Variables conflict between Makefile and C code - c

Question

In my C Makefile I have the following lines:
ifeq ($(VERBOSE_PRINT), TRUE)
CFLAGS += -D TRUE
else
CFLAGS += -D FALSE
endif
As you can tell by it's name, this flag indicated whether I should print debug statements or not.
In other place at my C code, I'm defining TRUE and FALSE with
#define FALSE 0
#define TRUE !FALSE
Then when I'm trying to compile my code, I get the following error:
types.h:6:0: error: "FALSE" redefined [-Werror]
#define FALSE 0
<command-line>:0:0: note: this is the location of the previous definition
If I delete the definition of TRUE and FALSE from my C code, my program get's crazy.
What's going on and how can I solve this conflict?
Thanks ahead.

Answer 1

You can't use the same name for two different incompatible things, so you'll need to change one of them. Most likely you want to change your Makefile, since what you have makes no sense. Something like:
ifeq ($(VERBOSE_PRINT), TRUE)
CFLAGS += -DDEBUG=1
else
CFLAGS += -DDEBUG=0
endif
Then in your C code you can have
#if DEBUG
printf( ... debugging messages ... )
#endif

Answer 2

On the compiler command line, -DXYZ is equivalent to -DXYZ=1 (POSIX c99). This means that with -DFALSE on the command line, you've got both #define FALSE 1 and #define FALSE 0, which is a non-benign redefinition. Benign redefinition of a macro is OK (meaning that the sequence of replacement tokens is the same in both the current and new definition — see C11 §6.10.3 Macro replacement ¶1-2).
You need to use a different variable in your code to decide whether to do printing:
#ifdef VERBOSE_PRINT
printf("Debugging!\n");
#endif
You can then use -DVERBOSE_PRINT on the command line to enable printing and leave it out to disable it. Alternatively, you can use:
#if VERBOSE_PRINT
printf("Debugging!\n");
#endif
Then your makefile can contain:
ifeq ($(VERBOSE_PRINT), TRUE)
CFLAGS += -DVERBOSE_PRINT=TRUE
else
CFLAGS += -DVERBOSE_PRINT=FALSE
endif
This technique is less common. It will work as if you have #define VERBOSE_PRINT 0 if you don't specify a value on the compiler command line (C11 §6.10.1 Conditional inclusion ¶4).