CFLAGS vs CPPFLAGS

Question

I understand that CFLAGS  or CXXFLAGS for C    are for the compiler  whereas CPPFLAGS is used by the preprocessor   But I still don t understand the difference   I need to specify an include path for a header file that is included with  include -- because  include is a preprocessor directive  is the preprocessor  CPPFLAGS  the only thing I care about   Under what circumstances do I need to give the compiler an extra include path   In general  if the preprocessor finds and includes needed header files  why does it ever need to be told about extra include directories   What use is CFLAGS at all      In my case  I actually found that BOTH of these allow me to compile my program  which adds to the confusion    I can use CFLAGS OR CPPFLAGS to accomplish my goal  in autoconf context at least    What gives

User · Accepted Answer

The implicit make rule for compiling a C program is

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

where the $() syntax expands the variables. As both CPPFLAGS and CFLAGS are used in the compiler call, which you use to define include paths is a matter of personal taste. For instance if foo.c is a file in the current directory

make foo.o CPPFLAGS="-I/usr/include"
make foo.o CFLAGS="-I/usr/include"

will both call your compiler in exactly the same way, namely

gcc -I/usr/include -c -o foo.o foo.c

The difference between the two comes into play when you have multiple languages which need the same include path, for instance if you have bar.cpp then try

make bar.o CPPFLAGS="-I/usr/include"
make bar.o CFLAGS="-I/usr/include"

then the compilations will be

g++ -I/usr/include -c -o bar.o bar.cpp
g++ -c -o bar.o bar.cpp

as the C++ implicit rule also uses the CPPFLAGS variable.

This difference gives you a good guide for which to use - if you want the flag to be used for all languages put it in CPPFLAGS, if it's for a specific language put it in CFLAGS, CXXFLAGS etc. Examples of the latter type include standard compliance or warning flags - you wouldn't want to pass -std=c99 to your C++ compiler!

You might then end up with something like this in your makefile

CPPFLAGS=-I/usr/include
CFLAGS=-std=c99
CXXFLAGS=-Weffc++

User · Answer

You are after implicit make rules

User · Answer

To add to those who have mentioned the implicit rules  it s best to see what make has defined implicitly and for your env using  make -p  For instance    o    c       COMPILE c    OUTPUT OPTION    lt   which expands COMPILE c     CXX    CXXFLAGS    CPPFLAGS    TARGET ARCH  -c  This will also print   environment data  Here  you will find GCC s include path among other useful info  C INCLUDE PATH  usr include  In make  when it comes to search  the paths are many  the light is one    or something to that effect   C INCLUDE PATH is system-wide  set it in your shell s   rc    CPPFLAGS  is for the preprocessor include path  If you need to add a general search path for make  use   VPATH   my dir to search      or even more specific vpath   c src vpath   h include  make uses VPATH as a general search path so use cautiously  If a file exists in more than one location listed in VPATH  make will take the first occurrence in the list

User · Answer

The CPPFLAGS macro is the one to use to specify  include directories   Both CPPFLAGS and CFLAGS work in your case because the make 1  rule combines both preprocessing and compiling in one command  so both macros are used in the command    You don t need to specify   as an include-directory if you use the form  include         You also don t need to specify the standard compiler include directory   You do need to specify all other include-directories

[makefile] CFLAGS vs CPPFLAGS

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