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.