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The C preprocessor implements the macro language used to transform C, C++, and Objective-C programs before they are compiled. It can also be useful on its own.

Copyright © 1987-2025 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation. A copy of the license is included in the section entitled “GNU Free Documentation License”.

This manual contains no Invariant Sections. The Front-Cover Texts are (a) (see below), and the Back-Cover Texts are (b) (see below).

(a) The FSF’s Front-Cover Text is:

A GNU Manual

(b) The FSF’s Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.

• Overview
• Header Files
• Macros
• Conditionals
• Diagnostics
• Line Control
• Pragmas
• Binary Resource Inclusion
• Other Directives
• Preprocessor Output
• Traditional Mode
• Implementation Details
• Invocation
• Environment Variables
• GNU Free Documentation License
• Index of Directives
• Option Index
• Concept Index