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Conditionals

In a macro processor, a conditional is a directive that allows a part of the program to be ignored during compilation, on some conditions. In the C preprocessor, a conditional can test either an arithmetic expression or whether a name is defined as a macro.

See the following documentation for more detailed discussion of conditionals.

- Why Conditionals are Used

- Syntax of Conditionals
- The #if Directive
- The #else Directive
- The #elif Directive

- Keeping Deleted Code for Future Reference
- Conditionals and Macros
- Assertions
- The #error and #warning Directives

A conditional in the C preprocessor resembles in some ways an if statement in C, but it is important to understand the difference between them.

The condition in an if statement is tested during the execution of your program. Its purpose is to allow your program to behave differently from run to run, depending on the data it is operating on. The condition in a preprocessing conditional directive is tested when your program is compiled. Its purpose is to allow different code to be included in the program depending on the situation at the time of compilation.