How to undefine a define at commandline using gcc

Solution 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.

Solution 2:

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 ...

Solution 3:

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.

Solution 4:

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.

Solution 5:

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.