Symbols

-D
- -D name
  
  Predefine name as a macro, with definition 1.
  
  E.g.:
- -D name=value
  
  Predefine name as a macro, with definition value. The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition will be truncated by embedded newline characters.
-U

Cancel any previous definition of name, either built-in or provided with a -D option.

-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options.