#undef directive

The #undef directive removes a previously defined definition of the macro name that follows it. That is, it "undefines" a macro. The general form for #undef is

#undef macro-name

For example,

#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. #undef is used principally to allow macro names to be localized to only those sections of code that need them.