RTEMS GNU Tools On-Line Library

Undefining

The C Preprocessor

1.4.6: 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 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.

The C Preprocessor