gcc可以undefine一个macro,但功能很有限

GOOGLE：  gcc how to undefine a macro using  parameter；

A1： 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.
 
A2： Yeah, -U only works with macros also defined on the command line. You can't undefine a macro defined in code from the CL.