The and Preprocessor Operators

#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.