How to undefine a define at commandline using gcc

Question

How do I at compile time undefine a compiler macro using gcc. I tried some compile args to gcc like -D but I can't get to see the "not defined" message.

Thanks

#include <iostream>

#define MYDEF


int main(){
#ifdef MYDEF
  std::cout<<"defined\n";
#else
  std::cout<<"not defined\n";
#endif

}

Answer 1

You can use the -U option with gcc, but it won't undefine a macro defined in your source code. As far as I know, there's no way to do that.

Answer 2

You can resort to filtering source code and give this back to gcc for compilation, like this pseudo code:

grep -v "define MYDEF" yourFile.c | gcc -o yourFile.o -xc -

Hope it helps.

Answer 3

You should wrap the MYDEF definition in a preprocessor macro, the presence of which (defined on the command line) would then prevent MYDEF from being defined. A bit convoluted to be sure but you can then control the build in the way you want from the command line (or Makefile). Example:

#ifndef DONT_DEFINE_MYDEF
#define MYDEF
#endif

Then from the command line when you don't want MYDEF:

gcc -DDONT_DEFINE_MYDEF ...

Answer 4

The code use case is not right. As I see, you have hard coded #define in the file. If compiler initially assumes MYDEF undefined, it will define it once it start processing the file.

You should remove the line #define MYDEF. And I hope your test case will work, if you pass MYDEF to -D and -U.

Answer 5

http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Preprocessor-Options.html#Preprocessor-Options

The -U options seemed like what you could have needed... but then again you can't override a definition contained in your source code without resorting to more preprocessor directives.

Answer 6

Here is one possibility that doesn't completely cover your use case but which I found to be helpful in my case.

If your MYDEF were #defined in a separate header file #included from the .c file you could force the definition of the #include guard macro with the -D option (thus preventing the MYDEF #definition) then either actively #define (still with the -D option) MYDEF to something else or just leave it undefined.

It is clear that anything else defined in the header file would also be missing but this was for me a solution to forcedly undefine a macro without changing the third-party code.