A conditional in the C preprocessor begins with a conditional directive: `#if', `#ifdef' or `#ifndef'. See 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. |