So we understand that:

• make had implicit rules to make main_c and main_cpp from main_c.c and main_cpp.cpp
• CFLAGS and CPPFLAGS were used as part of the implicit rule for .c compilation
• CXXFLAGS and CPPFLAGS were used as part of the implicit rule for .cpp compilation
• CCFLAGS is not used

Those variables are only used in make's implicit rules automatically: if compilation had used our own explicit rules, then we would have to explicitly use those variables as in:

main_c: main_c.c
    $(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $<

main_cpp: main_c.c
    $(CXX) $(CXXFLAGS) $(CPPFLAGS) -o $@ $<

to achieve a similar affect to the implicit rules.

We could also name those variables however we want: but since Make already treats them magically in the implicit rules, those make good name choices.

Tested in Ubuntu 16.04, GNU Make 4.1.