How to undefine a define at commandline using gcc

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
  
}

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.

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 ...

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.

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.

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.

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.