#undef

The #undef directive removes a previously defined macro name. The general form is

#undef macro-name

For example, in the following code,

#define LEN 100
#define WIDTH 100

char array[LEN][WIDTH];

#undef LEN
#undef WIDTH
/* at this point both LEN and WIDTH are
   undefined */

both LEN and WIDTH are defined until the #undef statements are encountered.