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GNU Info (cpp-295.info)UndefiningUndefining 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 example, #define FOO 4 x = FOO; #undef FOO x = FOO; expands into 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. automatically generated by info2www version 1.2.2.9 |