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 · 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) $<

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

User · Answer

You are after implicit make rules

User · Answer

The implicit make rule for compiling a C program is    o   c       CC    CPPFLAGS    CFLAGS  -c -o      lt    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

[makefile] CFLAGS vs CPPFLAGS

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