#undef

With the # undef a macro can be overridden.
Status “defined” or “undefined” is an important characteristic of an identifier, regardless of its actual definition.
The # ifdef and # ifndef can be used to check whether an identifier is currently defined or not.

Syntax

#undef macro_name

  #include <stdio.h>
 
  void)
  {
    #define TESTDEFINE 1
      printf("%d\n", TESTDEFINE);
 
    #undef TESTDEFINE
 
    #define TESTDEFINE 42
      printf("%d\n", TESTDEFINE);
 
    return 0;
  }

   1
   42