How to Cancel any previous definition of name, either built in or provided with a ‘-D’ option in gcc

Question

#include <stdio.h>
#define DEBUG

#ifdef DEBUG
#define MAGIC 5
#endif

int main(void){
        printf("\n magic is %d",MAGIC);
        return 0;
    }

Now i want to undef DEBUG so this program should give compilation error

gcc test.c -U DEBUG

But it does not give any error and works fine. It means -U does not work.

so how can I undef any name at compile time in gcc?

Answer 1

man GCC says,

 -D and -U options are processed in the order they are given on the command line.

Seems you cannot undefine a MACRO from CLI which is defined in program.

Answer 2

#include <stdio.h>

#define DEBUG

// add some new lines
#if defined(CUD_DEBUG) && defined(DEBUG)
#undef DEBUG
#endif

#ifdef DEBUG
#define MAGIC 5
#endif

int main(void)
{
    printf("\n magic is %d", MAGIC);
    return 0;
}

To compile that by command:

gcc test.cc -DCUD_DEBUG

CUD_DEBUG means compiler undefine debug.

Answer 3

You should write your code like this

#include <stdio.h>
//#define DEBUG

#ifdef DEBUG
#define MAGIC 5
#else
#define MAGIC 1
#endif

int main(void){
        printf("\n magic is %d",MAGIC);
        return 0;
    }

Now gcc test.c -U DEBUG will work

Answer 4

Don't define DEBUG inside the program. Instead, define DEBUG while compilation as gcc test.c -DDEBUG.

If you want to get an error at compile time then don't define DEBUG at compile time as gcc test.c:

test.c: In function "main":
test.c:8: error: "MAGIC" undeclared (first use in this function)
test.c:8: error: (Each undeclared identifier is reported only once
test.c:8: error: for each function it appears in.)