The simplest sort of conditional is
#ifdef MACRO controlled text #endif /* MACRO */
This block is called a conditional group.
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.
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 ‘
#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 ‘
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 ‘
Sometimes you wish to use some code if a macro is not defined.
You can do this by writing ‘
#ifndef’ instead of ‘
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.
-Ucommand-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.
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.