CFLAGS CCFLAGS CXXFLAGS - what exactly do these variables control

Question

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   lt  -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

User · Answer

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

User · Answer

As you noticed  these are Makefile  macros or variables   not compiler options  They implement a set of conventions   Macros is an old name for them  still used by some  GNU make doc calls them variables    The only reason that the names matter is the default make rules  visible via make -p  which use some of them   If you write all your own rules  you get to pick all your own macro names   In a vanilla gnu make  there s no such thing as CCFLAGS  There are CFLAGS  CPPFLAGS  and CXXFLAGS  CFLAGS for the C compiler  CXXFLAGS for C    and CPPFLAGS for both   Why is CPPFLAGS in both  Conventionally  it s the home of preprocessor flags  -D  -U  and both c and c   use them  Now  the assumption that everyone wants the same define environment for c and c   is perhaps questionable  but traditional     P S  As noted by James Moore  some projects use CPPFLAGS for flags to the C   compiler  not flags to the C preprocessor  The Android NDK  for one huge example

User · Answer

Minimal example And just to make what Mizux said as a minimal example  main c c  include  lt stdio h gt   int main void        puts  quot hello quot       main cpp cpp  include  lt iostream gt   int main void        std  cout  lt  lt   quot hello quot   lt  lt  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      lt   main cpp  main c c       CXX    CXXFLAGS    CPPFLAGS  -o      lt   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

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

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