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Undefining Macros

To undefine a macro means to cancel its definition. This is done with the #undef directive. #undef is followed by the macro name to be undefined.

Like definition, undefinition occurs at a specific point in the source file, and it applies starting from that point. The name ceases to be a macro name, and from that point on it is treated by the preprocessor as if it had never been a macro name. For clarification, use the following example.

    #define FOO 4 x = FOO; #undef FOO x = FOO;

This input expands into the following output.

    x =4;
    x = FOO;

In this example, FOO had better be a variable or function as well as (temporarily) a macro, in order for the result of the expansion to be valid C code. The same form of #undef directive will cancel definitions with arguments or definitions that don’t expect arguments. The #undef directive has no effect when used on a name not currently defined as a macro.