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.
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