On Wed, 2019-01-23 at 13:02 -0600, Peng Yu wrote: > In Option Summary of the man page of gcc, I see many sections about > options, like the following. What should be in CFLAGS, CXXFLAGS, > CPPFLAGS and LDFLAGS, respectively? > > For example, should -g be used in CPPFLAGS or both CFLAGS and > CXXFLAGS? Should it be used with LDFLAGS? What about -O2? Thanks. You probably should consider asking this on help-make@xxxxxxx instead. Anyway. CPPFLAGS are for preprocessor options, only. So, -I, -D, -U, etc. You should not put things like -g there. The idea behind separating this is that you may want to run tools that invoke the preprocessor, such as lint or other tools, but won't understand compiler flags. Usually in makefiles linking is done with the compiler front-end, not ld (the linker) directly. And usually the CFLAGS or CXXFLAGS flags are added to the link recipe. So, you don't need to add them into LDFLAGS. LDFLAGS should be for link-time only options such as -L, -Wl,..., etc. But do NOT put -l (library) options here, they go into LDLIBS. Of course you can create other variables and use them, like: DEBUG = -ggdb3 -Og CFLAGS = $(DEBUG) ... CXXFLAGS = $(DEBUG) ... Finally, if you're creating a makefile for use by people who might want to customize your build you shouldn't be putting any but the most basic flags in the CFLAGS or CXXFLAGS variables, so that people can override them on the make command line without breaking the build: CFLAGS = -g -O2 OTHER_CFLAGS = -Wall -Werror ... then write your rule to use both $(CFLAGS) and $(OTHER_CFLAGS), so people can run "make CFLAGS=-g" or something and things still work.
