The preprocessor directive #define can be used to create compile-time flags. You have two choices: you can simply tell the preprocessor that the flag is defined, without specifying a value:

or you can give it a value (which is the typical C way to define a constant):

In either case, the label can now be tested by the preprocessor to see if it has been defined:

This will yield a true result, and the code following the #ifdef will be included in the package sent to the compiler. This inclusion stops when the preprocessor encounters the statement

or

Any non-comment after the #endif on the same line is illegal, even though some compilers may accept it. The #ifdef/#endif pairs may be nested within each other.

The complement of #define is #undef (short for “un-define”), which will make an #ifdef statement using the same variable yield a false result. #undef will also cause the preprocessor to stop using a macro. The complement of #ifdef is #ifndef, which will yield a true if the label has not been defined (this is the one we will use in header files).

There are other useful features in the C preprocessor. You should check your local documentation for the full set.