A conditional in the C preprocessor begins with a "conditional directive": `#if', `#ifdef' or `#ifndef'. *Note Conditionals-Macros::, for information on `#ifdef' and `#ifndef'; only `#if' is explained here.
If | Basic conditionals using `#if' and `#endif'. |
Else | Including some text if the condition fails. |
Elif | Testing several alternative possibilities. |