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   In a macro processor, a "conditional" is a command 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.

   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 preprocessor conditional command 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

* Uses
Conditional UsesWhat conditionals are for.
* Syntax
Conditional SyntaxHow conditionals are written.
* Deletion
Deleted CodeMaking code into a comment.
* Macros
Conditionals-MacrosWhy conditionals are used with macros.
* Assertions
How and why to use assertions.
* Errors
#error CommandDetecting inconsistent compilation parameters.

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