| #ifdef

| Conditional Syntax | Next

The simplest sort of conditional is

    #ifdef MACRO

    controlled text

    #endif /* MACRO */

This block is called a conditional group. controlled text will be included in the output of the preprocessor if and only if MACRO is defined. We say that the conditional succeeds if MACRO is defined, fails if it is not.
The controlled text inside of a conditional can include preprocessing directives. They are executed only if the conditional succeeds. You can nest conditional groups inside other conditional groups, but they must be completely nested. In other words, `#endif` always matches the nearest `#ifdef` (or `#ifndef`, or `#if`). Also, you cannot start a conditional group in one file and end it in another.
Even if a conditional fails, the controlled text inside it is still run through initial transformations and tokenization. Therefore, it must all be lexically valid C. Normally the only way this matters is that all comments and string literals inside a failing conditional group must still be properly ended.
The comment following the `#endif` is not required, but it is a good practice if there is a lot of controlled text, because it helps people match the `#endif` to the corresponding `#ifdef`. Older programs sometimes put MACRO directly after the `#endif` without enclosing it in a comment. This is invalid code according to the C standard. CPP accepts it with a warning. It never affects which `#ifndef` the `#endif` matches.
Sometimes you wish to use some code if a macro is not defined. You can do this by writing `#ifndef` instead of `#ifdef`. One common use of `#ifndef` is to include code only the first time a header file is included. See Once-Only Headers.
Macro definitions can vary between compilations for several reasons. Here are some samples.

- Some macros are predefined on each kind of machine (see System-specific Predefined Macros). This allows you to provide code specially tuned for a particular machine.
- System header files define more macros, associated with the features they implement. You can test these macros with conditionals to avoid using a system feature on a machine where it is not implemented.
- Macros can be defined or undefined with the '-D' and '-U' command line options when you compile the program. You can arrange to compile the same source file into two different programs by choosing a macro name to specify which program you want, writing conditionals to test whether or how this macro is defined, and then controlling the state of the macro with command line options, perhaps set in the Makefile. See Invocation.
- Your program might have a special header file (often called `config.h`) that is adjusted when the program is compiled. It can define or not define macros depending on the features of the system and the desired capabilities of the program. The adjustment can be automated by a tool such as `autoconf`, or done by hand.