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\input texinfo

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@setfilename cpp.info

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@settitle The C Preprocessor

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@c @smallbook

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@c @cropmarks

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@c @finalout

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@include gcc-common.texi

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@copying

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@c man begin COPYRIGHT

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Copyright @copyright{} 1987-2016 Free Software Foundation, Inc.

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Permission is granted to copy, distribute and/or modify this document

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under the terms of the GNU Free Documentation License, Version 1.3 or

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any later version published by the Free Software Foundation. A copy of

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the license is included in the

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@c man end

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section entitled ``GNU Free Documentation License''.

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@ignore

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@c man begin COPYRIGHT

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man page gfdl(7).

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@c man end

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@end ignore

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@c man begin COPYRIGHT

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This manual contains no Invariant Sections. The Front-Cover Texts are

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(a) (see below), and the Back-Cover Texts are (b) (see below).

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(a) The FSF's Front-Cover Text is:

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A GNU Manual

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(b) The FSF's Back-Cover Text is:

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You have freedom to copy and modify this GNU Manual, like GNU

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software. Copies published by the Free Software Foundation raise

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funds for GNU development.

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@c man end

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@end copying

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@c Create a separate index for command line options.

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@defcodeindex op

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@syncodeindex vr op

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@c Used in cppopts.texi and cppenv.texi.

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@set cppmanual

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@ifinfo

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@dircategory Software development

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@direntry

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* Cpp: (cpp). The GNU C preprocessor.

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@end direntry

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@end ifinfo

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@titlepage

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@title The C Preprocessor

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@versionsubtitle

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@author Richard M. Stallman, Zachary Weinberg

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@page

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@c There is a fill at the bottom of the page, so we need a filll to

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@c override it.

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@vskip 0pt plus 1filll

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@insertcopying

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@end titlepage

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@contents

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@page

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@ifnottex

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@node Top

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@top

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The C preprocessor implements the macro language used to transform C,

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C++, and Objective-C programs before they are compiled. It can also be

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useful on its own.

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@menu

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* Overview::

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* Header Files::

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* Macros::

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* Conditionals::

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* Diagnostics::

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* Line Control::

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* Pragmas::

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* Other Directives::

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* Preprocessor Output::

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* Traditional Mode::

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* Implementation Details::

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* Invocation::

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* Environment Variables::

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* GNU Free Documentation License::

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* Index of Directives::

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* Option Index::

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* Concept Index::

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@detailmenu

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--- The Detailed Node Listing ---

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Overview

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* Character sets::

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* Initial processing::

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* Tokenization::

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* The preprocessing language::

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Header Files

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* Include Syntax::

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* Include Operation::

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* Search Path::

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* Once-Only Headers::

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* Alternatives to Wrapper #ifndef::

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* Computed Includes::

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* Wrapper Headers::

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* System Headers::

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Macros

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* Object-like Macros::

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* Function-like Macros::

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* Macro Arguments::

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* Stringification::

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* Concatenation::

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* Variadic Macros::

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* Predefined Macros::

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* Undefining and Redefining Macros::

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* Directives Within Macro Arguments::

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* Macro Pitfalls::

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Predefined Macros

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* Standard Predefined Macros::

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* Common Predefined Macros::

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* System-specific Predefined Macros::

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* C++ Named Operators::

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Macro Pitfalls

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* Misnesting::

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* Operator Precedence Problems::

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* Swallowing the Semicolon::

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* Duplication of Side Effects::

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* Self-Referential Macros::

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* Argument Prescan::

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* Newlines in Arguments::

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Conditionals

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* Conditional Uses::

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* Conditional Syntax::

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* Deleted Code::

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Conditional Syntax

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* Ifdef::

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* If::

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* Defined::

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* Else::

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* Elif::

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Implementation Details

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* Implementation-defined behavior::

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* Implementation limits::

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* Obsolete Features::

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* Differences from previous versions::

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Obsolete Features

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* Obsolete Features::

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@end detailmenu

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@end menu

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@insertcopying

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@end ifnottex

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@node Overview

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@chapter Overview

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@c man begin DESCRIPTION

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The C preprocessor, often known as @dfn{cpp}, is a @dfn{macro processor}

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that is used automatically by the C compiler to transform your program

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before compilation. It is called a macro processor because it allows

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you to define @dfn{macros}, which are brief abbreviations for longer

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constructs.

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The C preprocessor is intended to be used only with C, C++, and

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Objective-C source code. In the past, it has been abused as a general

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text processor. It will choke on input which does not obey C's lexical

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rules. For example, apostrophes will be interpreted as the beginning of

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character constants, and cause errors. Also, you cannot rely on it

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preserving characteristics of the input which are not significant to

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C-family languages. If a Makefile is preprocessed, all the hard tabs

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will be removed, and the Makefile will not work.

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Having said that, you can often get away with using cpp on things which

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are not C@. Other Algol-ish programming languages are often safe

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(Pascal, Ada, etc.) So is assembly, with caution. @option{-traditional-cpp}

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mode preserves more white space, and is otherwise more permissive. Many

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of the problems can be avoided by writing C or C++ style comments

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instead of native language comments, and keeping macros simple.

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Wherever possible, you should use a preprocessor geared to the language

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you are writing in. Modern versions of the GNU assembler have macro

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facilities. Most high level programming languages have their own

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conditional compilation and inclusion mechanism. If all else fails,

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try a true general text processor, such as GNU M4.

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C preprocessors vary in some details. This manual discusses the GNU C

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preprocessor, which provides a small superset of the features of ISO

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Standard C@. In its default mode, the GNU C preprocessor does not do a

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few things required by the standard. These are features which are

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rarely, if ever, used, and may cause surprising changes to the meaning

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of a program which does not expect them. To get strict ISO Standard C,

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you should use the @option{-std=c90}, @option{-std=c99} or

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@option{-std=c11} options, depending

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on which version of the standard you want. To get all the mandatory

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diagnostics, you must also use @option{-pedantic}. @xref{Invocation}.

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This manual describes the behavior of the ISO preprocessor. To

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minimize gratuitous differences, where the ISO preprocessor's

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behavior does not conflict with traditional semantics, the

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traditional preprocessor should behave the same way. The various

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differences that do exist are detailed in the section @ref{Traditional

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Mode}.

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For clarity, unless noted otherwise, references to @samp{CPP} in this

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manual refer to GNU CPP@.

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@c man end

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@menu

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* Character sets::

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* Initial processing::

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* Tokenization::

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* The preprocessing language::

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@end menu

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@node Character sets

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@section Character sets

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Source code character set processing in C and related languages is

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rather complicated. The C standard discusses two character sets, but

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there are really at least four.

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The files input to CPP might be in any character set at all. CPP's

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very first action, before it even looks for line boundaries, is to

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convert the file into the character set it uses for internal

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processing. That set is what the C standard calls the @dfn{source}

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character set. It must be isomorphic with ISO 10646, also known as

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Unicode. CPP uses the UTF-8 encoding of Unicode.

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The character sets of the input files are specified using the

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@option{-finput-charset=} option.

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All preprocessing work (the subject of the rest of this manual) is

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carried out in the source character set. If you request textual

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output from the preprocessor with the @option{-E} option, it will be

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in UTF-8.

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After preprocessing is complete, string and character constants are

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converted again, into the @dfn{execution} character set. This

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character set is under control of the user; the default is UTF-8,

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matching the source character set. Wide string and character

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constants have their own character set, which is not called out

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specifically in the standard. Again, it is under control of the user.

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The default is UTF-16 or UTF-32, whichever fits in the target's

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@code{wchar_t} type, in the target machine's byte

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order.@footnote{UTF-16 does not meet the requirements of the C

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standard for a wide character set, but the choice of 16-bit

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@code{wchar_t} is enshrined in some system ABIs so we cannot fix

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this.} Octal and hexadecimal escape sequences do not undergo

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conversion; @t{'\x12'} has the value 0x12 regardless of the currently

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selected execution character set. All other escapes are replaced by

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the character in the source character set that they represent, then

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converted to the execution character set, just like unescaped

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characters.

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In identifiers, characters outside the ASCII range can only be

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specified with the @samp{\u} and @samp{\U} escapes, not used

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directly. If strict ISO C90 conformance is specified with an option

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such as @option{-std=c90}, or @option{-fno-extended-identifiers} is

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used, then those escapes are not permitted in identifiers.

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@node Initial processing

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@section Initial processing

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The preprocessor performs a series of textual transformations on its

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input. These happen before all other processing. Conceptually, they

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happen in a rigid order, and the entire file is run through each

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transformation before the next one begins. CPP actually does them

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all at once, for performance reasons. These transformations correspond

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roughly to the first three ``phases of translation'' described in the C

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standard.

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@enumerate

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@item

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@cindex line endings

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The input file is read into memory and broken into lines.

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Different systems use different conventions to indicate the end of a

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line. GCC accepts the ASCII control sequences @kbd{LF}, @kbd{@w{CR

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LF}} and @kbd{CR} as end-of-line markers. These are the canonical

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sequences used by Unix, DOS and VMS, and the classic Mac OS (before

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OSX) respectively. You may therefore safely copy source code written

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on any of those systems to a different one and use it without

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conversion. (GCC may lose track of the current line number if a file

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doesn't consistently use one convention, as sometimes happens when it

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is edited on computers with different conventions that share a network

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file system.)

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If the last line of any input file lacks an end-of-line marker, the end

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of the file is considered to implicitly supply one. The C standard says

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that this condition provokes undefined behavior, so GCC will emit a

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warning message.

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@item

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@cindex trigraphs

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@anchor{trigraphs}If trigraphs are enabled, they are replaced by their

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corresponding single characters. By default GCC ignores trigraphs,

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but if you request a strictly conforming mode with the @option{-std}

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option, or you specify the @option{-trigraphs} option, then it

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converts them.

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These are nine three-character sequences, all starting with @samp{??},

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that are defined by ISO C to stand for single characters. They permit

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obsolete systems that lack some of C's punctuation to use C@. For

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example, @samp{??/} stands for @samp{\}, so @t{'??/n'} is a character

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constant for a newline.

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Trigraphs are not popular and many compilers implement them

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incorrectly. Portable code should not rely on trigraphs being either

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converted or ignored. With @option{-Wtrigraphs} GCC will warn you

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when a trigraph may change the meaning of your program if it were

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converted. @xref{Wtrigraphs}.

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In a string constant, you can prevent a sequence of question marks

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from being confused with a trigraph by inserting a backslash between

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the question marks, or by separating the string literal at the

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trigraph and making use of string literal concatenation. @t{"(??\?)"}

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is the string @samp{(???)}, not @samp{(?]}. Traditional C compilers

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do not recognize these idioms.

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The nine trigraphs and their replacements are

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@smallexample

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Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-

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Replacement: [ ] @{ @} # \ ^ | ~

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@end smallexample

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@item

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@cindex continued lines

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@cindex backslash-newline

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Continued lines are merged into one long line.

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A continued line is a line which ends with a backslash, @samp{\}. The

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backslash is removed and the following line is joined with the current

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one. No space is inserted, so you may split a line anywhere, even in

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the middle of a word. (It is generally more readable to split lines

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only at white space.)

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The trailing backslash on a continued line is commonly referred to as a

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@dfn{backslash-newline}.

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If there is white space between a backslash and the end of a line, that

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is still a continued line. However, as this is usually the result of an

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editing mistake, and many compilers will not accept it as a continued

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line, GCC will warn you about it.

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@item

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@cindex comments

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@cindex line comments

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@cindex block comments

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All comments are replaced with single spaces.

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There are two kinds of comments. @dfn{Block comments} begin with

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@samp{/*} and continue until the next @samp{*/}. Block comments do not

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nest:

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@smallexample

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/* @r{this is} /* @r{one comment} */ @r{text outside comment}

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@end smallexample

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@dfn{Line comments} begin with @samp{//} and continue to the end of the

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current line. Line comments do not nest either, but it does not matter,

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because they would end in the same place anyway.

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@smallexample

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// @r{this is} // @r{one comment}

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@r{text outside comment}

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@end smallexample

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@end enumerate

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It is safe to put line comments inside block comments, or vice versa.

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@smallexample

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@group

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/* @r{block comment}

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// @r{contains line comment}

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@r{yet more comment}

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*/ @r{outside comment}

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// @r{line comment} /* @r{contains block comment} */

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@end group

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@end smallexample

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But beware of commenting out one end of a block comment with a line

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comment.

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@smallexample

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@group

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// @r{l.c.} /* @r{block comment begins}

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@r{oops! this isn't a comment anymore} */

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@end group

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@end smallexample

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Comments are not recognized within string literals.

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@t{@w{"/* blah */"}} is the string constant @samp{@w{/* blah */}}, not

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an empty string.

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Line comments are not in the 1989 edition of the C standard, but they

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are recognized by GCC as an extension. In C++ and in the 1999 edition

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of the C standard, they are an official part of the language.

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Since these transformations happen before all other processing, you can

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split a line mechanically with backslash-newline anywhere. You can

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comment out the end of a line. You can continue a line comment onto the

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next line with backslash-newline. You can even split @samp{/*},

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@samp{*/}, and @samp{//} onto multiple lines with backslash-newline.

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For example:

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@smallexample

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@group

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/\

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*

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*/ # /*

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*/ defi\

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ne FO\

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O 10\

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20

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@end group

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@end smallexample

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@noindent

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is equivalent to @code{@w{#define FOO 1020}}. All these tricks are

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extremely confusing and should not be used in code intended to be

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readable.

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There is no way to prevent a backslash at the end of a line from being

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interpreted as a backslash-newline. This cannot affect any correct

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program, however.

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@node Tokenization

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@section Tokenization

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@cindex tokens

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@cindex preprocessing tokens

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After the textual transformations are finished, the input file is

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converted into a sequence of @dfn{preprocessing tokens}. These mostly

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correspond to the syntactic tokens used by the C compiler, but there are

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a few differences. White space separates tokens; it is not itself a

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token of any kind. Tokens do not have to be separated by white space,

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but it is often necessary to avoid ambiguities.

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When faced with a sequence of characters that has more than one possible

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tokenization, the preprocessor is greedy. It always makes each token,

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starting from the left, as big as possible before moving on to the next

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token. For instance, @code{a+++++b} is interpreted as

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@code{@w{a ++ ++ + b}}, not as @code{@w{a ++ + ++ b}}, even though the

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latter tokenization could be part of a valid C program and the former

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could not.

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Once the input file is broken into tokens, the token boundaries never

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change, except when the @samp{##} preprocessing operator is used to paste

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tokens together. @xref{Concatenation}. For example,

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@smallexample

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@group

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#define foo() bar

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foo()baz

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@expansion{} bar baz

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@emph{not}

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@expansion{} barbaz

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@end group

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@end smallexample

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The compiler does not re-tokenize the preprocessor's output. Each

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preprocessing token becomes one compiler token.

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@cindex identifiers

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Preprocessing tokens fall into five broad classes: identifiers,

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preprocessing numbers, string literals, punctuators, and other. An

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@dfn{identifier} is the same as an identifier in C: any sequence of

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letters, digits, or underscores, which begins with a letter or

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underscore. Keywords of C have no significance to the preprocessor;

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they are ordinary identifiers. You can define a macro whose name is a

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keyword, for instance. The only identifier which can be considered a

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preprocessing keyword is @code{defined}. @xref{Defined}.

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This is mostly true of other languages which use the C preprocessor.

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However, a few of the keywords of C++ are significant even in the

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preprocessor. @xref{C++ Named Operators}.

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In the 1999 C standard, identifiers may contain letters which are not

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part of the ``basic source character set'', at the implementation's

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discretion (such as accented Latin letters, Greek letters, or Chinese

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ideograms). This may be done with an extended character set, or the

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@samp{\u} and @samp{\U} escape sequences. GCC only accepts such

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characters in the @samp{\u} and @samp{\U} forms.

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As an extension, GCC treats @samp{$} as a letter. This is for

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compatibility with some systems, such as VMS, where @samp{$} is commonly

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used in system-defined function and object names. @samp{$} is not a

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letter in strictly conforming mode, or if you specify the @option{-$}

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option. @xref{Invocation}.

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@cindex numbers

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@cindex preprocessing numbers

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A @dfn{preprocessing number} has a rather bizarre definition. The

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category includes all the normal integer and floating point constants

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one expects of C, but also a number of other things one might not

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initially recognize as a number. Formally, preprocessing numbers begin

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with an optional period, a required decimal digit, and then continue

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with any sequence of letters, digits, underscores, periods, and

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exponents. Exponents are the two-character sequences @samp{e+},

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@samp{e-}, @samp{E+}, @samp{E-}, @samp{p+}, @samp{p-}, @samp{P+}, and

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@samp{P-}. (The exponents that begin with @samp{p} or @samp{P} are new

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to C99. They are used for hexadecimal floating-point constants.)

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The purpose of this unusual definition is to isolate the preprocessor

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from the full complexity of numeric constants. It does not have to

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distinguish between lexically valid and invalid floating-point numbers,

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which is complicated. The definition also permits you to split an

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identifier at any position and get exactly two tokens, which can then be

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pasted back together with the @samp{##} operator.

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It's possible for preprocessing numbers to cause programs to be

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misinterpreted. For example, @code{0xE+12} is a preprocessing number

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which does not translate to any valid numeric constant, therefore a

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syntax error. It does not mean @code{@w{0xE + 12}}, which is what you

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might have intended.

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@cindex string literals

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@cindex string constants

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@cindex character constants

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@cindex header file names

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@c the @: prevents makeinfo from turning '' into ".

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@dfn{String literals} are string constants, character constants, and

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header file names (the argument of @samp{#include}).@footnote{The C

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standard uses the term @dfn{string literal} to refer only to what we are

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calling @dfn{string constants}.} String constants and character

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constants are straightforward: @t{"@dots{}"} or @t{'@dots{}'}. In

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either case embedded quotes should be escaped with a backslash:

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@t{'\'@:'} is the character constant for @samp{'}. There is no limit on

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the length of a character constant, but the value of a character

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constant that contains more than one character is

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implementation-defined. @xref{Implementation Details}.

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Header file names either look like string constants, @t{"@dots{}"}, or are

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written with angle brackets instead, @t{<@dots{}>}. In either case,

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backslash is an ordinary character. There is no way to escape the

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closing quote or angle bracket. The preprocessor looks for the header

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file in different places depending on which form you use. @xref{Include

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Operation}.

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No string literal may extend past the end of a line. Older versions

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of GCC accepted multi-line string constants. You may use continued

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lines instead, or string constant concatenation. @xref{Differences

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from previous versions}.

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@cindex punctuators

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@cindex digraphs

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@cindex alternative tokens

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@dfn{Punctuators} are all the usual bits of punctuation which are

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meaningful to C and C++. All but three of the punctuation characters in

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ASCII are C punctuators. The exceptions are @samp{@@}, @samp{$}, and

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@samp{`}. In addition, all the two- and three-character operators are

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punctuators. There are also six @dfn{digraphs}, which the C++ standard

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calls @dfn{alternative tokens}, which are merely alternate ways to spell

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other punctuators. This is a second attempt to work around missing

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punctuation in obsolete systems. It has no negative side effects,

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unlike trigraphs, but does not cover as much ground. The digraphs and

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their corresponding normal punctuators are:

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@smallexample

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Digraph: <% %> <: :> %: %:%:

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Punctuator: @{ @} [ ] # ##

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@end smallexample

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@cindex other tokens

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Any other single character is considered ``other''. It is passed on to

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the preprocessor's output unmolested. The C compiler will almost

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certainly reject source code containing ``other'' tokens. In ASCII, the

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only other characters are @samp{@@}, @samp{$}, @samp{`}, and control

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characters other than NUL (all bits zero). (Note that @samp{$} is

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normally considered a letter.) All characters with the high bit set

596

(numeric range 0x7F--0xFF) are also ``other'' in the present

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implementation. This will change when proper support for international

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character sets is added to GCC@.

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NUL is a special case because of the high probability that its

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appearance is accidental, and because it may be invisible to the user

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(many terminals do not display NUL at all). Within comments, NULs are

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silently ignored, just as any other character would be. In running

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text, NUL is considered white space. For example, these two directives

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have the same meaning.

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@smallexample

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#define X^@@1

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#define X 1

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@end smallexample

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@noindent

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(where @samp{^@@} is ASCII NUL)@. Within string or character constants,

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NULs are preserved. In the latter two cases the preprocessor emits a

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warning message.

616

617

@node The preprocessing language

618

@section The preprocessing language

619

@cindex directives

620

@cindex preprocessing directives

621

@cindex directive line

622

@cindex directive name

623

624

After tokenization, the stream of tokens may simply be passed straight

625

to the compiler's parser. However, if it contains any operations in the

626

@dfn{preprocessing language}, it will be transformed first. This stage

627

corresponds roughly to the standard's ``translation phase 4'' and is

628

what most people think of as the preprocessor's job.

629

630

The preprocessing language consists of @dfn{directives} to be executed

631

and @dfn{macros} to be expanded. Its primary capabilities are:

632

633

@itemize @bullet

634

@item

635

Inclusion of header files. These are files of declarations that can be

636

substituted into your program.

637

638

@item

639

Macro expansion. You can define @dfn{macros}, which are abbreviations

640

for arbitrary fragments of C code. The preprocessor will replace the

641

macros with their definitions throughout the program. Some macros are

642

automatically defined for you.

643

644

@item

645

Conditional compilation. You can include or exclude parts of the

646

program according to various conditions.

647

648

@item

649

Line control. If you use a program to combine or rearrange source files

650

into an intermediate file which is then compiled, you can use line

651

control to inform the compiler where each source line originally came

652

from.

653

654

@item

655

Diagnostics. You can detect problems at compile time and issue errors

656

or warnings.

657

@end itemize

658

659

There are a few more, less useful, features.

660

661

Except for expansion of predefined macros, all these operations are

662

triggered with @dfn{preprocessing directives}. Preprocessing directives

663

are lines in your program that start with @samp{#}. Whitespace is

664

allowed before and after the @samp{#}. The @samp{#} is followed by an

665

identifier, the @dfn{directive name}. It specifies the operation to

666

perform. Directives are commonly referred to as @samp{#@var{name}}

667

where @var{name} is the directive name. For example, @samp{#define} is

668

the directive that defines a macro.

669

670

The @samp{#} which begins a directive cannot come from a macro

671

expansion. Also, the directive name is not macro expanded. Thus, if

672

@code{foo} is defined as a macro expanding to @code{define}, that does

673

not make @samp{#foo} a valid preprocessing directive.

674

675

The set of valid directive names is fixed. Programs cannot define new

676

preprocessing directives.

677

678

Some directives require arguments; these make up the rest of the

679

directive line and must be separated from the directive name by

680

whitespace. For example, @samp{#define} must be followed by a macro

681

name and the intended expansion of the macro.

682

683

A preprocessing directive cannot cover more than one line. The line

684

may, however, be continued with backslash-newline, or by a block comment

685

which extends past the end of the line. In either case, when the

686

directive is processed, the continuations have already been merged with

687

the first line to make one long line.

688

689

@node Header Files

690

@chapter Header Files

691

692

@cindex header file

693

A header file is a file containing C declarations and macro definitions

694

(@pxref{Macros}) to be shared between several source files. You request

695

the use of a header file in your program by @dfn{including} it, with the

696

C preprocessing directive @samp{#include}.

697

698

Header files serve two purposes.

699

700

@itemize @bullet

701

@item

702

@cindex system header files

703

System header files declare the interfaces to parts of the operating

704

system. You include them in your program to supply the definitions and

705

declarations you need to invoke system calls and libraries.

706

707

@item

708

Your own header files contain declarations for interfaces between the

709

source files of your program. Each time you have a group of related

710

declarations and macro definitions all or most of which are needed in

711

several different source files, it is a good idea to create a header

712

file for them.

713

@end itemize

714

715

Including a header file produces the same results as copying the header

716

file into each source file that needs it. Such copying would be

717

time-consuming and error-prone. With a header file, the related

718

declarations appear in only one place. If they need to be changed, they

719

can be changed in one place, and programs that include the header file

720

will automatically use the new version when next recompiled. The header

721

file eliminates the labor of finding and changing all the copies as well

722

as the risk that a failure to find one copy will result in

723

inconsistencies within a program.

724

725

In C, the usual convention is to give header files names that end with

726

@file{.h}. It is most portable to use only letters, digits, dashes, and

727

underscores in header file names, and at most one dot.

728

729

@menu

730

* Include Syntax::

731

* Include Operation::

732

* Search Path::

733

* Once-Only Headers::

734

* Alternatives to Wrapper #ifndef::

735

* Computed Includes::

736

* Wrapper Headers::

737

* System Headers::

738

@end menu

739

740

@node Include Syntax

741

@section Include Syntax

742

743

@findex #include

744

Both user and system header files are included using the preprocessing

745

directive @samp{#include}. It has two variants:

746

747

@table @code

748

@item #include <@var{file}>

749

This variant is used for system header files. It searches for a file

750

named @var{file} in a standard list of system directories. You can prepend

751

directories to this list with the @option{-I} option (@pxref{Invocation}).

752

753

@item #include "@var{file}"

754

This variant is used for header files of your own program. It

755

searches for a file named @var{file} first in the directory containing

756

the current file, then in the quote directories and then the same

757

directories used for @code{<@var{file}>}. You can prepend directories

758

to the list of quote directories with the @option{-iquote} option.

759

@end table

760

761

The argument of @samp{#include}, whether delimited with quote marks or

762

angle brackets, behaves like a string constant in that comments are not

763

recognized, and macro names are not expanded. Thus, @code{@w{#include

764

<x/*y>}} specifies inclusion of a system header file named @file{x/*y}.

765

766

However, if backslashes occur within @var{file}, they are considered

767

ordinary text characters, not escape characters. None of the character

768

escape sequences appropriate to string constants in C are processed.

769

Thus, @code{@w{#include "x\n\\y"}} specifies a filename containing three

770

backslashes. (Some systems interpret @samp{\} as a pathname separator.

771

All of these also interpret @samp{/} the same way. It is most portable

772

to use only @samp{/}.)

773

774

It is an error if there is anything (other than comments) on the line

775

after the file name.

776

777

@node Include Operation

778

@section Include Operation

779

780

The @samp{#include} directive works by directing the C preprocessor to

781

scan the specified file as input before continuing with the rest of the

782

current file. The output from the preprocessor contains the output

783

already generated, followed by the output resulting from the included

784

file, followed by the output that comes from the text after the

785

@samp{#include} directive. For example, if you have a header file

786

@file{header.h} as follows,

787

788

@smallexample

789

char *test (void);

790

@end smallexample

791

792

@noindent

793

and a main program called @file{program.c} that uses the header file,

794

like this,

795

796

@smallexample

797

int x;

798

#include "header.h"

799

800

int

801

main (void)

802

@{

803

puts (test ());

804

@}

805

@end smallexample

806

807

@noindent

808

the compiler will see the same token stream as it would if

809

@file{program.c} read

810

811

@smallexample

812

int x;

813

char *test (void);

814

815

int

816

main (void)

817

@{

818

puts (test ());

819

@}

820

@end smallexample

821

822

Included files are not limited to declarations and macro definitions;

823

those are merely the typical uses. Any fragment of a C program can be

824

included from another file. The include file could even contain the

825

beginning of a statement that is concluded in the containing file, or

826

the end of a statement that was started in the including file. However,

827

an included file must consist of complete tokens. Comments and string

828

literals which have not been closed by the end of an included file are

829

invalid. For error recovery, they are considered to end at the end of

830

the file.

831

832

To avoid confusion, it is best if header files contain only complete

833

syntactic units---function declarations or definitions, type

834

declarations, etc.

835

836

The line following the @samp{#include} directive is always treated as a

837

separate line by the C preprocessor, even if the included file lacks a

838

final newline.

839

840

@node Search Path

841

@section Search Path

842

843

GCC looks in several different places for headers. On a normal Unix

844

system, if you do not instruct it otherwise, it will look for headers

845

requested with @code{@w{#include <@var{file}>}} in:

846

847

@smallexample

848

/usr/local/include

849

@var{libdir}/gcc/@var{target}/@var{version}/include

850

/usr/@var{target}/include

851

/usr/include

852

@end smallexample

853

854

For C++ programs, it will also look in

855

@file{@var{libdir}/../include/c++/@var{version}},

856

first. In the above, @var{target} is the canonical name of the system

857

GCC was configured to compile code for; often but not always the same as

858

the canonical name of the system it runs on. @var{version} is the

859

version of GCC in use.

860

861

You can add to this list with the @option{-I@var{dir}} command-line

862

option. All the directories named by @option{-I} are searched, in

863

left-to-right order, @emph{before} the default directories. The only

864

exception is when @file{dir} is already searched by default. In

865

this case, the option is ignored and the search order for system

866

directories remains unchanged.

867

868

Duplicate directories are removed from the quote and bracket search

869

chains before the two chains are merged to make the final search chain.

870

Thus, it is possible for a directory to occur twice in the final search

871

chain if it was specified in both the quote and bracket chains.

872

873

You can prevent GCC from searching any of the default directories with

874

the @option{-nostdinc} option. This is useful when you are compiling an

875

operating system kernel or some other program that does not use the

876

standard C library facilities, or the standard C library itself.

877

@option{-I} options are not ignored as described above when

878

@option{-nostdinc} is in effect.

879

880

GCC looks for headers requested with @code{@w{#include "@var{file}"}}

881

first in the directory containing the current file, then in the

882

directories as specified by @option{-iquote} options, then in the same

883

places it would have looked for a header requested with angle

884

brackets. For example, if @file{/usr/include/sys/stat.h} contains

885

@code{@w{#include "types.h"}}, GCC looks for @file{types.h} first in

886

@file{/usr/include/sys}, then in its usual search path.

887

888

@samp{#line} (@pxref{Line Control}) does not change GCC's idea of the

889

directory containing the current file.

890

891

You may put @option{-I-} at any point in your list of @option{-I} options.

892

This has two effects. First, directories appearing before the

893

@option{-I-} in the list are searched only for headers requested with

894

quote marks. Directories after @option{-I-} are searched for all

895

headers. Second, the directory containing the current file is not

896

searched for anything, unless it happens to be one of the directories

897

named by an @option{-I} switch. @option{-I-} is deprecated, @option{-iquote}

898

should be used instead.

899

900

@option{-I. -I-} is not the same as no @option{-I} options at all, and does

901

not cause the same behavior for @samp{<>} includes that @samp{""}

902

includes get with no special options. @option{-I.} searches the

903

compiler's current working directory for header files. That may or may

904

not be the same as the directory containing the current file.

905

906

If you need to look for headers in a directory named @file{-}, write

907

@option{-I./-}.

908

909

There are several more ways to adjust the header search path. They are

910

generally less useful. @xref{Invocation}.

911

912

@node Once-Only Headers

913

@section Once-Only Headers

914

@cindex repeated inclusion

915

@cindex including just once

916

@cindex wrapper @code{#ifndef}

917

918

If a header file happens to be included twice, the compiler will process

919

its contents twice. This is very likely to cause an error, e.g.@: when the

920

compiler sees the same structure definition twice. Even if it does not,

921

it will certainly waste time.

922

923

The standard way to prevent this is to enclose the entire real contents

924

of the file in a conditional, like this:

925

926

@smallexample

927

@group

928

/* File foo. */

929

#ifndef FILE_FOO_SEEN

930

#define FILE_FOO_SEEN

931

932

@var{the entire file}

933

934

#endif /* !FILE_FOO_SEEN */

935

@end group

936

@end smallexample

937

938

This construct is commonly known as a @dfn{wrapper #ifndef}.

939

When the header is included again, the conditional will be false,

940

because @code{FILE_FOO_SEEN} is defined. The preprocessor will skip

941

over the entire contents of the file, and the compiler will not see it

942

twice.

943

944

CPP optimizes even further. It remembers when a header file has a

945

wrapper @samp{#ifndef}. If a subsequent @samp{#include} specifies that

946

header, and the macro in the @samp{#ifndef} is still defined, it does

947

not bother to rescan the file at all.

948

949

You can put comments outside the wrapper. They will not interfere with

950

this optimization.

951

952

@cindex controlling macro

953

@cindex guard macro

954

The macro @code{FILE_FOO_SEEN} is called the @dfn{controlling macro} or

955

@dfn{guard macro}. In a user header file, the macro name should not

956

begin with @samp{_}. In a system header file, it should begin with

957

@samp{__} to avoid conflicts with user programs. In any kind of header

958

file, the macro name should contain the name of the file and some

959

additional text, to avoid conflicts with other header files.

960

961

@node Alternatives to Wrapper #ifndef

962

@section Alternatives to Wrapper #ifndef

963

964

CPP supports two more ways of indicating that a header file should be

965

read only once. Neither one is as portable as a wrapper @samp{#ifndef}

966

and we recommend you do not use them in new programs, with the caveat

967

that @samp{#import} is standard practice in Objective-C.

968

969

@findex #import

970

CPP supports a variant of @samp{#include} called @samp{#import} which

971

includes a file, but does so at most once. If you use @samp{#import}

972

instead of @samp{#include}, then you don't need the conditionals

973

inside the header file to prevent multiple inclusion of the contents.

974

@samp{#import} is standard in Objective-C, but is considered a

975

deprecated extension in C and C++.

976

977

@samp{#import} is not a well designed feature. It requires the users of

978

a header file to know that it should only be included once. It is much

979

better for the header file's implementor to write the file so that users

980

don't need to know this. Using a wrapper @samp{#ifndef} accomplishes

981

this goal.

982

983

In the present implementation, a single use of @samp{#import} will

984

prevent the file from ever being read again, by either @samp{#import} or

985

@samp{#include}. You should not rely on this; do not use both

986

@samp{#import} and @samp{#include} to refer to the same header file.

987

988

Another way to prevent a header file from being included more than once

989

is with the @samp{#pragma once} directive. If @samp{#pragma once} is

990

seen when scanning a header file, that file will never be read again, no

991

matter what.

992

993

@samp{#pragma once} does not have the problems that @samp{#import} does,

994

but it is not recognized by all preprocessors, so you cannot rely on it

995

in a portable program.

996

997

@node Computed Includes

998

@section Computed Includes

999

@cindex computed includes

1000

@cindex macros in include

1001

1002

Sometimes it is necessary to select one of several different header

1003

files to be included into your program. They might specify

1004

configuration parameters to be used on different sorts of operating

1005

systems, for instance. You could do this with a series of conditionals,

1006

1007

@smallexample

1008

#if SYSTEM_1

1009

# include "system_1.h"

1010

#elif SYSTEM_2

1011

# include "system_2.h"

1012

#elif SYSTEM_3

1013

@dots{}

1014

#endif

1015

@end smallexample

1016

1017

That rapidly becomes tedious. Instead, the preprocessor offers the

1018

ability to use a macro for the header name. This is called a

1019

@dfn{computed include}. Instead of writing a header name as the direct

1020

argument of @samp{#include}, you simply put a macro name there instead:

1021

1022

@smallexample

1023

#define SYSTEM_H "system_1.h"

1024

@dots{}

1025

#include SYSTEM_H

1026

@end smallexample

1027

1028

@noindent

1029

@code{SYSTEM_H} will be expanded, and the preprocessor will look for

1030

@file{system_1.h} as if the @samp{#include} had been written that way

1031

originally. @code{SYSTEM_H} could be defined by your Makefile with a

1032

@option{-D} option.

1033

1034

You must be careful when you define the macro. @samp{#define} saves

1035

tokens, not text. The preprocessor has no way of knowing that the macro

1036

will be used as the argument of @samp{#include}, so it generates

1037

ordinary tokens, not a header name. This is unlikely to cause problems

1038

if you use double-quote includes, which are close enough to string

1039

constants. If you use angle brackets, however, you may have trouble.

1040

1041

The syntax of a computed include is actually a bit more general than the

1042

above. If the first non-whitespace character after @samp{#include} is

1043

not @samp{"} or @samp{<}, then the entire line is macro-expanded

1044

like running text would be.

1045

1046

If the line expands to a single string constant, the contents of that

1047

string constant are the file to be included. CPP does not re-examine the

1048

string for embedded quotes, but neither does it process backslash

1049

escapes in the string. Therefore

1050

1051

@smallexample

1052

#define HEADER "a\"b"

1053

#include HEADER

1054

@end smallexample

1055

1056

@noindent

1057

looks for a file named @file{a\"b}. CPP searches for the file according

1058

to the rules for double-quoted includes.

1059

1060

If the line expands to a token stream beginning with a @samp{<} token

1061

and including a @samp{>} token, then the tokens between the @samp{<} and

1062

the first @samp{>} are combined to form the filename to be included.

1063

Any whitespace between tokens is reduced to a single space; then any

1064

space after the initial @samp{<} is retained, but a trailing space

1065

before the closing @samp{>} is ignored. CPP searches for the file

1066

according to the rules for angle-bracket includes.

1067

1068

In either case, if there are any tokens on the line after the file name,

1069

an error occurs and the directive is not processed. It is also an error

1070

if the result of expansion does not match either of the two expected

1071

forms.

1072

1073

These rules are implementation-defined behavior according to the C

1074

standard. To minimize the risk of different compilers interpreting your

1075

computed includes differently, we recommend you use only a single

1076

object-like macro which expands to a string constant. This will also

1077

minimize confusion for people reading your program.

1078

1079

@node Wrapper Headers

1080

@section Wrapper Headers

1081

@cindex wrapper headers

1082

@cindex overriding a header file

1083

@findex #include_next

1084

1085

Sometimes it is necessary to adjust the contents of a system-provided

1086

header file without editing it directly. GCC's @command{fixincludes}

1087

operation does this, for example. One way to do that would be to create

1088

a new header file with the same name and insert it in the search path

1089

before the original header. That works fine as long as you're willing

1090

to replace the old header entirely. But what if you want to refer to

1091

the old header from the new one?

1092

1093

You cannot simply include the old header with @samp{#include}. That

1094

will start from the beginning, and find your new header again. If your

1095

header is not protected from multiple inclusion (@pxref{Once-Only

1096

Headers}), it will recurse infinitely and cause a fatal error.

1097

1098

You could include the old header with an absolute pathname:

1099

@smallexample

1100

#include "/usr/include/old-header.h"

1101

@end smallexample

1102

@noindent

1103

This works, but is not clean; should the system headers ever move, you

1104

would have to edit the new headers to match.

1105

1106

There is no way to solve this problem within the C standard, but you can

1107

use the GNU extension @samp{#include_next}. It means, ``Include the

1108

@emph{next} file with this name''. This directive works like

1109

@samp{#include} except in searching for the specified file: it starts

1110

searching the list of header file directories @emph{after} the directory

1111

in which the current file was found.

1112

1113

Suppose you specify @option{-I /usr/local/include}, and the list of

1114

directories to search also includes @file{/usr/include}; and suppose

1115

both directories contain @file{signal.h}. Ordinary @code{@w{#include

1116

<signal.h>}} finds the file under @file{/usr/local/include}. If that

1117

file contains @code{@w{#include_next <signal.h>}}, it starts searching

1118

after that directory, and finds the file in @file{/usr/include}.

1119

1120

@samp{#include_next} does not distinguish between @code{<@var{file}>}

1121

and @code{"@var{file}"} inclusion, nor does it check that the file you

1122

specify has the same name as the current file. It simply looks for the

1123

file named, starting with the directory in the search path after the one

1124

where the current file was found.

1125

1126

The use of @samp{#include_next} can lead to great confusion. We

1127

recommend it be used only when there is no other alternative. In

1128

particular, it should not be used in the headers belonging to a specific

1129

program; it should be used only to make global corrections along the

1130

lines of @command{fixincludes}.

1131

1132

@node System Headers

1133

@section System Headers

1134

@cindex system header files

1135

1136

The header files declaring interfaces to the operating system and

1137

runtime libraries often cannot be written in strictly conforming C@.

1138

Therefore, GCC gives code found in @dfn{system headers} special

1139

treatment. All warnings, other than those generated by @samp{#warning}

1140

(@pxref{Diagnostics}), are suppressed while GCC is processing a system

1141

header. Macros defined in a system header are immune to a few warnings

1142

wherever they are expanded. This immunity is granted on an ad-hoc

1143

basis, when we find that a warning generates lots of false positives

1144

because of code in macros defined in system headers.

1145

1146

Normally, only the headers found in specific directories are considered

1147

system headers. These directories are determined when GCC is compiled.

1148

There are, however, two ways to make normal headers into system headers.

1149

1150

The @option{-isystem} command-line option adds its argument to the list of

1151

directories to search for headers, just like @option{-I}. Any headers

1152

found in that directory will be considered system headers.

1153

1154

All directories named by @option{-isystem} are searched @emph{after} all

1155

directories named by @option{-I}, no matter what their order was on the

1156

command line. If the same directory is named by both @option{-I} and

1157

@option{-isystem}, the @option{-I} option is ignored. GCC provides an

1158

informative message when this occurs if @option{-v} is used.

1159

1160

@findex #pragma GCC system_header

1161

There is also a directive, @code{@w{#pragma GCC system_header}}, which

1162

tells GCC to consider the rest of the current include file a system

1163

header, no matter where it was found. Code that comes before the

1164

@samp{#pragma} in the file will not be affected. @code{@w{#pragma GCC

1165

system_header}} has no effect in the primary source file.

1166

1167

On very old systems, some of the pre-defined system header directories

1168

get even more special treatment. GNU C++ considers code in headers

1169

found in those directories to be surrounded by an @code{@w{extern "C"}}

1170

block. There is no way to request this behavior with a @samp{#pragma},

1171

or from the command line.

1172

1173

@node Macros

1174

@chapter Macros

1175

1176

A @dfn{macro} is a fragment of code which has been given a name.

1177

Whenever the name is used, it is replaced by the contents of the macro.

1178

There are two kinds of macros. They differ mostly in what they look

1179

like when they are used. @dfn{Object-like} macros resemble data objects

1180

when used, @dfn{function-like} macros resemble function calls.

1181

1182

You may define any valid identifier as a macro, even if it is a C

1183

keyword. The preprocessor does not know anything about keywords. This

1184

can be useful if you wish to hide a keyword such as @code{const} from an

1185

older compiler that does not understand it. However, the preprocessor

1186

operator @code{defined} (@pxref{Defined}) can never be defined as a

1187

macro, and C++'s named operators (@pxref{C++ Named Operators}) cannot be

1188

macros when you are compiling C++.

1189

1190

@menu

1191

* Object-like Macros::

1192

* Function-like Macros::

1193

* Macro Arguments::

1194

* Stringification::

1195

* Concatenation::

1196

* Variadic Macros::

1197

* Predefined Macros::

1198

* Undefining and Redefining Macros::

1199

* Directives Within Macro Arguments::

1200

* Macro Pitfalls::

1201

@end menu

1202

1203

@node Object-like Macros

1204

@section Object-like Macros

1205

@cindex object-like macro

1206

@cindex symbolic constants

1207

@cindex manifest constants

1208

1209

An @dfn{object-like macro} is a simple identifier which will be replaced

1210

by a code fragment. It is called object-like because it looks like a

1211

data object in code that uses it. They are most commonly used to give

1212

symbolic names to numeric constants.

1213

1214

@findex #define

1215

You create macros with the @samp{#define} directive. @samp{#define} is

1216

followed by the name of the macro and then the token sequence it should

1217

be an abbreviation for, which is variously referred to as the macro's

1218

@dfn{body}, @dfn{expansion} or @dfn{replacement list}. For example,

1219

1220

@smallexample

1221

#define BUFFER_SIZE 1024

1222

@end smallexample

1223

1224

@noindent

1225

defines a macro named @code{BUFFER_SIZE} as an abbreviation for the

1226

token @code{1024}. If somewhere after this @samp{#define} directive

1227

there comes a C statement of the form

1228

1229

@smallexample

1230

foo = (char *) malloc (BUFFER_SIZE);

1231

@end smallexample

1232

1233

@noindent

1234

then the C preprocessor will recognize and @dfn{expand} the macro

1235

@code{BUFFER_SIZE}. The C compiler will see the same tokens as it would

1236

if you had written

1237

1238

@smallexample

1239

foo = (char *) malloc (1024);

1240

@end smallexample

1241

1242

By convention, macro names are written in uppercase. Programs are

1243

easier to read when it is possible to tell at a glance which names are

1244

macros.

1245

1246

The macro's body ends at the end of the @samp{#define} line. You may

1247

continue the definition onto multiple lines, if necessary, using

1248

backslash-newline. When the macro is expanded, however, it will all

1249

come out on one line. For example,

1250

1251

@smallexample

1252

#define NUMBERS 1, \

1253

2, \

1254

3

1255

int x[] = @{ NUMBERS @};

1256

@expansion{} int x[] = @{ 1, 2, 3 @};

1257

@end smallexample

1258

1259

@noindent

1260

The most common visible consequence of this is surprising line numbers

1261

in error messages.

1262

1263

There is no restriction on what can go in a macro body provided it

1264

decomposes into valid preprocessing tokens. Parentheses need not

1265

balance, and the body need not resemble valid C code. (If it does not,

1266

you may get error messages from the C compiler when you use the macro.)

1267

1268

The C preprocessor scans your program sequentially. Macro definitions

1269

take effect at the place you write them. Therefore, the following input

1270

to the C preprocessor

1271

1272

@smallexample

1273

foo = X;

1274

#define X 4

1275

bar = X;

1276

@end smallexample

1277

1278

@noindent

1279

produces

1280

1281

@smallexample

1282

foo = X;

1283

bar = 4;

1284

@end smallexample

1285

1286

When the preprocessor expands a macro name, the macro's expansion

1287

replaces the macro invocation, then the expansion is examined for more

1288

macros to expand. For example,

1289

1290

@smallexample

1291

@group

1292

#define TABLESIZE BUFSIZE

1293

#define BUFSIZE 1024

1294

TABLESIZE

1295

@expansion{} BUFSIZE

1296

@expansion{} 1024

1297

@end group

1298

@end smallexample

1299

1300

@noindent

1301

@code{TABLESIZE} is expanded first to produce @code{BUFSIZE}, then that

1302

macro is expanded to produce the final result, @code{1024}.

1303

1304

Notice that @code{BUFSIZE} was not defined when @code{TABLESIZE} was

1305

defined. The @samp{#define} for @code{TABLESIZE} uses exactly the

1306

expansion you specify---in this case, @code{BUFSIZE}---and does not

1307

check to see whether it too contains macro names. Only when you

1308

@emph{use} @code{TABLESIZE} is the result of its expansion scanned for

1309

more macro names.

1310

1311

This makes a difference if you change the definition of @code{BUFSIZE}

1312

at some point in the source file. @code{TABLESIZE}, defined as shown,

1313

will always expand using the definition of @code{BUFSIZE} that is

1314

currently in effect:

1315

1316

@smallexample

1317

#define BUFSIZE 1020

1318

#define TABLESIZE BUFSIZE

1319

#undef BUFSIZE

1320

#define BUFSIZE 37

1321

@end smallexample

1322

1323

@noindent

1324

Now @code{TABLESIZE} expands (in two stages) to @code{37}.

1325

1326

If the expansion of a macro contains its own name, either directly or

1327

via intermediate macros, it is not expanded again when the expansion is

1328

examined for more macros. This prevents infinite recursion.

1329

@xref{Self-Referential Macros}, for the precise details.

1330

1331

@node Function-like Macros

1332

@section Function-like Macros

1333

@cindex function-like macros

1334

1335

You can also define macros whose use looks like a function call. These

1336

are called @dfn{function-like macros}. To define a function-like macro,

1337

you use the same @samp{#define} directive, but you put a pair of

1338

parentheses immediately after the macro name. For example,

1339

1340

@smallexample

1341

#define lang_init() c_init()

1342

lang_init()

1343

@expansion{} c_init()

1344

@end smallexample

1345

1346

A function-like macro is only expanded if its name appears with a pair

1347

of parentheses after it. If you write just the name, it is left alone.

1348

This can be useful when you have a function and a macro of the same

1349

name, and you wish to use the function sometimes.

1350

1351

@smallexample

1352

extern void foo(void);

1353

#define foo() /* @r{optimized inline version} */

1354

@dots{}

1355

foo();

1356

funcptr = foo;

1357

@end smallexample

1358

1359

Here the call to @code{foo()} will use the macro, but the function

1360

pointer will get the address of the real function. If the macro were to

1361

be expanded, it would cause a syntax error.

1362

1363

If you put spaces between the macro name and the parentheses in the

1364

macro definition, that does not define a function-like macro, it defines

1365

an object-like macro whose expansion happens to begin with a pair of

1366

parentheses.

1367

1368

@smallexample

1369

#define lang_init () c_init()

1370

lang_init()

1371

@expansion{} () c_init()()

1372

@end smallexample

1373

1374

The first two pairs of parentheses in this expansion come from the

1375

macro. The third is the pair that was originally after the macro

1376

invocation. Since @code{lang_init} is an object-like macro, it does not

1377

consume those parentheses.

1378

1379

@node Macro Arguments

1380

@section Macro Arguments

1381

@cindex arguments

1382

@cindex macros with arguments

1383

@cindex arguments in macro definitions

1384

1385

Function-like macros can take @dfn{arguments}, just like true functions.

1386

To define a macro that uses arguments, you insert @dfn{parameters}

1387

between the pair of parentheses in the macro definition that make the

1388

macro function-like. The parameters must be valid C identifiers,

1389

separated by commas and optionally whitespace.

1390

1391

To invoke a macro that takes arguments, you write the name of the macro

1392

followed by a list of @dfn{actual arguments} in parentheses, separated

1393

by commas. The invocation of the macro need not be restricted to a

1394

single logical line---it can cross as many lines in the source file as

1395

you wish. The number of arguments you give must match the number of

1396

parameters in the macro definition. When the macro is expanded, each

1397

use of a parameter in its body is replaced by the tokens of the

1398

corresponding argument. (You need not use all of the parameters in the

1399

macro body.)

1400

1401

As an example, here is a macro that computes the minimum of two numeric

1402

values, as it is defined in many C programs, and some uses.

1403

1404

@smallexample

1405

#define min(X, Y) ((X) < (Y) ? (X) : (Y))

1406

x = min(a, b); @expansion{} x = ((a) < (b) ? (a) : (b));

1407

y = min(1, 2); @expansion{} y = ((1) < (2) ? (1) : (2));

1408

z = min(a + 28, *p); @expansion{} z = ((a + 28) < (*p) ? (a + 28) : (*p));

1409

@end smallexample

1410

1411

@noindent

1412

(In this small example you can already see several of the dangers of

1413

macro arguments. @xref{Macro Pitfalls}, for detailed explanations.)

1414

1415

Leading and trailing whitespace in each argument is dropped, and all

1416

whitespace between the tokens of an argument is reduced to a single

1417

space. Parentheses within each argument must balance; a comma within

1418

such parentheses does not end the argument. However, there is no

1419

requirement for square brackets or braces to balance, and they do not

1420

prevent a comma from separating arguments. Thus,

1421

1422

@smallexample

1423

macro (array[x = y, x + 1])

1424

@end smallexample

1425

1426

@noindent

1427

passes two arguments to @code{macro}: @code{array[x = y} and @code{x +

1428

1]}. If you want to supply @code{array[x = y, x + 1]} as an argument,

1429

you can write it as @code{array[(x = y, x + 1)]}, which is equivalent C

1430

code.

1431

1432

All arguments to a macro are completely macro-expanded before they are

1433

substituted into the macro body. After substitution, the complete text

1434

is scanned again for macros to expand, including the arguments. This rule

1435

may seem strange, but it is carefully designed so you need not worry

1436

about whether any function call is actually a macro invocation. You can

1437

run into trouble if you try to be too clever, though. @xref{Argument

1438

Prescan}, for detailed discussion.

1439

1440

For example, @code{min (min (a, b), c)} is first expanded to

1441

1442

@smallexample

1443

min (((a) < (b) ? (a) : (b)), (c))

1444

@end smallexample

1445

1446

@noindent

1447

and then to

1448

1449

@smallexample

1450

@group

1451

((((a) < (b) ? (a) : (b))) < (c)

1452

? (((a) < (b) ? (a) : (b)))

1453

: (c))

1454

@end group

1455

@end smallexample

1456

1457

@noindent

1458

(Line breaks shown here for clarity would not actually be generated.)

1459

1460

@cindex empty macro arguments

1461

You can leave macro arguments empty; this is not an error to the

1462

preprocessor (but many macros will then expand to invalid code).

1463

You cannot leave out arguments entirely; if a macro takes two arguments,

1464

there must be exactly one comma at the top level of its argument list.

1465

Here are some silly examples using @code{min}:

1466

1467

@smallexample

1468

min(, b) @expansion{} (( ) < (b) ? ( ) : (b))

1469

min(a, ) @expansion{} ((a ) < ( ) ? (a ) : ( ))

1470

min(,) @expansion{} (( ) < ( ) ? ( ) : ( ))

1471

min((,),) @expansion{} (((,)) < ( ) ? ((,)) : ( ))

1472

1473

min() @error{} macro "min" requires 2 arguments, but only 1 given

1474

min(,,) @error{} macro "min" passed 3 arguments, but takes just 2

1475

@end smallexample

1476

1477

Whitespace is not a preprocessing token, so if a macro @code{foo} takes

1478

one argument, @code{@w{foo ()}} and @code{@w{foo ( )}} both supply it an

1479

empty argument. Previous GNU preprocessor implementations and

1480

documentation were incorrect on this point, insisting that a

1481

function-like macro that takes a single argument be passed a space if an

1482

empty argument was required.

1483

1484

Macro parameters appearing inside string literals are not replaced by

1485

their corresponding actual arguments.

1486

1487

@smallexample

1488

#define foo(x) x, "x"

1489

foo(bar) @expansion{} bar, "x"

1490

@end smallexample

1491

1492

@node Stringification

1493

@section Stringification

1494

@cindex stringification

1495

@cindex @samp{#} operator

1496

1497

Sometimes you may want to convert a macro argument into a string

1498

constant. Parameters are not replaced inside string constants, but you

1499

can use the @samp{#} preprocessing operator instead. When a macro

1500

parameter is used with a leading @samp{#}, the preprocessor replaces it

1501

with the literal text of the actual argument, converted to a string

1502

constant. Unlike normal parameter replacement, the argument is not

1503

macro-expanded first. This is called @dfn{stringification}.

1504

1505

There is no way to combine an argument with surrounding text and

1506

stringify it all together. Instead, you can write a series of adjacent

1507

string constants and stringified arguments. The preprocessor will

1508

replace the stringified arguments with string constants. The C

1509

compiler will then combine all the adjacent string constants into one

1510

long string.

1511

1512

Here is an example of a macro definition that uses stringification:

1513

1514

@smallexample

1515

@group

1516

#define WARN_IF(EXP) \

1517

do @{ if (EXP) \

1518

fprintf (stderr, "Warning: " #EXP "\n"); @} \

1519

while (0)

1520

WARN_IF (x == 0);

1521

@expansion{} do @{ if (x == 0)

1522

fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0);

1523

@end group

1524

@end smallexample

1525

1526

@noindent

1527

The argument for @code{EXP} is substituted once, as-is, into the

1528

@code{if} statement, and once, stringified, into the argument to

1529

@code{fprintf}. If @code{x} were a macro, it would be expanded in the

1530

@code{if} statement, but not in the string.

1531

1532

The @code{do} and @code{while (0)} are a kludge to make it possible to

1533

write @code{WARN_IF (@var{arg});}, which the resemblance of

1534

@code{WARN_IF} to a function would make C programmers want to do; see

1535

@ref{Swallowing the Semicolon}.

1536

1537

Stringification in C involves more than putting double-quote characters

1538

around the fragment. The preprocessor backslash-escapes the quotes

1539

surrounding embedded string constants, and all backslashes within string and

1540

character constants, in order to get a valid C string constant with the

1541

proper contents. Thus, stringifying @code{@w{p = "foo\n";}} results in

1542

@t{@w{"p = \"foo\\n\";"}}. However, backslashes that are not inside string

1543

or character constants are not duplicated: @samp{\n} by itself

1544

stringifies to @t{"\n"}.

1545

1546

All leading and trailing whitespace in text being stringified is

1547

ignored. Any sequence of whitespace in the middle of the text is

1548

converted to a single space in the stringified result. Comments are

1549

replaced by whitespace long before stringification happens, so they

1550

never appear in stringified text.

1551

1552

There is no way to convert a macro argument into a character constant.

1553

1554

If you want to stringify the result of expansion of a macro argument,

1555

you have to use two levels of macros.

1556

1557

@smallexample

1558

#define xstr(s) str(s)

1559

#define str(s) #s

1560

#define foo 4

1561

str (foo)

1562

@expansion{} "foo"

1563

xstr (foo)

1564

@expansion{} xstr (4)

1565

@expansion{} str (4)

1566

@expansion{} "4"

1567

@end smallexample

1568

1569

@code{s} is stringified when it is used in @code{str}, so it is not

1570

macro-expanded first. But @code{s} is an ordinary argument to

1571

@code{xstr}, so it is completely macro-expanded before @code{xstr}

1572

itself is expanded (@pxref{Argument Prescan}). Therefore, by the time

1573

@code{str} gets to its argument, it has already been macro-expanded.

1574

1575

@node Concatenation

1576

@section Concatenation

1577

@cindex concatenation

1578

@cindex token pasting

1579

@cindex token concatenation

1580

@cindex @samp{##} operator

1581

1582

It is often useful to merge two tokens into one while expanding macros.

1583

This is called @dfn{token pasting} or @dfn{token concatenation}. The

1584

@samp{##} preprocessing operator performs token pasting. When a macro

1585

is expanded, the two tokens on either side of each @samp{##} operator

1586

are combined into a single token, which then replaces the @samp{##} and

1587

the two original tokens in the macro expansion. Usually both will be

1588

identifiers, or one will be an identifier and the other a preprocessing

1589

number. When pasted, they make a longer identifier. This isn't the

1590

only valid case. It is also possible to concatenate two numbers (or a

1591

number and a name, such as @code{1.5} and @code{e3}) into a number.

1592

Also, multi-character operators such as @code{+=} can be formed by

1593

token pasting.

1594

1595

However, two tokens that don't together form a valid token cannot be

1596

pasted together. For example, you cannot concatenate @code{x} with

1597

@code{+} in either order. If you try, the preprocessor issues a warning

1598

and emits the two tokens. Whether it puts white space between the

1599

tokens is undefined. It is common to find unnecessary uses of @samp{##}

1600

in complex macros. If you get this warning, it is likely that you can

1601

simply remove the @samp{##}.

1602

1603

Both the tokens combined by @samp{##} could come from the macro body,

1604

but you could just as well write them as one token in the first place.

1605

Token pasting is most useful when one or both of the tokens comes from a

1606

macro argument. If either of the tokens next to an @samp{##} is a

1607

parameter name, it is replaced by its actual argument before @samp{##}

1608

executes. As with stringification, the actual argument is not

1609

macro-expanded first. If the argument is empty, that @samp{##} has no

1610

effect.

1611

1612

Keep in mind that the C preprocessor converts comments to whitespace

1613

before macros are even considered. Therefore, you cannot create a

1614

comment by concatenating @samp{/} and @samp{*}. You can put as much

1615

whitespace between @samp{##} and its operands as you like, including

1616

comments, and you can put comments in arguments that will be

1617

concatenated. However, it is an error if @samp{##} appears at either

1618

end of a macro body.

1619

1620

Consider a C program that interprets named commands. There probably

1621

needs to be a table of commands, perhaps an array of structures declared

1622

as follows:

1623

1624

@smallexample

1625

@group

1626

struct command

1627

@{

1628

char *name;

1629

void (*function) (void);

1630

@};

1631

@end group

1632

1633

@group

1634

struct command commands[] =

1635

@{

1636

@{ "quit", quit_command @},

1637

@{ "help", help_command @},

1638

@dots{}

1639

@};

1640

@end group

1641

@end smallexample

1642

1643

It would be cleaner not to have to give each command name twice, once in

1644

the string constant and once in the function name. A macro which takes the

1645

name of a command as an argument can make this unnecessary. The string

1646

constant can be created with stringification, and the function name by

1647

concatenating the argument with @samp{_command}. Here is how it is done:

1648

1649

@smallexample

1650

#define COMMAND(NAME) @{ #NAME, NAME ## _command @}

1651

1652

struct command commands[] =

1653

@{

1654

COMMAND (quit),

1655

COMMAND (help),

1656

@dots{}

1657

@};

1658

@end smallexample

1659

1660

@node Variadic Macros

1661

@section Variadic Macros

1662

@cindex variable number of arguments

1663

@cindex macros with variable arguments

1664

@cindex variadic macros

1665

1666

A macro can be declared to accept a variable number of arguments much as

1667

a function can. The syntax for defining the macro is similar to that of

1668

a function. Here is an example:

1669

1670

@smallexample

1671

#define eprintf(@dots{}) fprintf (stderr, __VA_ARGS__)

1672

@end smallexample

1673

1674

This kind of macro is called @dfn{variadic}. When the macro is invoked,

1675

all the tokens in its argument list after the last named argument (this

1676

macro has none), including any commas, become the @dfn{variable

1677

argument}. This sequence of tokens replaces the identifier

1678

@code{@w{__VA_ARGS__}} in the macro body wherever it appears. Thus, we

1679

have this expansion:

1680

1681

@smallexample

1682

eprintf ("%s:%d: ", input_file, lineno)

1683

@expansion{} fprintf (stderr, "%s:%d: ", input_file, lineno)

1684

@end smallexample

1685

1686

The variable argument is completely macro-expanded before it is inserted

1687

into the macro expansion, just like an ordinary argument. You may use

1688

the @samp{#} and @samp{##} operators to stringify the variable argument

1689

or to paste its leading or trailing token with another token. (But see

1690

below for an important special case for @samp{##}.)

1691

1692

If your macro is complicated, you may want a more descriptive name for

1693

the variable argument than @code{@w{__VA_ARGS__}}. CPP permits

1694

this, as an extension. You may write an argument name immediately

1695

before the @samp{@dots{}}; that name is used for the variable argument.

1696

The @code{eprintf} macro above could be written

1697

1698

@smallexample

1699

#define eprintf(args@dots{}) fprintf (stderr, args)

1700

@end smallexample

1701

1702

@noindent

1703

using this extension. You cannot use @code{@w{__VA_ARGS__}} and this

1704

extension in the same macro.

1705

1706

You can have named arguments as well as variable arguments in a variadic

1707

macro. We could define @code{eprintf} like this, instead:

1708

1709

@smallexample

1710

#define eprintf(format, @dots{}) fprintf (stderr, format, __VA_ARGS__)

1711

@end smallexample

1712

1713

@noindent

1714

This formulation looks more descriptive, but unfortunately it is less

1715

flexible: you must now supply at least one argument after the format

1716

string. In standard C, you cannot omit the comma separating the named

1717

argument from the variable arguments. Furthermore, if you leave the

1718

variable argument empty, you will get a syntax error, because

1719

there will be an extra comma after the format string.

1720

1721

@smallexample

1722

eprintf("success!\n", );

1723

@expansion{} fprintf(stderr, "success!\n", );

1724

@end smallexample

1725

1726

GNU CPP has a pair of extensions which deal with this problem. First,

1727

you are allowed to leave the variable argument out entirely:

1728

1729

@smallexample

1730

eprintf ("success!\n")

1731

@expansion{} fprintf(stderr, "success!\n", );

1732

@end smallexample

1733

1734

@noindent

1735

Second, the @samp{##} token paste operator has a special meaning when

1736

placed between a comma and a variable argument. If you write

1737

1738

@smallexample

1739

#define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__)

1740

@end smallexample

1741

1742

@noindent

1743

and the variable argument is left out when the @code{eprintf} macro is

1744

used, then the comma before the @samp{##} will be deleted. This does

1745

@emph{not} happen if you pass an empty argument, nor does it happen if

1746

the token preceding @samp{##} is anything other than a comma.

1747

1748

@smallexample

1749

eprintf ("success!\n")

1750

@expansion{} fprintf(stderr, "success!\n");

1751

@end smallexample

1752

1753

@noindent

1754

The above explanation is ambiguous about the case where the only macro

1755

parameter is a variable arguments parameter, as it is meaningless to

1756

try to distinguish whether no argument at all is an empty argument or

1757

a missing argument. In this case the C99 standard is clear that the

1758

comma must remain, however the existing GCC extension used to swallow

1759

the comma. So CPP retains the comma when conforming to a specific C

1760

standard, and drops it otherwise.

1761

1762

C99 mandates that the only place the identifier @code{@w{__VA_ARGS__}}

1763

can appear is in the replacement list of a variadic macro. It may not

1764

be used as a macro name, macro argument name, or within a different type

1765

of macro. It may also be forbidden in open text; the standard is

1766

ambiguous. We recommend you avoid using it except for its defined

1767

purpose.

1768

1769

Variadic macros are a new feature in C99. GNU CPP has supported them

1770

for a long time, but only with a named variable argument

1771

(@samp{args@dots{}}, not @samp{@dots{}} and @code{@w{__VA_ARGS__}}). If you are

1772

concerned with portability to previous versions of GCC, you should use

1773

only named variable arguments. On the other hand, if you are concerned

1774

with portability to other conforming implementations of C99, you should

1775

use only @code{@w{__VA_ARGS__}}.

1776

1777

Previous versions of CPP implemented the comma-deletion extension

1778

much more generally. We have restricted it in this release to minimize

1779

the differences from C99. To get the same effect with both this and

1780

previous versions of GCC, the token preceding the special @samp{##} must

1781

be a comma, and there must be white space between that comma and

1782

whatever comes immediately before it:

1783

1784

@smallexample

1785

#define eprintf(format, args@dots{}) fprintf (stderr, format , ##args)

1786

@end smallexample

1787

1788

@noindent

1789

@xref{Differences from previous versions}, for the gory details.

1790

1791

@node Predefined Macros

1792

@section Predefined Macros

1793

1794

@cindex predefined macros

1795

Several object-like macros are predefined; you use them without

1796

supplying their definitions. They fall into three classes: standard,

1797

common, and system-specific.

1798

1799

In C++, there is a fourth category, the named operators. They act like

1800

predefined macros, but you cannot undefine them.

1801

1802

@menu

1803

* Standard Predefined Macros::

1804

* Common Predefined Macros::

1805

* System-specific Predefined Macros::

1806

* C++ Named Operators::

1807

@end menu

1808

1809

@node Standard Predefined Macros

1810

@subsection Standard Predefined Macros

1811

@cindex standard predefined macros.

1812

1813

The standard predefined macros are specified by the relevant

1814

language standards, so they are available with all compilers that

1815

implement those standards. Older compilers may not provide all of

1816

them. Their names all start with double underscores.

1817

1818

@table @code

1819

@item __FILE__

1820

This macro expands to the name of the current input file, in the form of

1821

a C string constant. This is the path by which the preprocessor opened

1822

the file, not the short name specified in @samp{#include} or as the

1823

input file name argument. For example,

1824

@code{"/usr/local/include/myheader.h"} is a possible expansion of this

1825

macro.

1826

1827

@item __LINE__

1828

This macro expands to the current input line number, in the form of a

1829

decimal integer constant. While we call it a predefined macro, it's

1830

a pretty strange macro, since its ``definition'' changes with each

1831

new line of source code.

1832

@end table

1833

1834

@code{__FILE__} and @code{__LINE__} are useful in generating an error

1835

message to report an inconsistency detected by the program; the message

1836

can state the source line at which the inconsistency was detected. For

1837

example,

1838

1839

@smallexample

1840

fprintf (stderr, "Internal error: "

1841

"negative string length "

1842

"%d at %s, line %d.",

1843

length, __FILE__, __LINE__);

1844

@end smallexample

1845

1846

An @samp{#include} directive changes the expansions of @code{__FILE__}

1847

and @code{__LINE__} to correspond to the included file. At the end of

1848

that file, when processing resumes on the input file that contained

1849

the @samp{#include} directive, the expansions of @code{__FILE__} and

1850

@code{__LINE__} revert to the values they had before the

1851

@samp{#include} (but @code{__LINE__} is then incremented by one as

1852

processing moves to the line after the @samp{#include}).

1853

1854

A @samp{#line} directive changes @code{__LINE__}, and may change

1855

@code{__FILE__} as well. @xref{Line Control}.

1856

1857

C99 introduces @code{__func__}, and GCC has provided @code{__FUNCTION__}

1858

for a long time. Both of these are strings containing the name of the

1859

current function (there are slight semantic differences; see the GCC

1860

manual). Neither of them is a macro; the preprocessor does not know the

1861

name of the current function. They tend to be useful in conjunction

1862

with @code{__FILE__} and @code{__LINE__}, though.

1863

1864

@table @code

1865

1866

@item __DATE__

1867

This macro expands to a string constant that describes the date on which

1868

the preprocessor is being run. The string constant contains eleven

1869

characters and looks like @code{@w{"Feb 12 1996"}}. If the day of the

1870

month is less than 10, it is padded with a space on the left.

1871

1872

If GCC cannot determine the current date, it will emit a warning message

1873

(once per compilation) and @code{__DATE__} will expand to

1874

@code{@w{"??? ?? ????"}}.

1875

1876

@item __TIME__

1877

This macro expands to a string constant that describes the time at

1878

which the preprocessor is being run. The string constant contains

1879

eight characters and looks like @code{"23:59:01"}.

1880

1881

If GCC cannot determine the current time, it will emit a warning message

1882

(once per compilation) and @code{__TIME__} will expand to

1883

@code{"??:??:??"}.

1884

1885

@item __STDC__

1886

In normal operation, this macro expands to the constant 1, to signify

1887

that this compiler conforms to ISO Standard C@. If GNU CPP is used with

1888

a compiler other than GCC, this is not necessarily true; however, the

1889

preprocessor always conforms to the standard unless the

1890

@option{-traditional-cpp} option is used.

1891

1892

This macro is not defined if the @option{-traditional-cpp} option is used.

1893

1894

On some hosts, the system compiler uses a different convention, where

1895

@code{__STDC__} is normally 0, but is 1 if the user specifies strict

1896

conformance to the C Standard. CPP follows the host convention when

1897

processing system header files, but when processing user files

1898

@code{__STDC__} is always 1. This has been reported to cause problems;

1899

for instance, some versions of Solaris provide X Windows headers that

1900

expect @code{__STDC__} to be either undefined or 1. @xref{Invocation}.

1901

1902

@item __STDC_VERSION__

1903

This macro expands to the C Standard's version number, a long integer

1904

constant of the form @code{@var{yyyy}@var{mm}L} where @var{yyyy} and

1905

@var{mm} are the year and month of the Standard version. This signifies

1906

which version of the C Standard the compiler conforms to. Like

1907

@code{__STDC__}, this is not necessarily accurate for the entire

1908

implementation, unless GNU CPP is being used with GCC@.

1909

1910

The value @code{199409L} signifies the 1989 C standard as amended in

1911

1994, which is the current default; the value @code{199901L} signifies

1912

the 1999 revision of the C standard. Support for the 1999 revision is

1913

not yet complete.

1914

1915

This macro is not defined if the @option{-traditional-cpp} option is

1916

used, nor when compiling C++ or Objective-C@.

1917

1918

@item __STDC_HOSTED__

1919

This macro is defined, with value 1, if the compiler's target is a

1920

@dfn{hosted environment}. A hosted environment has the complete

1921

facilities of the standard C library available.

1922

1923

@item __cplusplus

1924

This macro is defined when the C++ compiler is in use. You can use

1925

@code{__cplusplus} to test whether a header is compiled by a C compiler

1926

or a C++ compiler. This macro is similar to @code{__STDC_VERSION__}, in

1927

that it expands to a version number. Depending on the language standard

1928

selected, the value of the macro is

1929

@code{199711L} for the 1998 C++ standard,

1930

@code{201103L} for the 2011 C++ standard,

1931

@code{201402L} for the 2014 C++ standard,

1932

or an unspecified value strictly larger than @code{201402L} for the

1933

experimental languages enabled by @option{-std=c++1z} and

1934

@option{-std=gnu++1z}.

1935

1936

@item __OBJC__

1937

This macro is defined, with value 1, when the Objective-C compiler is in

1938

use. You can use @code{__OBJC__} to test whether a header is compiled

1939

by a C compiler or an Objective-C compiler.

1940

1941

@item __ASSEMBLER__

1942

This macro is defined with value 1 when preprocessing assembly

1943

language.

1944

1945

@end table

1946

1947

@node Common Predefined Macros

1948

@subsection Common Predefined Macros

1949

@cindex common predefined macros

1950

1951

The common predefined macros are GNU C extensions. They are available

1952

with the same meanings regardless of the machine or operating system on

1953

which you are using GNU C or GNU Fortran. Their names all start with

1954

double underscores.

1955

1956

@table @code

1957

1958

@item __COUNTER__

1959

This macro expands to sequential integral values starting from 0. In

1960

conjunction with the @code{##} operator, this provides a convenient means to

1961

generate unique identifiers. Care must be taken to ensure that

1962

@code{__COUNTER__} is not expanded prior to inclusion of precompiled headers

1963

which use it. Otherwise, the precompiled headers will not be used.

1964

1965

@item __GFORTRAN__

1966

The GNU Fortran compiler defines this.

1967

1968

@item __GNUC__

1969

@itemx __GNUC_MINOR__

1970

@itemx __GNUC_PATCHLEVEL__

1971

These macros are defined by all GNU compilers that use the C

1972

preprocessor: C, C++, Objective-C and Fortran. Their values are the major

1973

version, minor version, and patch level of the compiler, as integer

1974

constants. For example, GCC 3.2.1 will define @code{__GNUC__} to 3,

1975

@code{__GNUC_MINOR__} to 2, and @code{__GNUC_PATCHLEVEL__} to 1. These

1976

macros are also defined if you invoke the preprocessor directly.

1977

1978

@code{__GNUC_PATCHLEVEL__} is new to GCC 3.0; it is also present in the

1979

widely-used development snapshots leading up to 3.0 (which identify

1980

themselves as GCC 2.96 or 2.97, depending on which snapshot you have).

1981

1982

If all you need to know is whether or not your program is being compiled

1983

by GCC, or a non-GCC compiler that claims to accept the GNU C dialects,

1984

you can simply test @code{__GNUC__}. If you need to write code

1985

which depends on a specific version, you must be more careful. Each

1986

time the minor version is increased, the patch level is reset to zero;

1987

each time the major version is increased (which happens rarely), the

1988

minor version and patch level are reset. If you wish to use the

1989

predefined macros directly in the conditional, you will need to write it

1990

like this:

1991

1992

@smallexample

1993

/* @r{Test for GCC > 3.2.0} */

1994

#if __GNUC__ > 3 || \

1995

(__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \

1996

(__GNUC_MINOR__ == 2 && \

1997

__GNUC_PATCHLEVEL__ > 0))

1998

@end smallexample

1999

2000

@noindent

2001

Another approach is to use the predefined macros to

2002

calculate a single number, then compare that against a threshold:

2003

2004

@smallexample

2005

#define GCC_VERSION (__GNUC__ * 10000 \

2006

+ __GNUC_MINOR__ * 100 \

2007

+ __GNUC_PATCHLEVEL__)

2008

@dots{}

2009

/* @r{Test for GCC > 3.2.0} */

2010

#if GCC_VERSION > 30200

2011

@end smallexample

2012

2013

@noindent

2014

Many people find this form easier to understand.

2015

2016

@item __GNUG__

2017

The GNU C++ compiler defines this. Testing it is equivalent to

2018

testing @code{@w{(__GNUC__ && __cplusplus)}}.

2019

2020

@item __STRICT_ANSI__

2021

GCC defines this macro if and only if the @option{-ansi} switch, or a

2022

@option{-std} switch specifying strict conformance to some version of ISO C

2023

or ISO C++, was specified when GCC was invoked. It is defined to @samp{1}.

2024

This macro exists primarily to direct GNU libc's header files to

2025

restrict their definitions to the minimal set found in the 1989 C

2026

standard.

2027

2028

@item __BASE_FILE__

2029

This macro expands to the name of the main input file, in the form

2030

of a C string constant. This is the source file that was specified

2031

on the command line of the preprocessor or C compiler.

2032

2033

@item __INCLUDE_LEVEL__

2034

This macro expands to a decimal integer constant that represents the

2035

depth of nesting in include files. The value of this macro is

2036

incremented on every @samp{#include} directive and decremented at the

2037

end of every included file. It starts out at 0, its value within the

2038

base file specified on the command line.

2039

2040

@item __ELF__

2041

This macro is defined if the target uses the ELF object format.

2042

2043

@item __VERSION__

2044

This macro expands to a string constant which describes the version of

2045

the compiler in use. You should not rely on its contents having any

2046

particular form, but it can be counted on to contain at least the

2047

release number.

2048

2049

@item __OPTIMIZE__

2050

@itemx __OPTIMIZE_SIZE__

2051

@itemx __NO_INLINE__

2052

These macros describe the compilation mode. @code{__OPTIMIZE__} is

2053

defined in all optimizing compilations. @code{__OPTIMIZE_SIZE__} is

2054

defined if the compiler is optimizing for size, not speed.

2055

@code{__NO_INLINE__} is defined if no functions will be inlined into

2056

their callers (when not optimizing, or when inlining has been

2057

specifically disabled by @option{-fno-inline}).

2058

2059

These macros cause certain GNU header files to provide optimized

2060

definitions, using macros or inline functions, of system library

2061

functions. You should not use these macros in any way unless you make

2062

sure that programs will execute with the same effect whether or not they

2063

are defined. If they are defined, their value is 1.

2064

2065

@item __GNUC_GNU_INLINE__

2066

GCC defines this macro if functions declared @code{inline} will be

2067

handled in GCC's traditional gnu90 mode. Object files will contain

2068

externally visible definitions of all functions declared @code{inline}

2069

without @code{extern} or @code{static}. They will not contain any

2070

definitions of any functions declared @code{extern inline}.

2071

2072

@item __GNUC_STDC_INLINE__

2073

GCC defines this macro if functions declared @code{inline} will be

2074

handled according to the ISO C99 standard. Object files will contain

2075

externally visible definitions of all functions declared @code{extern

2076

inline}. They will not contain definitions of any functions declared

2077

@code{inline} without @code{extern}.

2078

2079

If this macro is defined, GCC supports the @code{gnu_inline} function

2080

attribute as a way to always get the gnu90 behavior. Support for

2081

this and @code{__GNUC_GNU_INLINE__} was added in GCC 4.1.3. If

2082

neither macro is defined, an older version of GCC is being used:

2083

@code{inline} functions will be compiled in gnu90 mode, and the

2084

@code{gnu_inline} function attribute will not be recognized.

2085

2086

@item __CHAR_UNSIGNED__

2087

GCC defines this macro if and only if the data type @code{char} is

2088

unsigned on the target machine. It exists to cause the standard header

2089

file @file{limits.h} to work correctly. You should not use this macro

2090

yourself; instead, refer to the standard macros defined in @file{limits.h}.

2091

2092

@item __WCHAR_UNSIGNED__

2093

Like @code{__CHAR_UNSIGNED__}, this macro is defined if and only if the

2094

data type @code{wchar_t} is unsigned and the front-end is in C++ mode.

2095

2096

@item __REGISTER_PREFIX__

2097

This macro expands to a single token (not a string constant) which is

2098

the prefix applied to CPU register names in assembly language for this

2099

target. You can use it to write assembly that is usable in multiple

2100

environments. For example, in the @code{m68k-aout} environment it

2101

expands to nothing, but in the @code{m68k-coff} environment it expands

2102

to a single @samp{%}.

2103

2104

@item __USER_LABEL_PREFIX__

2105

This macro expands to a single token which is the prefix applied to

2106

user labels (symbols visible to C code) in assembly. For example, in

2107

the @code{m68k-aout} environment it expands to an @samp{_}, but in the

2108

@code{m68k-coff} environment it expands to nothing.

2109

2110

This macro will have the correct definition even if

2111

@option{-f(no-)underscores} is in use, but it will not be correct if

2112

target-specific options that adjust this prefix are used (e.g.@: the

2113

OSF/rose @option{-mno-underscores} option).

2114

2115

@item __SIZE_TYPE__

2116

@itemx __PTRDIFF_TYPE__

2117

@itemx __WCHAR_TYPE__

2118

@itemx __WINT_TYPE__

2119

@itemx __INTMAX_TYPE__

2120

@itemx __UINTMAX_TYPE__

2121

@itemx __SIG_ATOMIC_TYPE__

2122

@itemx __INT8_TYPE__

2123

@itemx __INT16_TYPE__

2124

@itemx __INT32_TYPE__

2125

@itemx __INT64_TYPE__

2126

@itemx __UINT8_TYPE__

2127

@itemx __UINT16_TYPE__

2128

@itemx __UINT32_TYPE__

2129

@itemx __UINT64_TYPE__

2130

@itemx __INT_LEAST8_TYPE__

2131

@itemx __INT_LEAST16_TYPE__

2132

@itemx __INT_LEAST32_TYPE__

2133

@itemx __INT_LEAST64_TYPE__

2134

@itemx __UINT_LEAST8_TYPE__

2135

@itemx __UINT_LEAST16_TYPE__

2136

@itemx __UINT_LEAST32_TYPE__

2137

@itemx __UINT_LEAST64_TYPE__

2138

@itemx __INT_FAST8_TYPE__

2139

@itemx __INT_FAST16_TYPE__

2140

@itemx __INT_FAST32_TYPE__

2141

@itemx __INT_FAST64_TYPE__

2142

@itemx __UINT_FAST8_TYPE__

2143

@itemx __UINT_FAST16_TYPE__

2144

@itemx __UINT_FAST32_TYPE__

2145

@itemx __UINT_FAST64_TYPE__

2146

@itemx __INTPTR_TYPE__

2147

@itemx __UINTPTR_TYPE__

2148

These macros are defined to the correct underlying types for the

2149

@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, @code{wint_t},

2150

@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t},

2151

@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t},

2152

@code{uint16_t}, @code{uint32_t}, @code{uint64_t},

2153

@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t},

2154

@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t},

2155

@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t},

2156

@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t},

2157

@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t},

2158

@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} typedefs,

2159

respectively. They exist to make the standard header files

2160

@file{stddef.h}, @file{stdint.h}, and @file{wchar.h} work correctly.

2161

You should not use these macros directly; instead, include the

2162

appropriate headers and use the typedefs. Some of these macros may

2163

not be defined on particular systems if GCC does not provide a

2164

@file{stdint.h} header on those systems.

2165

2166

@item __CHAR_BIT__

2167

Defined to the number of bits used in the representation of the

2168

@code{char} data type. It exists to make the standard header given

2169

numerical limits work correctly. You should not use

2170

this macro directly; instead, include the appropriate headers.

2171

2172

@item __SCHAR_MAX__

2173

@itemx __WCHAR_MAX__

2174

@itemx __SHRT_MAX__

2175

@itemx __INT_MAX__

2176

@itemx __LONG_MAX__

2177

@itemx __LONG_LONG_MAX__

2178

@itemx __WINT_MAX__

2179

@itemx __SIZE_MAX__

2180

@itemx __PTRDIFF_MAX__

2181

@itemx __INTMAX_MAX__

2182

@itemx __UINTMAX_MAX__

2183

@itemx __SIG_ATOMIC_MAX__

2184

@itemx __INT8_MAX__

2185

@itemx __INT16_MAX__

2186

@itemx __INT32_MAX__

2187

@itemx __INT64_MAX__

2188

@itemx __UINT8_MAX__

2189

@itemx __UINT16_MAX__

2190

@itemx __UINT32_MAX__

2191

@itemx __UINT64_MAX__

2192

@itemx __INT_LEAST8_MAX__

2193

@itemx __INT_LEAST16_MAX__

2194

@itemx __INT_LEAST32_MAX__

2195

@itemx __INT_LEAST64_MAX__

2196

@itemx __UINT_LEAST8_MAX__

2197

@itemx __UINT_LEAST16_MAX__

2198

@itemx __UINT_LEAST32_MAX__

2199

@itemx __UINT_LEAST64_MAX__

2200

@itemx __INT_FAST8_MAX__

2201

@itemx __INT_FAST16_MAX__

2202

@itemx __INT_FAST32_MAX__

2203

@itemx __INT_FAST64_MAX__

2204

@itemx __UINT_FAST8_MAX__

2205

@itemx __UINT_FAST16_MAX__

2206

@itemx __UINT_FAST32_MAX__

2207

@itemx __UINT_FAST64_MAX__

2208

@itemx __INTPTR_MAX__

2209

@itemx __UINTPTR_MAX__

2210

@itemx __WCHAR_MIN__

2211

@itemx __WINT_MIN__

2212

@itemx __SIG_ATOMIC_MIN__

2213

Defined to the maximum value of the @code{signed char}, @code{wchar_t},

2214

@code{signed short},

2215

@code{signed int}, @code{signed long}, @code{signed long long},

2216

@code{wint_t}, @code{size_t}, @code{ptrdiff_t},

2217

@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t},

2218

@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t},

2219

@code{uint16_t}, @code{uint32_t}, @code{uint64_t},

2220

@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t},

2221

@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t},

2222

@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t},

2223

@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t},

2224

@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t},

2225

@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} types and

2226

to the minimum value of the @code{wchar_t}, @code{wint_t}, and

2227

@code{sig_atomic_t} types respectively. They exist to make the

2228

standard header given numerical limits work correctly. You should not

2229

use these macros directly; instead, include the appropriate headers.

2230

Some of these macros may not be defined on particular systems if GCC

2231

does not provide a @file{stdint.h} header on those systems.

2232

2233

@item __INT8_C

2234

@itemx __INT16_C

2235

@itemx __INT32_C

2236

@itemx __INT64_C

2237

@itemx __UINT8_C

2238

@itemx __UINT16_C

2239

@itemx __UINT32_C

2240

@itemx __UINT64_C

2241

@itemx __INTMAX_C

2242

@itemx __UINTMAX_C

2243

Defined to implementations of the standard @file{stdint.h} macros with

2244

the same names without the leading @code{__}. They exist the make the

2245

implementation of that header work correctly. You should not use

2246

these macros directly; instead, include the appropriate headers. Some

2247

of these macros may not be defined on particular systems if GCC does

2248

not provide a @file{stdint.h} header on those systems.

2249

2250

@item __SIZEOF_INT__

2251

@itemx __SIZEOF_LONG__

2252

@itemx __SIZEOF_LONG_LONG__

2253

@itemx __SIZEOF_SHORT__

2254

@itemx __SIZEOF_POINTER__

2255

@itemx __SIZEOF_FLOAT__

2256

@itemx __SIZEOF_DOUBLE__

2257

@itemx __SIZEOF_LONG_DOUBLE__

2258

@itemx __SIZEOF_SIZE_T__

2259

@itemx __SIZEOF_WCHAR_T__

2260

@itemx __SIZEOF_WINT_T__

2261

@itemx __SIZEOF_PTRDIFF_T__

2262

Defined to the number of bytes of the C standard data types: @code{int},

2263

@code{long}, @code{long long}, @code{short}, @code{void *}, @code{float},

2264

@code{double}, @code{long double}, @code{size_t}, @code{wchar_t}, @code{wint_t}

2265

and @code{ptrdiff_t}.

2266

2267

@item __BYTE_ORDER__

2268

@itemx __ORDER_LITTLE_ENDIAN__

2269

@itemx __ORDER_BIG_ENDIAN__

2270

@itemx __ORDER_PDP_ENDIAN__

2271

@code{__BYTE_ORDER__} is defined to one of the values

2272

@code{__ORDER_LITTLE_ENDIAN__}, @code{__ORDER_BIG_ENDIAN__}, or

2273

@code{__ORDER_PDP_ENDIAN__} to reflect the layout of multi-byte and

2274

multi-word quantities in memory. If @code{__BYTE_ORDER__} is equal to

2275

@code{__ORDER_LITTLE_ENDIAN__} or @code{__ORDER_BIG_ENDIAN__}, then

2276

multi-byte and multi-word quantities are laid out identically: the

2277

byte (word) at the lowest address is the least significant or most

2278

significant byte (word) of the quantity, respectively. If

2279

@code{__BYTE_ORDER__} is equal to @code{__ORDER_PDP_ENDIAN__}, then

2280

bytes in 16-bit words are laid out in a little-endian fashion, whereas

2281

the 16-bit subwords of a 32-bit quantity are laid out in big-endian

2282

fashion.

2283

2284

You should use these macros for testing like this:

2285

2286

@smallexample

2287

/* @r{Test for a little-endian machine} */

2288

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__

2289

@end smallexample

2290

2291

@item __FLOAT_WORD_ORDER__

2292

@code{__FLOAT_WORD_ORDER__} is defined to one of the values

2293

@code{__ORDER_LITTLE_ENDIAN__} or @code{__ORDER_BIG_ENDIAN__} to reflect

2294

the layout of the words of multi-word floating-point quantities.

2295

2296

@item __DEPRECATED

2297

This macro is defined, with value 1, when compiling a C++ source file

2298

with warnings about deprecated constructs enabled. These warnings are

2299

enabled by default, but can be disabled with @option{-Wno-deprecated}.

2300

2301

@item __EXCEPTIONS

2302

This macro is defined, with value 1, when compiling a C++ source file

2303

with exceptions enabled. If @option{-fno-exceptions} is used when

2304

compiling the file, then this macro is not defined.

2305

2306

@item __GXX_RTTI

2307

This macro is defined, with value 1, when compiling a C++ source file

2308

with runtime type identification enabled. If @option{-fno-rtti} is

2309

used when compiling the file, then this macro is not defined.

2310

2311

@item __USING_SJLJ_EXCEPTIONS__

2312

This macro is defined, with value 1, if the compiler uses the old

2313

mechanism based on @code{setjmp} and @code{longjmp} for exception

2314

handling.

2315

2316

@item __GXX_EXPERIMENTAL_CXX0X__

2317

This macro is defined when compiling a C++ source file with the option

2318

@option{-std=c++0x} or @option{-std=gnu++0x}. It indicates that some

2319

features likely to be included in C++0x are available. Note that these

2320

features are experimental, and may change or be removed in future

2321

versions of GCC.

2322

2323

@item __GXX_WEAK__

2324

This macro is defined when compiling a C++ source file. It has the

2325

value 1 if the compiler will use weak symbols, COMDAT sections, or

2326

other similar techniques to collapse symbols with ``vague linkage''

2327

that are defined in multiple translation units. If the compiler will

2328

not collapse such symbols, this macro is defined with value 0. In

2329

general, user code should not need to make use of this macro; the

2330

purpose of this macro is to ease implementation of the C++ runtime

2331

library provided with G++.

2332

2333

@item __NEXT_RUNTIME__

2334

This macro is defined, with value 1, if (and only if) the NeXT runtime

2335

(as in @option{-fnext-runtime}) is in use for Objective-C@. If the GNU

2336

runtime is used, this macro is not defined, so that you can use this

2337

macro to determine which runtime (NeXT or GNU) is being used.

2338

2339

@item __LP64__

2340

@itemx _LP64

2341

These macros are defined, with value 1, if (and only if) the compilation

2342

is for a target where @code{long int} and pointer both use 64-bits and

2343

@code{int} uses 32-bit.

2344

2345

@item __SSP__

2346

This macro is defined, with value 1, when @option{-fstack-protector} is in

2347

use.

2348

2349

@item __SSP_ALL__

2350

This macro is defined, with value 2, when @option{-fstack-protector-all} is

2351

in use.

2352

2353

@item __SSP_STRONG__

2354

This macro is defined, with value 3, when @option{-fstack-protector-strong} is

2355

in use.

2356

2357

@item __SSP_EXPLICIT__

2358

This macro is defined, with value 4, when @option{-fstack-protector-explicit} is

2359

in use.

2360

2361

@item __SANITIZE_ADDRESS__

2362

This macro is defined, with value 1, when @option{-fsanitize=address}

2363

or @option{-fsanitize=kernel-address} are in use.

2364

2365

@item __TIMESTAMP__

2366

This macro expands to a string constant that describes the date and time

2367

of the last modification of the current source file. The string constant

2368

contains abbreviated day of the week, month, day of the month, time in

2369

hh:mm:ss form, year and looks like @code{@w{"Sun Sep 16 01:03:52 1973"}}.

2370

If the day of the month is less than 10, it is padded with a space on the left.

2371

2372

If GCC cannot determine the current date, it will emit a warning message

2373

(once per compilation) and @code{__TIMESTAMP__} will expand to

2374

@code{@w{"??? ??? ?? ??:??:?? ????"}}.

2375

2376

@item __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1

2377

@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2

2378

@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4

2379

@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8

2380

@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16

2381

These macros are defined when the target processor supports atomic compare

2382

and swap operations on operands 1, 2, 4, 8 or 16 bytes in length, respectively.

2383

2384

@item __GCC_HAVE_DWARF2_CFI_ASM

2385

This macro is defined when the compiler is emitting DWARF CFI directives

2386

to the assembler. When this is defined, it is possible to emit those same

2387

directives in inline assembly.

2388

2389

@item __FP_FAST_FMA

2390

@itemx __FP_FAST_FMAF

2391

@itemx __FP_FAST_FMAL

2392

These macros are defined with value 1 if the backend supports the

2393

@code{fma}, @code{fmaf}, and @code{fmal} builtin functions, so that

2394

the include file @file{math.h} can define the macros

2395

@code{FP_FAST_FMA}, @code{FP_FAST_FMAF}, and @code{FP_FAST_FMAL}

2396

for compatibility with the 1999 C standard.

2397

2398

@item __GCC_IEC_559

2399

This macro is defined to indicate the intended level of support for

2400

IEEE 754 (IEC 60559) floating-point arithmetic. It expands to a

2401

nonnegative integer value. If 0, it indicates that the combination of

2402

the compiler configuration and the command-line options is not

2403

intended to support IEEE 754 arithmetic for @code{float} and

2404

@code{double} as defined in C99 and C11 Annex F (for example, that the

2405

standard rounding modes and exceptions are not supported, or that

2406

optimizations are enabled that conflict with IEEE 754 semantics). If

2407

1, it indicates that IEEE 754 arithmetic is intended to be supported;

2408

this does not mean that all relevant language features are supported

2409

by GCC. If 2 or more, it additionally indicates support for IEEE

2410

754-2008 (in particular, that the binary encodings for quiet and

2411

signaling NaNs are as specified in IEEE 754-2008).

2412

2413

This macro does not indicate the default state of command-line options

2414

that control optimizations that C99 and C11 permit to be controlled by

2415

standard pragmas, where those standards do not require a particular

2416

default state. It does not indicate whether optimizations respect

2417

signaling NaN semantics (the macro for that is

2418

@code{__SUPPORT_SNAN__}). It does not indicate support for decimal

2419

floating point or the IEEE 754 binary16 and binary128 types.

2420

2421

@item __GCC_IEC_559_COMPLEX

2422

This macro is defined to indicate the intended level of support for

2423

IEEE 754 (IEC 60559) floating-point arithmetic for complex numbers, as

2424

defined in C99 and C11 Annex G. It expands to a nonnegative integer

2425

value. If 0, it indicates that the combination of the compiler

2426

configuration and the command-line options is not intended to support

2427

Annex G requirements (for example, because @option{-fcx-limited-range}

2428

was used). If 1 or more, it indicates that it is intended to support

2429

those requirements; this does not mean that all relevant language

2430

features are supported by GCC.

2431

2432

@item __NO_MATH_ERRNO__

2433

This macro is defined if @option{-fno-math-errno} is used, or enabled

2434

by another option such as @option{-ffast-math} or by default.

2435

@end table

2436

2437

@node System-specific Predefined Macros

2438

@subsection System-specific Predefined Macros

2439

2440

@cindex system-specific predefined macros

2441

@cindex predefined macros, system-specific

2442

@cindex reserved namespace

2443

2444

The C preprocessor normally predefines several macros that indicate what

2445

type of system and machine is in use. They are obviously different on

2446

each target supported by GCC@. This manual, being for all systems and

2447

machines, cannot tell you what their names are, but you can use

2448

@command{cpp -dM} to see them all. @xref{Invocation}. All system-specific

2449

predefined macros expand to a constant value, so you can test them with

2450

either @samp{#ifdef} or @samp{#if}.

2451

2452

The C standard requires that all system-specific macros be part of the

2453

@dfn{reserved namespace}. All names which begin with two underscores,

2454

or an underscore and a capital letter, are reserved for the compiler and

2455

library to use as they wish. However, historically system-specific

2456

macros have had names with no special prefix; for instance, it is common

2457

to find @code{unix} defined on Unix systems. For all such macros, GCC

2458

provides a parallel macro with two underscores added at the beginning

2459

and the end. If @code{unix} is defined, @code{__unix__} will be defined

2460

too. There will never be more than two underscores; the parallel of

2461

@code{_mips} is @code{__mips__}.

2462

2463

When the @option{-ansi} option, or any @option{-std} option that

2464

requests strict conformance, is given to the compiler, all the

2465

system-specific predefined macros outside the reserved namespace are

2466

suppressed. The parallel macros, inside the reserved namespace, remain

2467

defined.

2468

2469

We are slowly phasing out all predefined macros which are outside the

2470

reserved namespace. You should never use them in new programs, and we

2471

encourage you to correct older code to use the parallel macros whenever

2472

you find it. We don't recommend you use the system-specific macros that

2473

are in the reserved namespace, either. It is better in the long run to

2474

check specifically for features you need, using a tool such as

2475

@command{autoconf}.

2476

2477

@node C++ Named Operators

2478

@subsection C++ Named Operators

2479

@cindex named operators

2480

@cindex C++ named operators

2481

@cindex @file{iso646.h}

2482

2483

In C++, there are eleven keywords which are simply alternate spellings

2484

of operators normally written with punctuation. These keywords are

2485

treated as such even in the preprocessor. They function as operators in

2486

@samp{#if}, and they cannot be defined as macros or poisoned. In C, you

2487

can request that those keywords take their C++ meaning by including

2488

@file{iso646.h}. That header defines each one as a normal object-like

2489

macro expanding to the appropriate punctuator.

2490

2491

These are the named operators and their corresponding punctuators:

2492

2493

@multitable {Named Operator} {Punctuator}

2494

@item Named Operator @tab Punctuator

2495

@item @code{and} @tab @code{&&}

2496

@item @code{and_eq} @tab @code{&=}

2497

@item @code{bitand} @tab @code{&}

2498

@item @code{bitor} @tab @code{|}

2499

@item @code{compl} @tab @code{~}

2500

@item @code{not} @tab @code{!}

2501

@item @code{not_eq} @tab @code{!=}

2502

@item @code{or} @tab @code{||}

2503

@item @code{or_eq} @tab @code{|=}

2504

@item @code{xor} @tab @code{^}

2505

@item @code{xor_eq} @tab @code{^=}

2506

@end multitable

2507

2508

@node Undefining and Redefining Macros

2509

@section Undefining and Redefining Macros

2510

@cindex undefining macros

2511

@cindex redefining macros

2512

@findex #undef

2513

2514

If a macro ceases to be useful, it may be @dfn{undefined} with the

2515

@samp{#undef} directive. @samp{#undef} takes a single argument, the

2516

name of the macro to undefine. You use the bare macro name, even if the

2517

macro is function-like. It is an error if anything appears on the line

2518

after the macro name. @samp{#undef} has no effect if the name is not a

2519

macro.

2520

2521

@smallexample

2522

#define FOO 4

2523

x = FOO; @expansion{} x = 4;

2524

#undef FOO

2525

x = FOO; @expansion{} x = FOO;

2526

@end smallexample

2527

2528

Once a macro has been undefined, that identifier may be @dfn{redefined}

2529

as a macro by a subsequent @samp{#define} directive. The new definition

2530

need not have any resemblance to the old definition.

2531

2532

However, if an identifier which is currently a macro is redefined, then

2533

the new definition must be @dfn{effectively the same} as the old one.

2534

Two macro definitions are effectively the same if:

2535

@itemize @bullet

2536

@item Both are the same type of macro (object- or function-like).

2537

@item All the tokens of the replacement list are the same.

2538

@item If there are any parameters, they are the same.

2539

@item Whitespace appears in the same places in both. It need not be

2540

exactly the same amount of whitespace, though. Remember that comments

2541

count as whitespace.

2542

@end itemize

2543

2544

@noindent

2545

These definitions are effectively the same:

2546

@smallexample

2547

#define FOUR (2 + 2)

2548

#define FOUR (2 + 2)

2549

#define FOUR (2 /* @r{two} */ + 2)

2550

@end smallexample

2551

@noindent

2552

but these are not:

2553

@smallexample

2554

#define FOUR (2 + 2)

2555

#define FOUR ( 2+2 )

2556

#define FOUR (2 * 2)

2557

#define FOUR(score,and,seven,years,ago) (2 + 2)

2558

@end smallexample

2559

2560

If a macro is redefined with a definition that is not effectively the

2561

same as the old one, the preprocessor issues a warning and changes the

2562

macro to use the new definition. If the new definition is effectively

2563

the same, the redefinition is silently ignored. This allows, for

2564

instance, two different headers to define a common macro. The

2565

preprocessor will only complain if the definitions do not match.

2566

2567

@node Directives Within Macro Arguments

2568

@section Directives Within Macro Arguments

2569

@cindex macro arguments and directives

2570

2571

Occasionally it is convenient to use preprocessor directives within

2572

the arguments of a macro. The C and C++ standards declare that

2573

behavior in these cases is undefined.

2574

2575

Versions of CPP prior to 3.2 would reject such constructs with an

2576

error message. This was the only syntactic difference between normal

2577

functions and function-like macros, so it seemed attractive to remove

2578

this limitation, and people would often be surprised that they could

2579

not use macros in this way. Moreover, sometimes people would use

2580

conditional compilation in the argument list to a normal library

2581

function like @samp{printf}, only to find that after a library upgrade

2582

@samp{printf} had changed to be a function-like macro, and their code

2583

would no longer compile. So from version 3.2 we changed CPP to

2584

successfully process arbitrary directives within macro arguments in

2585

exactly the same way as it would have processed the directive were the

2586

function-like macro invocation not present.

2587

2588

If, within a macro invocation, that macro is redefined, then the new

2589

definition takes effect in time for argument pre-expansion, but the

2590

original definition is still used for argument replacement. Here is a

2591

pathological example:

2592

2593

@smallexample

2594

#define f(x) x x

2595

f (1

2596

#undef f

2597

#define f 2

2598

f)

2599

@end smallexample

2600

2601

@noindent

2602

which expands to

2603

2604

@smallexample

2605

1 2 1 2

2606

@end smallexample

2607

2608

@noindent

2609

with the semantics described above.

2610

2611

@node Macro Pitfalls

2612

@section Macro Pitfalls

2613

@cindex problems with macros

2614

@cindex pitfalls of macros

2615

2616

In this section we describe some special rules that apply to macros and

2617

macro expansion, and point out certain cases in which the rules have

2618

counter-intuitive consequences that you must watch out for.

2619

2620

@menu

2621

* Misnesting::

2622

* Operator Precedence Problems::

2623

* Swallowing the Semicolon::

2624

* Duplication of Side Effects::

2625

* Self-Referential Macros::

2626

* Argument Prescan::

2627

* Newlines in Arguments::

2628

@end menu

2629

2630

@node Misnesting

2631

@subsection Misnesting

2632

2633

When a macro is called with arguments, the arguments are substituted

2634

into the macro body and the result is checked, together with the rest of

2635

the input file, for more macro calls. It is possible to piece together

2636

a macro call coming partially from the macro body and partially from the

2637

arguments. For example,

2638

2639

@smallexample

2640

#define twice(x) (2*(x))

2641

#define call_with_1(x) x(1)

2642

call_with_1 (twice)

2643

@expansion{} twice(1)

2644

@expansion{} (2*(1))

2645

@end smallexample

2646

2647

Macro definitions do not have to have balanced parentheses. By writing

2648

an unbalanced open parenthesis in a macro body, it is possible to create

2649

a macro call that begins inside the macro body but ends outside of it.

2650

For example,

2651

2652

@smallexample

2653

#define strange(file) fprintf (file, "%s %d",

2654

@dots{}

2655

strange(stderr) p, 35)

2656

@expansion{} fprintf (stderr, "%s %d", p, 35)

2657

@end smallexample

2658

2659

The ability to piece together a macro call can be useful, but the use of

2660

unbalanced open parentheses in a macro body is just confusing, and

2661

should be avoided.

2662

2663

@node Operator Precedence Problems

2664

@subsection Operator Precedence Problems

2665

@cindex parentheses in macro bodies

2666

2667

You may have noticed that in most of the macro definition examples shown

2668

above, each occurrence of a macro argument name had parentheses around

2669

it. In addition, another pair of parentheses usually surround the

2670

entire macro definition. Here is why it is best to write macros that

2671

way.

2672

2673

Suppose you define a macro as follows,

2674

2675

@smallexample

2676

#define ceil_div(x, y) (x + y - 1) / y

2677

@end smallexample

2678

2679

@noindent

2680

whose purpose is to divide, rounding up. (One use for this operation is

2681

to compute how many @code{int} objects are needed to hold a certain

2682

number of @code{char} objects.) Then suppose it is used as follows:

2683

2684

@smallexample

2685

a = ceil_div (b & c, sizeof (int));

2686

@expansion{} a = (b & c + sizeof (int) - 1) / sizeof (int);

2687

@end smallexample

2688

2689

@noindent

2690

This does not do what is intended. The operator-precedence rules of

2691

C make it equivalent to this:

2692

2693

@smallexample

2694

a = (b & (c + sizeof (int) - 1)) / sizeof (int);

2695

@end smallexample

2696

2697

@noindent

2698

What we want is this:

2699

2700

@smallexample

2701

a = ((b & c) + sizeof (int) - 1)) / sizeof (int);

2702

@end smallexample

2703

2704

@noindent

2705

Defining the macro as

2706

2707

@smallexample

2708

#define ceil_div(x, y) ((x) + (y) - 1) / (y)

2709

@end smallexample

2710

2711

@noindent

2712

provides the desired result.

2713

2714

Unintended grouping can result in another way. Consider @code{sizeof

2715

ceil_div(1, 2)}. That has the appearance of a C expression that would

2716

compute the size of the type of @code{ceil_div (1, 2)}, but in fact it

2717

means something very different. Here is what it expands to:

2718

2719

@smallexample

2720

sizeof ((1) + (2) - 1) / (2)

2721

@end smallexample

2722

2723

@noindent

2724

This would take the size of an integer and divide it by two. The

2725

precedence rules have put the division outside the @code{sizeof} when it

2726

was intended to be inside.

2727

2728

Parentheses around the entire macro definition prevent such problems.

2729

Here, then, is the recommended way to define @code{ceil_div}:

2730

2731

@smallexample

2732

#define ceil_div(x, y) (((x) + (y) - 1) / (y))

2733

@end smallexample

2734

2735

@node Swallowing the Semicolon

2736

@subsection Swallowing the Semicolon

2737

@cindex semicolons (after macro calls)

2738

2739

Often it is desirable to define a macro that expands into a compound

2740

statement. Consider, for example, the following macro, that advances a

2741

pointer (the argument @code{p} says where to find it) across whitespace

2742

characters:

2743

2744

@smallexample

2745

#define SKIP_SPACES(p, limit) \

2746

@{ char *lim = (limit); \

2747

while (p < lim) @{ \

2748

if (*p++ != ' ') @{ \

2749

p--; break; @}@}@}

2750

@end smallexample

2751

2752

@noindent

2753

Here backslash-newline is used to split the macro definition, which must

2754

be a single logical line, so that it resembles the way such code would

2755

be laid out if not part of a macro definition.

2756

2757

A call to this macro might be @code{SKIP_SPACES (p, lim)}. Strictly

2758

speaking, the call expands to a compound statement, which is a complete

2759

statement with no need for a semicolon to end it. However, since it

2760

looks like a function call, it minimizes confusion if you can use it

2761

like a function call, writing a semicolon afterward, as in

2762

@code{SKIP_SPACES (p, lim);}

2763

2764

This can cause trouble before @code{else} statements, because the

2765

semicolon is actually a null statement. Suppose you write

2766

2767

@smallexample

2768

if (*p != 0)

2769

SKIP_SPACES (p, lim);

2770

else @dots{}

2771

@end smallexample

2772

2773

@noindent

2774

The presence of two statements---the compound statement and a null

2775

statement---in between the @code{if} condition and the @code{else}

2776

makes invalid C code.

2777

2778

The definition of the macro @code{SKIP_SPACES} can be altered to solve

2779

this problem, using a @code{do @dots{} while} statement. Here is how:

2780

2781

@smallexample

2782

#define SKIP_SPACES(p, limit) \

2783

do @{ char *lim = (limit); \

2784

while (p < lim) @{ \

2785

if (*p++ != ' ') @{ \

2786

p--; break; @}@}@} \

2787

while (0)

2788

@end smallexample

2789

2790

Now @code{SKIP_SPACES (p, lim);} expands into

2791

2792

@smallexample

2793

do @{@dots{}@} while (0);

2794

@end smallexample

2795

2796

@noindent

2797

which is one statement. The loop executes exactly once; most compilers

2798

generate no extra code for it.

2799

2800

@node Duplication of Side Effects

2801

@subsection Duplication of Side Effects

2802

2803

@cindex side effects (in macro arguments)

2804

@cindex unsafe macros

2805

Many C programs define a macro @code{min}, for ``minimum'', like this:

2806

2807

@smallexample

2808

#define min(X, Y) ((X) < (Y) ? (X) : (Y))

2809

@end smallexample

2810

2811

When you use this macro with an argument containing a side effect,

2812

as shown here,

2813

2814

@smallexample

2815

next = min (x + y, foo (z));

2816

@end smallexample

2817

2818

@noindent

2819

it expands as follows:

2820

2821

@smallexample

2822

next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));

2823

@end smallexample

2824

2825

@noindent

2826

where @code{x + y} has been substituted for @code{X} and @code{foo (z)}

2827

for @code{Y}.

2828

2829

The function @code{foo} is used only once in the statement as it appears

2830

in the program, but the expression @code{foo (z)} has been substituted

2831

twice into the macro expansion. As a result, @code{foo} might be called

2832

two times when the statement is executed. If it has side effects or if

2833

it takes a long time to compute, the results might not be what you

2834

intended. We say that @code{min} is an @dfn{unsafe} macro.

2835

2836

The best solution to this problem is to define @code{min} in a way that

2837

computes the value of @code{foo (z)} only once. The C language offers

2838

no standard way to do this, but it can be done with GNU extensions as

2839

follows:

2840

2841

@smallexample

2842

#define min(X, Y) \

2843

(@{ typeof (X) x_ = (X); \

2844

typeof (Y) y_ = (Y); \

2845

(x_ < y_) ? x_ : y_; @})

2846

@end smallexample

2847

2848

The @samp{(@{ @dots{} @})} notation produces a compound statement that

2849

acts as an expression. Its value is the value of its last statement.

2850

This permits us to define local variables and assign each argument to

2851

one. The local variables have underscores after their names to reduce

2852

the risk of conflict with an identifier of wider scope (it is impossible

2853

to avoid this entirely). Now each argument is evaluated exactly once.

2854

2855

If you do not wish to use GNU C extensions, the only solution is to be

2856

careful when @emph{using} the macro @code{min}. For example, you can

2857

calculate the value of @code{foo (z)}, save it in a variable, and use

2858

that variable in @code{min}:

2859

2860

@smallexample

2861

@group

2862

#define min(X, Y) ((X) < (Y) ? (X) : (Y))

2863

@dots{}

2864

@{

2865

int tem = foo (z);

2866

next = min (x + y, tem);

2867

@}

2868

@end group

2869

@end smallexample

2870

2871

@noindent

2872

(where we assume that @code{foo} returns type @code{int}).

2873

2874

@node Self-Referential Macros

2875

@subsection Self-Referential Macros

2876

@cindex self-reference

2877

2878

A @dfn{self-referential} macro is one whose name appears in its

2879

definition. Recall that all macro definitions are rescanned for more

2880

macros to replace. If the self-reference were considered a use of the

2881

macro, it would produce an infinitely large expansion. To prevent this,

2882

the self-reference is not considered a macro call. It is passed into

2883

the preprocessor output unchanged. Consider an example:

2884

2885

@smallexample

2886

#define foo (4 + foo)

2887

@end smallexample

2888

2889

@noindent

2890

where @code{foo} is also a variable in your program.

2891

2892

Following the ordinary rules, each reference to @code{foo} will expand

2893

into @code{(4 + foo)}; then this will be rescanned and will expand into

2894

@code{(4 + (4 + foo))}; and so on until the computer runs out of memory.

2895

2896

The self-reference rule cuts this process short after one step, at

2897

@code{(4 + foo)}. Therefore, this macro definition has the possibly

2898

useful effect of causing the program to add 4 to the value of @code{foo}

2899

wherever @code{foo} is referred to.

2900

2901

In most cases, it is a bad idea to take advantage of this feature. A

2902

person reading the program who sees that @code{foo} is a variable will

2903

not expect that it is a macro as well. The reader will come across the

2904

identifier @code{foo} in the program and think its value should be that

2905

of the variable @code{foo}, whereas in fact the value is four greater.

2906

2907

One common, useful use of self-reference is to create a macro which

2908

expands to itself. If you write

2909

2910

@smallexample

2911

#define EPERM EPERM

2912

@end smallexample

2913

2914

@noindent

2915

then the macro @code{EPERM} expands to @code{EPERM}. Effectively, it is

2916

left alone by the preprocessor whenever it's used in running text. You

2917

can tell that it's a macro with @samp{#ifdef}. You might do this if you

2918

want to define numeric constants with an @code{enum}, but have

2919

@samp{#ifdef} be true for each constant.

2920

2921

If a macro @code{x} expands to use a macro @code{y}, and the expansion of

2922

@code{y} refers to the macro @code{x}, that is an @dfn{indirect

2923

self-reference} of @code{x}. @code{x} is not expanded in this case

2924

either. Thus, if we have

2925

2926

@smallexample

2927

#define x (4 + y)

2928

#define y (2 * x)

2929

@end smallexample

2930

2931

@noindent

2932

then @code{x} and @code{y} expand as follows:

2933

2934

@smallexample

2935

@group

2936

x @expansion{} (4 + y)

2937

@expansion{} (4 + (2 * x))

2938

2939

y @expansion{} (2 * x)

2940

@expansion{} (2 * (4 + y))

2941

@end group

2942

@end smallexample

2943

2944

@noindent

2945

Each macro is expanded when it appears in the definition of the other

2946

macro, but not when it indirectly appears in its own definition.

2947

2948

@node Argument Prescan

2949

@subsection Argument Prescan

2950

@cindex expansion of arguments

2951

@cindex macro argument expansion

2952

@cindex prescan of macro arguments

2953

2954

Macro arguments are completely macro-expanded before they are

2955

substituted into a macro body, unless they are stringified or pasted

2956

with other tokens. After substitution, the entire macro body, including

2957

the substituted arguments, is scanned again for macros to be expanded.

2958

The result is that the arguments are scanned @emph{twice} to expand

2959

macro calls in them.

2960

2961

Most of the time, this has no effect. If the argument contained any

2962

macro calls, they are expanded during the first scan. The result

2963

therefore contains no macro calls, so the second scan does not change

2964

it. If the argument were substituted as given, with no prescan, the

2965

single remaining scan would find the same macro calls and produce the

2966

same results.

2967

2968

You might expect the double scan to change the results when a

2969

self-referential macro is used in an argument of another macro

2970

(@pxref{Self-Referential Macros}): the self-referential macro would be

2971

expanded once in the first scan, and a second time in the second scan.

2972

However, this is not what happens. The self-references that do not

2973

expand in the first scan are marked so that they will not expand in the

2974

second scan either.

2975

2976

You might wonder, ``Why mention the prescan, if it makes no difference?

2977

And why not skip it and make the preprocessor faster?'' The answer is

2978

that the prescan does make a difference in three special cases:

2979

2980

@itemize @bullet

2981

@item

2982

Nested calls to a macro.

2983

2984

We say that @dfn{nested} calls to a macro occur when a macro's argument

2985

contains a call to that very macro. For example, if @code{f} is a macro

2986

that expects one argument, @code{f (f (1))} is a nested pair of calls to

2987

@code{f}. The desired expansion is made by expanding @code{f (1)} and

2988

substituting that into the definition of @code{f}. The prescan causes

2989

the expected result to happen. Without the prescan, @code{f (1)} itself

2990

would be substituted as an argument, and the inner use of @code{f} would

2991

appear during the main scan as an indirect self-reference and would not

2992

be expanded.

2993

2994

@item

2995

Macros that call other macros that stringify or concatenate.

2996

2997

If an argument is stringified or concatenated, the prescan does not

2998

occur. If you @emph{want} to expand a macro, then stringify or

2999

concatenate its expansion, you can do that by causing one macro to call

3000

another macro that does the stringification or concatenation. For

3001

instance, if you have

3002

3003

@smallexample

3004

#define AFTERX(x) X_ ## x

3005

#define XAFTERX(x) AFTERX(x)

3006

#define TABLESIZE 1024

3007

#define BUFSIZE TABLESIZE

3008

@end smallexample

3009

3010

then @code{AFTERX(BUFSIZE)} expands to @code{X_BUFSIZE}, and

3011

@code{XAFTERX(BUFSIZE)} expands to @code{X_1024}. (Not to

3012

@code{X_TABLESIZE}. Prescan always does a complete expansion.)

3013

3014

@item

3015

Macros used in arguments, whose expansions contain unshielded commas.

3016

3017

This can cause a macro expanded on the second scan to be called with the

3018

wrong number of arguments. Here is an example:

3019

3020

@smallexample

3021

#define foo a,b

3022

#define bar(x) lose(x)

3023

#define lose(x) (1 + (x))

3024

@end smallexample

3025

3026

We would like @code{bar(foo)} to turn into @code{(1 + (foo))}, which

3027

would then turn into @code{(1 + (a,b))}. Instead, @code{bar(foo)}

3028

expands into @code{lose(a,b)}, and you get an error because @code{lose}

3029

requires a single argument. In this case, the problem is easily solved

3030

by the same parentheses that ought to be used to prevent misnesting of

3031

arithmetic operations:

3032

3033

@smallexample

3034

#define foo (a,b)

3035

@exdent or

3036

#define bar(x) lose((x))

3037

@end smallexample

3038

3039

The extra pair of parentheses prevents the comma in @code{foo}'s

3040

definition from being interpreted as an argument separator.

3041

3042

@end itemize

3043

3044

@node Newlines in Arguments

3045

@subsection Newlines in Arguments

3046

@cindex newlines in macro arguments

3047

3048

The invocation of a function-like macro can extend over many logical

3049

lines. However, in the present implementation, the entire expansion

3050

comes out on one line. Thus line numbers emitted by the compiler or

3051

debugger refer to the line the invocation started on, which might be

3052

different to the line containing the argument causing the problem.

3053

3054

Here is an example illustrating this:

3055

3056

@smallexample

3057

#define ignore_second_arg(a,b,c) a; c

3058

3059

ignore_second_arg (foo (),

3060

ignored (),

3061

syntax error);

3062

@end smallexample

3063

3064

@noindent

3065

The syntax error triggered by the tokens @code{syntax error} results in

3066

an error message citing line three---the line of ignore_second_arg---

3067

even though the problematic code comes from line five.

3068

3069

We consider this a bug, and intend to fix it in the near future.

3070

3071

@node Conditionals

3072

@chapter Conditionals

3073

@cindex conditionals

3074

3075

A @dfn{conditional} is a directive that instructs the preprocessor to

3076

select whether or not to include a chunk of code in the final token

3077

stream passed to the compiler. Preprocessor conditionals can test

3078

arithmetic expressions, or whether a name is defined as a macro, or both

3079

simultaneously using the special @code{defined} operator.

3080

3081

A conditional in the C preprocessor resembles in some ways an @code{if}

3082

statement in C, but it is important to understand the difference between

3083

them. The condition in an @code{if} statement is tested during the

3084

execution of your program. Its purpose is to allow your program to

3085

behave differently from run to run, depending on the data it is

3086

operating on. The condition in a preprocessing conditional directive is

3087

tested when your program is compiled. Its purpose is to allow different

3088

code to be included in the program depending on the situation at the

3089

time of compilation.

3090

3091

However, the distinction is becoming less clear. Modern compilers often

3092

do test @code{if} statements when a program is compiled, if their

3093

conditions are known not to vary at run time, and eliminate code which

3094

can never be executed. If you can count on your compiler to do this,

3095

you may find that your program is more readable if you use @code{if}

3096

statements with constant conditions (perhaps determined by macros). Of

3097

course, you can only use this to exclude code, not type definitions or

3098

other preprocessing directives, and you can only do it if the code

3099

remains syntactically valid when it is not to be used.

3100

3101

GCC version 3 eliminates this kind of never-executed code even when

3102

not optimizing. Older versions did it only when optimizing.

3103

3104

@menu

3105

* Conditional Uses::

3106

* Conditional Syntax::

3107

* Deleted Code::

3108

@end menu

3109

3110

@node Conditional Uses

3111

@section Conditional Uses

3112

3113

There are three general reasons to use a conditional.

3114

3115

@itemize @bullet

3116

@item

3117

A program may need to use different code depending on the machine or

3118

operating system it is to run on. In some cases the code for one

3119

operating system may be erroneous on another operating system; for

3120

example, it might refer to data types or constants that do not exist on

3121

the other system. When this happens, it is not enough to avoid

3122

executing the invalid code. Its mere presence will cause the compiler

3123

to reject the program. With a preprocessing conditional, the offending

3124

code can be effectively excised from the program when it is not valid.

3125

3126

@item

3127

You may want to be able to compile the same source file into two

3128

different programs. One version might make frequent time-consuming

3129

consistency checks on its intermediate data, or print the values of

3130

those data for debugging, and the other not.

3131

3132

@item

3133

A conditional whose condition is always false is one way to exclude code

3134

from the program but keep it as a sort of comment for future reference.

3135

@end itemize

3136

3137

Simple programs that do not need system-specific logic or complex

3138

debugging hooks generally will not need to use preprocessing

3139

conditionals.

3140

3141

@node Conditional Syntax

3142

@section Conditional Syntax

3143

3144

@findex #if

3145

A conditional in the C preprocessor begins with a @dfn{conditional

3146

directive}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}.

3147

3148

@menu

3149

* Ifdef::

3150

* If::

3151

* Defined::

3152

* Else::

3153

* Elif::

3154

@end menu

3155

3156

@node Ifdef

3157

@subsection Ifdef

3158

@findex #ifdef

3159

@findex #endif

3160

3161

The simplest sort of conditional is

3162

3163

@smallexample

3164

@group

3165

#ifdef @var{MACRO}

3166

3167

@var{controlled text}

3168

3169

#endif /* @var{MACRO} */

3170

@end group

3171

@end smallexample

3172

3173

@cindex conditional group

3174

This block is called a @dfn{conditional group}. @var{controlled text}

3175

will be included in the output of the preprocessor if and only if

3176

@var{MACRO} is defined. We say that the conditional @dfn{succeeds} if

3177

@var{MACRO} is defined, @dfn{fails} if it is not.

3178

3179

The @var{controlled text} inside of a conditional can include

3180

preprocessing directives. They are executed only if the conditional

3181

succeeds. You can nest conditional groups inside other conditional

3182

groups, but they must be completely nested. In other words,

3183

@samp{#endif} always matches the nearest @samp{#ifdef} (or

3184

@samp{#ifndef}, or @samp{#if}). Also, you cannot start a conditional

3185

group in one file and end it in another.

3186

3187

Even if a conditional fails, the @var{controlled text} inside it is

3188

still run through initial transformations and tokenization. Therefore,

3189

it must all be lexically valid C@. Normally the only way this matters is

3190

that all comments and string literals inside a failing conditional group

3191

must still be properly ended.

3192

3193

The comment following the @samp{#endif} is not required, but it is a

3194

good practice if there is a lot of @var{controlled text}, because it

3195

helps people match the @samp{#endif} to the corresponding @samp{#ifdef}.

3196

Older programs sometimes put @var{MACRO} directly after the

3197

@samp{#endif} without enclosing it in a comment. This is invalid code

3198

according to the C standard. CPP accepts it with a warning. It

3199

never affects which @samp{#ifndef} the @samp{#endif} matches.

3200

3201

@findex #ifndef

3202

Sometimes you wish to use some code if a macro is @emph{not} defined.

3203

You can do this by writing @samp{#ifndef} instead of @samp{#ifdef}.

3204

One common use of @samp{#ifndef} is to include code only the first

3205

time a header file is included. @xref{Once-Only Headers}.

3206

3207

Macro definitions can vary between compilations for several reasons.

3208

Here are some samples.

3209

3210

@itemize @bullet

3211

@item

3212

Some macros are predefined on each kind of machine

3213

(@pxref{System-specific Predefined Macros}). This allows you to provide

3214

code specially tuned for a particular machine.

3215

3216

@item

3217

System header files define more macros, associated with the features

3218

they implement. You can test these macros with conditionals to avoid

3219

using a system feature on a machine where it is not implemented.

3220

3221

@item

3222

Macros can be defined or undefined with the @option{-D} and @option{-U}

3223

command-line options when you compile the program. You can arrange to

3224

compile the same source file into two different programs by choosing a

3225

macro name to specify which program you want, writing conditionals to

3226

test whether or how this macro is defined, and then controlling the

3227

state of the macro with command-line options, perhaps set in the

3228

Makefile. @xref{Invocation}.

3229

3230

@item

3231

Your program might have a special header file (often called

3232

@file{config.h}) that is adjusted when the program is compiled. It can

3233

define or not define macros depending on the features of the system and

3234

the desired capabilities of the program. The adjustment can be

3235

automated by a tool such as @command{autoconf}, or done by hand.

3236

@end itemize

3237

3238

@node If

3239

@subsection If

3240

3241

The @samp{#if} directive allows you to test the value of an arithmetic

3242

expression, rather than the mere existence of one macro. Its syntax is

3243

3244

@smallexample

3245

@group

3246

#if @var{expression}

3247

3248

@var{controlled text}

3249

3250

#endif /* @var{expression} */

3251

@end group

3252

@end smallexample

3253

3254

@var{expression} is a C expression of integer type, subject to stringent

3255

restrictions. It may contain

3256

3257

@itemize @bullet

3258

@item

3259

Integer constants.

3260

3261

@item

3262

Character constants, which are interpreted as they would be in normal

3263

code.

3264

3265

@item

3266

Arithmetic operators for addition, subtraction, multiplication,

3267

division, bitwise operations, shifts, comparisons, and logical

3268

operations (@code{&&} and @code{||}). The latter two obey the usual

3269

short-circuiting rules of standard C@.

3270

3271

@item

3272

Macros. All macros in the expression are expanded before actual

3273

computation of the expression's value begins.

3274

3275

@item

3276

Uses of the @code{defined} operator, which lets you check whether macros

3277

are defined in the middle of an @samp{#if}.

3278

3279

@item

3280

Identifiers that are not macros, which are all considered to be the

3281

number zero. This allows you to write @code{@w{#if MACRO}} instead of

3282

@code{@w{#ifdef MACRO}}, if you know that MACRO, when defined, will

3283

always have a nonzero value. Function-like macros used without their

3284

function call parentheses are also treated as zero.

3285

3286

In some contexts this shortcut is undesirable. The @option{-Wundef}

3287

option causes GCC to warn whenever it encounters an identifier which is

3288

not a macro in an @samp{#if}.

3289

@end itemize

3290

3291

The preprocessor does not know anything about types in the language.

3292

Therefore, @code{sizeof} operators are not recognized in @samp{#if}, and

3293

neither are @code{enum} constants. They will be taken as identifiers

3294

which are not macros, and replaced by zero. In the case of

3295

@code{sizeof}, this is likely to cause the expression to be invalid.

3296

3297

The preprocessor calculates the value of @var{expression}. It carries

3298

out all calculations in the widest integer type known to the compiler;

3299

on most machines supported by GCC this is 64 bits. This is not the same

3300

rule as the compiler uses to calculate the value of a constant

3301

expression, and may give different results in some cases. If the value

3302

comes out to be nonzero, the @samp{#if} succeeds and the @var{controlled

3303

text} is included; otherwise it is skipped.

3304

3305

@node Defined

3306

@subsection Defined

3307

3308

@cindex @code{defined}

3309

The special operator @code{defined} is used in @samp{#if} and

3310

@samp{#elif} expressions to test whether a certain name is defined as a

3311

macro. @code{defined @var{name}} and @code{defined (@var{name})} are

3312

both expressions whose value is 1 if @var{name} is defined as a macro at

3313

the current point in the program, and 0 otherwise. Thus, @code{@w{#if

3314

defined MACRO}} is precisely equivalent to @code{@w{#ifdef MACRO}}.

3315

3316

@code{defined} is useful when you wish to test more than one macro for

3317

existence at once. For example,

3318

3319

@smallexample

3320

#if defined (__vax__) || defined (__ns16000__)

3321

@end smallexample

3322

3323

@noindent

3324

would succeed if either of the names @code{__vax__} or

3325

@code{__ns16000__} is defined as a macro.

3326

3327

Conditionals written like this:

3328

3329

@smallexample

3330

#if defined BUFSIZE && BUFSIZE >= 1024

3331

@end smallexample

3332

3333

@noindent

3334

can generally be simplified to just @code{@w{#if BUFSIZE >= 1024}},

3335

since if @code{BUFSIZE} is not defined, it will be interpreted as having

3336

the value zero.

3337

3338

If the @code{defined} operator appears as a result of a macro expansion,

3339

the C standard says the behavior is undefined. GNU cpp treats it as a

3340

genuine @code{defined} operator and evaluates it normally. It will warn

3341

wherever your code uses this feature if you use the command-line option

3342

@option{-pedantic}, since other compilers may handle it differently.

3343

3344

@node Else

3345

@subsection Else

3346

3347

@findex #else

3348

The @samp{#else} directive can be added to a conditional to provide

3349

alternative text to be used if the condition fails. This is what it

3350

looks like:

3351

3352

@smallexample

3353

@group

3354

#if @var{expression}

3355

@var{text-if-true}

3356

#else /* Not @var{expression} */

3357

@var{text-if-false}

3358

#endif /* Not @var{expression} */

3359

@end group

3360

@end smallexample

3361

3362

@noindent

3363

If @var{expression} is nonzero, the @var{text-if-true} is included and

3364

the @var{text-if-false} is skipped. If @var{expression} is zero, the

3365

opposite happens.

3366

3367

You can use @samp{#else} with @samp{#ifdef} and @samp{#ifndef}, too.

3368

3369

@node Elif

3370

@subsection Elif

3371

3372

@findex #elif

3373

One common case of nested conditionals is used to check for more than two

3374

possible alternatives. For example, you might have

3375

3376

@smallexample

3377

#if X == 1

3378

@dots{}

3379

#else /* X != 1 */

3380

#if X == 2

3381

@dots{}

3382

#else /* X != 2 */

3383

@dots{}

3384

#endif /* X != 2 */

3385

#endif /* X != 1 */

3386

@end smallexample

3387

3388

Another conditional directive, @samp{#elif}, allows this to be

3389

abbreviated as follows:

3390

3391

@smallexample

3392

#if X == 1

3393

@dots{}

3394

#elif X == 2

3395

@dots{}

3396

#else /* X != 2 and X != 1*/

3397

@dots{}

3398

#endif /* X != 2 and X != 1*/

3399

@end smallexample

3400

3401

@samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the

3402

middle of a conditional group and subdivides it; it does not require a

3403

matching @samp{#endif} of its own. Like @samp{#if}, the @samp{#elif}

3404

directive includes an expression to be tested. The text following the

3405

@samp{#elif} is processed only if the original @samp{#if}-condition

3406

failed and the @samp{#elif} condition succeeds.

3407

3408

More than one @samp{#elif} can go in the same conditional group. Then

3409

the text after each @samp{#elif} is processed only if the @samp{#elif}

3410

condition succeeds after the original @samp{#if} and all previous

3411

@samp{#elif} directives within it have failed.

3412

3413

@samp{#else} is allowed after any number of @samp{#elif} directives, but

3414

@samp{#elif} may not follow @samp{#else}.

3415

3416

@node Deleted Code

3417

@section Deleted Code

3418

@cindex commenting out code

3419

3420

If you replace or delete a part of the program but want to keep the old

3421

code around for future reference, you often cannot simply comment it

3422

out. Block comments do not nest, so the first comment inside the old

3423

code will end the commenting-out. The probable result is a flood of

3424

syntax errors.

3425

3426

One way to avoid this problem is to use an always-false conditional

3427

instead. For instance, put @code{#if 0} before the deleted code and

3428

@code{#endif} after it. This works even if the code being turned

3429

off contains conditionals, but they must be entire conditionals

3430

(balanced @samp{#if} and @samp{#endif}).

3431

3432

Some people use @code{#ifdef notdef} instead. This is risky, because

3433

@code{notdef} might be accidentally defined as a macro, and then the

3434

conditional would succeed. @code{#if 0} can be counted on to fail.

3435

3436

Do not use @code{#if 0} for comments which are not C code. Use a real

3437

comment, instead. The interior of @code{#if 0} must consist of complete

3438

tokens; in particular, single-quote characters must balance. Comments

3439

often contain unbalanced single-quote characters (known in English as

3440

apostrophes). These confuse @code{#if 0}. They don't confuse

3441

@samp{/*}.

3442

3443

@node Diagnostics

3444

@chapter Diagnostics

3445

@cindex diagnostic

3446

@cindex reporting errors

3447

@cindex reporting warnings

3448

3449

@findex #error

3450

The directive @samp{#error} causes the preprocessor to report a fatal

3451

error. The tokens forming the rest of the line following @samp{#error}

3452

are used as the error message.

3453

3454

You would use @samp{#error} inside of a conditional that detects a

3455

combination of parameters which you know the program does not properly

3456

support. For example, if you know that the program will not run

3457

properly on a VAX, you might write

3458

3459

@smallexample

3460

@group

3461

#ifdef __vax__

3462

#error "Won't work on VAXen. See comments at get_last_object."

3463

#endif

3464

@end group

3465

@end smallexample

3466

3467

If you have several configuration parameters that must be set up by

3468

the installation in a consistent way, you can use conditionals to detect

3469

an inconsistency and report it with @samp{#error}. For example,

3470

3471

@smallexample

3472

#if !defined(FOO) && defined(BAR)

3473

#error "BAR requires FOO."

3474

#endif

3475

@end smallexample

3476

3477

@findex #warning

3478

The directive @samp{#warning} is like @samp{#error}, but causes the

3479

preprocessor to issue a warning and continue preprocessing. The tokens

3480

following @samp{#warning} are used as the warning message.

3481

3482

You might use @samp{#warning} in obsolete header files, with a message

3483

directing the user to the header file which should be used instead.

3484

3485

Neither @samp{#error} nor @samp{#warning} macro-expands its argument.

3486

Internal whitespace sequences are each replaced with a single space.

3487

The line must consist of complete tokens. It is wisest to make the

3488

argument of these directives be a single string constant; this avoids

3489

problems with apostrophes and the like.

3490

3491

@node Line Control

3492

@chapter Line Control

3493

@cindex line control

3494

3495

The C preprocessor informs the C compiler of the location in your source

3496

code where each token came from. Presently, this is just the file name

3497

and line number. All the tokens resulting from macro expansion are

3498

reported as having appeared on the line of the source file where the

3499

outermost macro was used. We intend to be more accurate in the future.

3500

3501

If you write a program which generates source code, such as the

3502

@command{bison} parser generator, you may want to adjust the preprocessor's

3503

notion of the current file name and line number by hand. Parts of the

3504

output from @command{bison} are generated from scratch, other parts come

3505

from a standard parser file. The rest are copied verbatim from

3506

@command{bison}'s input. You would like compiler error messages and

3507

symbolic debuggers to be able to refer to @code{bison}'s input file.

3508

3509

@findex #line

3510

@command{bison} or any such program can arrange this by writing

3511

@samp{#line} directives into the output file. @samp{#line} is a

3512

directive that specifies the original line number and source file name

3513

for subsequent input in the current preprocessor input file.

3514

@samp{#line} has three variants:

3515

3516

@table @code

3517

@item #line @var{linenum}

3518

@var{linenum} is a non-negative decimal integer constant. It specifies

3519

the line number which should be reported for the following line of

3520

input. Subsequent lines are counted from @var{linenum}.

3521

3522

@item #line @var{linenum} @var{filename}

3523

@var{linenum} is the same as for the first form, and has the same

3524

effect. In addition, @var{filename} is a string constant. The

3525

following line and all subsequent lines are reported to come from the

3526

file it specifies, until something else happens to change that.

3527

@var{filename} is interpreted according to the normal rules for a string

3528

constant: backslash escapes are interpreted. This is different from

3529

@samp{#include}.

3530

3531

Previous versions of CPP did not interpret escapes in @samp{#line};

3532

we have changed it because the standard requires they be interpreted,

3533

and most other compilers do.

3534

3535

@item #line @var{anything else}

3536

@var{anything else} is checked for macro calls, which are expanded.

3537

The result should match one of the above two forms.

3538

@end table

3539

3540

@samp{#line} directives alter the results of the @code{__FILE__} and

3541

@code{__LINE__} predefined macros from that point on. @xref{Standard

3542

Predefined Macros}. They do not have any effect on @samp{#include}'s

3543

idea of the directory containing the current file. This is a change

3544

from GCC 2.95. Previously, a file reading

3545

3546

@smallexample

3547

#line 1 "../src/gram.y"

3548

#include "gram.h"

3549

@end smallexample

3550

3551

would search for @file{gram.h} in @file{../src}, then the @option{-I}

3552

chain; the directory containing the physical source file would not be

3553

searched. In GCC 3.0 and later, the @samp{#include} is not affected by

3554

the presence of a @samp{#line} referring to a different directory.

3555

3556

We made this change because the old behavior caused problems when

3557

generated source files were transported between machines. For instance,

3558

it is common practice to ship generated parsers with a source release,

3559

so that people building the distribution do not need to have yacc or

3560

Bison installed. These files frequently have @samp{#line} directives

3561

referring to the directory tree of the system where the distribution was

3562

created. If GCC tries to search for headers in those directories, the

3563

build is likely to fail.

3564

3565

The new behavior can cause failures too, if the generated file is not

3566

in the same directory as its source and it attempts to include a header

3567

which would be visible searching from the directory containing the

3568

source file. However, this problem is easily solved with an additional

3569

@option{-I} switch on the command line. The failures caused by the old

3570

semantics could sometimes be corrected only by editing the generated

3571

files, which is difficult and error-prone.

3572

3573

@node Pragmas

3574

@chapter Pragmas

3575

3576

The @samp{#pragma} directive is the method specified by the C standard

3577

for providing additional information to the compiler, beyond what is

3578

conveyed in the language itself. Three forms of this directive

3579

(commonly known as @dfn{pragmas}) are specified by the 1999 C standard.

3580

A C compiler is free to attach any meaning it likes to other pragmas.

3581

3582

GCC has historically preferred to use extensions to the syntax of the

3583

language, such as @code{__attribute__}, for this purpose. However, GCC

3584

does define a few pragmas of its own. These mostly have effects on the

3585

entire translation unit or source file.

3586

3587

In GCC version 3, all GNU-defined, supported pragmas have been given a

3588

@code{GCC} prefix. This is in line with the @code{STDC} prefix on all

3589

pragmas defined by C99. For backward compatibility, pragmas which were

3590

recognized by previous versions are still recognized without the

3591

@code{GCC} prefix, but that usage is deprecated. Some older pragmas are

3592

deprecated in their entirety. They are not recognized with the

3593

@code{GCC} prefix. @xref{Obsolete Features}.

3594

3595

@cindex @code{_Pragma}

3596

C99 introduces the @code{@w{_Pragma}} operator. This feature addresses a

3597

major problem with @samp{#pragma}: being a directive, it cannot be

3598

produced as the result of macro expansion. @code{@w{_Pragma}} is an

3599

operator, much like @code{sizeof} or @code{defined}, and can be embedded

3600

in a macro.

3601

3602

Its syntax is @code{@w{_Pragma (@var{string-literal})}}, where

3603

@var{string-literal} can be either a normal or wide-character string

3604

literal. It is destringized, by replacing all @samp{\\} with a single

3605

@samp{\} and all @samp{\"} with a @samp{"}. The result is then

3606

processed as if it had appeared as the right hand side of a

3607

@samp{#pragma} directive. For example,

3608

3609

@smallexample

3610

_Pragma ("GCC dependency \"parse.y\"")

3611

@end smallexample

3612

3613

@noindent

3614

has the same effect as @code{#pragma GCC dependency "parse.y"}. The

3615

same effect could be achieved using macros, for example

3616

3617

@smallexample

3618

#define DO_PRAGMA(x) _Pragma (#x)

3619

DO_PRAGMA (GCC dependency "parse.y")

3620

@end smallexample

3621

3622

The standard is unclear on where a @code{_Pragma} operator can appear.

3623

The preprocessor does not accept it within a preprocessing conditional

3624

directive like @samp{#if}. To be safe, you are probably best keeping it

3625

out of directives other than @samp{#define}, and putting it on a line of

3626

its own.

3627

3628

This manual documents the pragmas which are meaningful to the

3629

preprocessor itself. Other pragmas are meaningful to the C or C++

3630

compilers. They are documented in the GCC manual.

3631

3632

GCC plugins may provide their own pragmas.

3633

3634

@ftable @code

3635

@item #pragma GCC dependency

3636

@code{#pragma GCC dependency} allows you to check the relative dates of

3637

the current file and another file. If the other file is more recent than

3638

the current file, a warning is issued. This is useful if the current

3639

file is derived from the other file, and should be regenerated. The

3640

other file is searched for using the normal include search path.

3641

Optional trailing text can be used to give more information in the

3642

warning message.

3643

3644

@smallexample

3645

#pragma GCC dependency "parse.y"

3646

#pragma GCC dependency "/usr/include/time.h" rerun fixincludes

3647

@end smallexample

3648

3649

@item #pragma GCC poison

3650

Sometimes, there is an identifier that you want to remove completely

3651

from your program, and make sure that it never creeps back in. To

3652

enforce this, you can @dfn{poison} the identifier with this pragma.

3653

@code{#pragma GCC poison} is followed by a list of identifiers to

3654

poison. If any of those identifiers appears anywhere in the source

3655

after the directive, it is a hard error. For example,

3656

3657

@smallexample

3658

#pragma GCC poison printf sprintf fprintf

3659

sprintf(some_string, "hello");

3660

@end smallexample

3661

3662

@noindent

3663

will produce an error.

3664

3665

If a poisoned identifier appears as part of the expansion of a macro

3666

which was defined before the identifier was poisoned, it will @emph{not}

3667

cause an error. This lets you poison an identifier without worrying

3668

about system headers defining macros that use it.

3669

3670

For example,

3671

3672

@smallexample

3673

#define strrchr rindex

3674

#pragma GCC poison rindex

3675

strrchr(some_string, 'h');

3676

@end smallexample

3677

3678

@noindent

3679

will not produce an error.

3680

3681

@item #pragma GCC system_header

3682

This pragma takes no arguments. It causes the rest of the code in the

3683

current file to be treated as if it came from a system header.

3684

@xref{System Headers}.

3685

3686

@item #pragma GCC warning

3687

@itemx #pragma GCC error

3688

@code{#pragma GCC warning "message"} causes the preprocessor to issue

3689

a warning diagnostic with the text @samp{message}. The message

3690

contained in the pragma must be a single string literal. Similarly,

3691

@code{#pragma GCC error "message"} issues an error message. Unlike

3692

the @samp{#warning} and @samp{#error} directives, these pragmas can be

3693

embedded in preprocessor macros using @samp{_Pragma}.

3694

3695

@end ftable

3696

3697

@node Other Directives

3698

@chapter Other Directives

3699

3700

@findex #ident

3701

@findex #sccs

3702

The @samp{#ident} directive takes one argument, a string constant. On

3703

some systems, that string constant is copied into a special segment of

3704

the object file. On other systems, the directive is ignored. The

3705

@samp{#sccs} directive is a synonym for @samp{#ident}.

3706

3707

These directives are not part of the C standard, but they are not

3708

official GNU extensions either. What historical information we have

3709

been able to find, suggests they originated with System V@.

3710

3711

@cindex null directive

3712

The @dfn{null directive} consists of a @samp{#} followed by a newline,

3713

with only whitespace (including comments) in between. A null directive

3714

is understood as a preprocessing directive but has no effect on the

3715

preprocessor output. The primary significance of the existence of the

3716

null directive is that an input line consisting of just a @samp{#} will

3717

produce no output, rather than a line of output containing just a

3718

@samp{#}. Supposedly some old C programs contain such lines.

3719

3720

@node Preprocessor Output

3721

@chapter Preprocessor Output

3722

3723

When the C preprocessor is used with the C, C++, or Objective-C

3724

compilers, it is integrated into the compiler and communicates a stream

3725

of binary tokens directly to the compiler's parser. However, it can

3726

also be used in the more conventional standalone mode, where it produces

3727

textual output.

3728

@c FIXME: Document the library interface.

3729

3730

@cindex output format

3731

The output from the C preprocessor looks much like the input, except

3732

that all preprocessing directive lines have been replaced with blank

3733

lines and all comments with spaces. Long runs of blank lines are

3734

discarded.

3735

3736

The ISO standard specifies that it is implementation defined whether a

3737

preprocessor preserves whitespace between tokens, or replaces it with

3738

e.g.@: a single space. In GNU CPP, whitespace between tokens is collapsed

3739

to become a single space, with the exception that the first token on a

3740

non-directive line is preceded with sufficient spaces that it appears in

3741

the same column in the preprocessed output that it appeared in the

3742

original source file. This is so the output is easy to read.

3743

@xref{Differences from previous versions}. CPP does not insert any

3744

whitespace where there was none in the original source, except where

3745

necessary to prevent an accidental token paste.

3746

3747

@cindex linemarkers

3748

Source file name and line number information is conveyed by lines

3749

of the form

3750

3751

@smallexample

3752

# @var{linenum} @var{filename} @var{flags}

3753

@end smallexample

3754

3755

@noindent

3756

These are called @dfn{linemarkers}. They are inserted as needed into

3757

the output (but never within a string or character constant). They mean

3758

that the following line originated in file @var{filename} at line

3759

@var{linenum}. @var{filename} will never contain any non-printing

3760

characters; they are replaced with octal escape sequences.

3761

3762

After the file name comes zero or more flags, which are @samp{1},

3763

@samp{2}, @samp{3}, or @samp{4}. If there are multiple flags, spaces

3764

separate them. Here is what the flags mean:

3765

3766

@table @samp

3767

@item 1

3768

This indicates the start of a new file.

3769

@item 2

3770

This indicates returning to a file (after having included another file).

3771

@item 3

3772

This indicates that the following text comes from a system header file,

3773

so certain warnings should be suppressed.

3774

@item 4

3775

This indicates that the following text should be treated as being

3776

wrapped in an implicit @code{extern "C"} block.

3777

@c maybe cross reference NO_IMPLICIT_EXTERN_C

3778

@end table

3779

3780

As an extension, the preprocessor accepts linemarkers in non-assembler

3781

input files. They are treated like the corresponding @samp{#line}

3782

directive, (@pxref{Line Control}), except that trailing flags are

3783

permitted, and are interpreted with the meanings described above. If

3784

multiple flags are given, they must be in ascending order.

3785

3786

Some directives may be duplicated in the output of the preprocessor.

3787

These are @samp{#ident} (always), @samp{#pragma} (only if the

3788

preprocessor does not handle the pragma itself), and @samp{#define} and

3789

@samp{#undef} (with certain debugging options). If this happens, the

3790

@samp{#} of the directive will always be in the first column, and there

3791

will be no space between the @samp{#} and the directive name. If macro

3792

expansion happens to generate tokens which might be mistaken for a

3793

duplicated directive, a space will be inserted between the @samp{#} and

3794

the directive name.

3795

3796

@node Traditional Mode

3797

@chapter Traditional Mode

3798

3799

Traditional (pre-standard) C preprocessing is rather different from

3800

the preprocessing specified by the standard. When GCC is given the

3801

@option{-traditional-cpp} option, it attempts to emulate a traditional

3802

preprocessor.

3803

3804

GCC versions 3.2 and later only support traditional mode semantics in

3805

the preprocessor, and not in the compiler front ends. This chapter

3806

outlines the traditional preprocessor semantics we implemented.

3807

3808

The implementation does not correspond precisely to the behavior of

3809

earlier versions of GCC, nor to any true traditional preprocessor.

3810

After all, inconsistencies among traditional implementations were a

3811

major motivation for C standardization. However, we intend that it

3812

should be compatible with true traditional preprocessors in all ways

3813

that actually matter.

3814

3815

@menu

3816

* Traditional lexical analysis::

3817

* Traditional macros::

3818

* Traditional miscellany::

3819

* Traditional warnings::

3820

@end menu

3821

3822

@node Traditional lexical analysis

3823

@section Traditional lexical analysis

3824

3825

The traditional preprocessor does not decompose its input into tokens

3826

the same way a standards-conforming preprocessor does. The input is

3827

simply treated as a stream of text with minimal internal form.

3828

3829

This implementation does not treat trigraphs (@pxref{trigraphs})

3830

specially since they were an invention of the standards committee. It

3831

handles arbitrarily-positioned escaped newlines properly and splices

3832

the lines as you would expect; many traditional preprocessors did not

3833

do this.

3834

3835

The form of horizontal whitespace in the input file is preserved in

3836

the output. In particular, hard tabs remain hard tabs. This can be

3837

useful if, for example, you are preprocessing a Makefile.

3838

3839

Traditional CPP only recognizes C-style block comments, and treats the

3840

@samp{/*} sequence as introducing a comment only if it lies outside

3841

quoted text. Quoted text is introduced by the usual single and double

3842

quotes, and also by an initial @samp{<} in a @code{#include}

3843

directive.

3844

3845

Traditionally, comments are completely removed and are not replaced

3846

with a space. Since a traditional compiler does its own tokenization

3847

of the output of the preprocessor, this means that comments can

3848

effectively be used as token paste operators. However, comments

3849

behave like separators for text handled by the preprocessor itself,

3850

since it doesn't re-lex its input. For example, in

3851

3852

@smallexample

3853

#if foo/**/bar

3854

@end smallexample

3855

3856

@noindent

3857

@samp{foo} and @samp{bar} are distinct identifiers and expanded

3858

separately if they happen to be macros. In other words, this

3859

directive is equivalent to

3860

3861

@smallexample

3862

#if foo bar

3863

@end smallexample

3864

3865

@noindent

3866

rather than

3867

3868

@smallexample

3869

#if foobar

3870

@end smallexample

3871

3872

Generally speaking, in traditional mode an opening quote need not have

3873

a matching closing quote. In particular, a macro may be defined with

3874

replacement text that contains an unmatched quote. Of course, if you

3875

attempt to compile preprocessed output containing an unmatched quote

3876

you will get a syntax error.

3877

3878

However, all preprocessing directives other than @code{#define}

3879

require matching quotes. For example:

3880

3881

@smallexample

3882

#define m This macro's fine and has an unmatched quote

3883

"/* This is not a comment. */

3884

/* @r{This is a comment. The following #include directive

3885

is ill-formed.} */

3886

#include <stdio.h

3887

@end smallexample

3888

3889

Just as for the ISO preprocessor, what would be a closing quote can be

3890

escaped with a backslash to prevent the quoted text from closing.

3891

3892

@node Traditional macros

3893

@section Traditional macros

3894

3895

The major difference between traditional and ISO macros is that the

3896

former expand to text rather than to a token sequence. CPP removes

3897

all leading and trailing horizontal whitespace from a macro's

3898

replacement text before storing it, but preserves the form of internal

3899

whitespace.

3900

3901

One consequence is that it is legitimate for the replacement text to

3902

contain an unmatched quote (@pxref{Traditional lexical analysis}). An

3903

unclosed string or character constant continues into the text

3904

following the macro call. Similarly, the text at the end of a macro's

3905

expansion can run together with the text after the macro invocation to

3906

produce a single token.

3907

3908

Normally comments are removed from the replacement text after the

3909

macro is expanded, but if the @option{-CC} option is passed on the

3910

command-line comments are preserved. (In fact, the current

3911

implementation removes comments even before saving the macro

3912

replacement text, but it careful to do it in such a way that the

3913

observed effect is identical even in the function-like macro case.)

3914

3915

The ISO stringification operator @samp{#} and token paste operator

3916

@samp{##} have no special meaning. As explained later, an effect

3917

similar to these operators can be obtained in a different way. Macro

3918

names that are embedded in quotes, either from the main file or after

3919

macro replacement, do not expand.

3920

3921

CPP replaces an unquoted object-like macro name with its replacement

3922

text, and then rescans it for further macros to replace. Unlike

3923

standard macro expansion, traditional macro expansion has no provision

3924

to prevent recursion. If an object-like macro appears unquoted in its

3925

replacement text, it will be replaced again during the rescan pass,

3926

and so on @emph{ad infinitum}. GCC detects when it is expanding

3927

recursive macros, emits an error message, and continues after the

3928

offending macro invocation.

3929

3930

@smallexample

3931

#define PLUS +

3932

#define INC(x) PLUS+x

3933

INC(foo);

3934

@expansion{} ++foo;

3935

@end smallexample

3936

3937

Function-like macros are similar in form but quite different in

3938

behavior to their ISO counterparts. Their arguments are contained

3939

within parentheses, are comma-separated, and can cross physical lines.

3940

Commas within nested parentheses are not treated as argument

3941

separators. Similarly, a quote in an argument cannot be left

3942

unclosed; a following comma or parenthesis that comes before the

3943

closing quote is treated like any other character. There is no

3944

facility for handling variadic macros.

3945

3946

This implementation removes all comments from macro arguments, unless

3947

the @option{-C} option is given. The form of all other horizontal

3948

whitespace in arguments is preserved, including leading and trailing

3949

whitespace. In particular

3950

3951

@smallexample

3952

f( )

3953

@end smallexample

3954

3955

@noindent

3956

is treated as an invocation of the macro @samp{f} with a single

3957

argument consisting of a single space. If you want to invoke a

3958

function-like macro that takes no arguments, you must not leave any

3959

whitespace between the parentheses.

3960

3961

If a macro argument crosses a new line, the new line is replaced with

3962

a space when forming the argument. If the previous line contained an

3963

unterminated quote, the following line inherits the quoted state.

3964

3965

Traditional preprocessors replace parameters in the replacement text

3966

with their arguments regardless of whether the parameters are within

3967

quotes or not. This provides a way to stringize arguments. For

3968

example

3969

3970

@smallexample

3971

#define str(x) "x"

3972

str(/* @r{A comment} */some text )

3973

@expansion{} "some text "

3974

@end smallexample

3975

3976

@noindent

3977

Note that the comment is removed, but that the trailing space is

3978

preserved. Here is an example of using a comment to effect token

3979

pasting.

3980

3981

@smallexample

3982

#define suffix(x) foo_/**/x

3983

suffix(bar)

3984

@expansion{} foo_bar

3985

@end smallexample

3986

3987

@node Traditional miscellany

3988

@section Traditional miscellany

3989

3990

Here are some things to be aware of when using the traditional

3991

preprocessor.

3992

3993

@itemize @bullet

3994

@item

3995

Preprocessing directives are recognized only when their leading

3996

@samp{#} appears in the first column. There can be no whitespace

3997

between the beginning of the line and the @samp{#}, but whitespace can

3998

follow the @samp{#}.

3999

4000

@item

4001

A true traditional C preprocessor does not recognize @samp{#error} or

4002

@samp{#pragma}, and may not recognize @samp{#elif}. CPP supports all

4003

the directives in traditional mode that it supports in ISO mode,

4004

including extensions, with the exception that the effects of

4005

@samp{#pragma GCC poison} are undefined.

4006

4007

@item

4008

__STDC__ is not defined.

4009

4010

@item

4011

If you use digraphs the behavior is undefined.

4012

4013

@item

4014

If a line that looks like a directive appears within macro arguments,

4015

the behavior is undefined.

4016

4017

@end itemize

4018

4019

@node Traditional warnings

4020

@section Traditional warnings

4021

You can request warnings about features that did not exist, or worked

4022

differently, in traditional C with the @option{-Wtraditional} option.

4023

GCC does not warn about features of ISO C which you must use when you

4024

are using a conforming compiler, such as the @samp{#} and @samp{##}

4025

operators.

4026

4027

Presently @option{-Wtraditional} warns about:

4028

4029

@itemize @bullet

4030

@item

4031

Macro parameters that appear within string literals in the macro body.

4032

In traditional C macro replacement takes place within string literals,

4033

but does not in ISO C@.

4034

4035

@item

4036

In traditional C, some preprocessor directives did not exist.

4037

Traditional preprocessors would only consider a line to be a directive

4038

if the @samp{#} appeared in column 1 on the line. Therefore

4039

@option{-Wtraditional} warns about directives that traditional C

4040

understands but would ignore because the @samp{#} does not appear as the

4041

first character on the line. It also suggests you hide directives like

4042

@samp{#pragma} not understood by traditional C by indenting them. Some

4043

traditional implementations would not recognize @samp{#elif}, so it

4044

suggests avoiding it altogether.

4045

4046

@item

4047

A function-like macro that appears without an argument list. In some

4048

traditional preprocessors this was an error. In ISO C it merely means

4049

that the macro is not expanded.

4050

4051

@item

4052

The unary plus operator. This did not exist in traditional C@.

4053

4054

@item

4055

The @samp{U} and @samp{LL} integer constant suffixes, which were not

4056

available in traditional C@. (Traditional C does support the @samp{L}

4057

suffix for simple long integer constants.) You are not warned about

4058

uses of these suffixes in macros defined in system headers. For

4059

instance, @code{UINT_MAX} may well be defined as @code{4294967295U}, but

4060

you will not be warned if you use @code{UINT_MAX}.

4061

4062

You can usually avoid the warning, and the related warning about

4063

constants which are so large that they are unsigned, by writing the

4064

integer constant in question in hexadecimal, with no U suffix. Take

4065

care, though, because this gives the wrong result in exotic cases.

4066

@end itemize

4067

4068

@node Implementation Details

4069

@chapter Implementation Details

4070

4071

Here we document details of how the preprocessor's implementation

4072

affects its user-visible behavior. You should try to avoid undue

4073

reliance on behavior described here, as it is possible that it will

4074

change subtly in future implementations.

4075

4076

Also documented here are obsolete features and changes from previous

4077

versions of CPP@.

4078

4079

@menu

4080

* Implementation-defined behavior::

4081

* Implementation limits::

4082

* Obsolete Features::

4083

* Differences from previous versions::

4084

@end menu

4085

4086

@node Implementation-defined behavior

4087

@section Implementation-defined behavior

4088

@cindex implementation-defined behavior

4089

4090

This is how CPP behaves in all the cases which the C standard

4091

describes as @dfn{implementation-defined}. This term means that the

4092

implementation is free to do what it likes, but must document its choice

4093

and stick to it.

4094

@c FIXME: Check the C++ standard for more implementation-defined stuff.

4095

4096

@itemize @bullet

4097

@need 1000

4098

@item The mapping of physical source file multi-byte characters to the

4099

execution character set.

4100

4101

The input character set can be specified using the

4102

@option{-finput-charset} option, while the execution character set may

4103

be controlled using the @option{-fexec-charset} and

4104

@option{-fwide-exec-charset} options.

4105

4106

@item Identifier characters.

4107

@anchor{Identifier characters}

4108

4109

The C and C++ standards allow identifiers to be composed of @samp{_}

4110

and the alphanumeric characters. C++ and C99 also allow universal

4111

character names, and C99 further permits implementation-defined

4112

characters.

4113

4114

GCC allows the @samp{$} character in identifiers as an extension for

4115

most targets. This is true regardless of the @option{std=} switch,

4116

since this extension cannot conflict with standards-conforming

4117

programs. When preprocessing assembler, however, dollars are not

4118

identifier characters by default.

4119

4120

Currently the targets that by default do not permit @samp{$} are AVR,

4121

IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX

4122

operating system.

4123

4124

You can override the default with @option{-fdollars-in-identifiers} or

4125

@option{fno-dollars-in-identifiers}. @xref{fdollars-in-identifiers}.

4126

4127

@item Non-empty sequences of whitespace characters.

4128

4129

In textual output, each whitespace sequence is collapsed to a single

4130

space. For aesthetic reasons, the first token on each non-directive

4131

line of output is preceded with sufficient spaces that it appears in the

4132

same column as it did in the original source file.

4133

4134

@item The numeric value of character constants in preprocessor expressions.

4135

4136

The preprocessor and compiler interpret character constants in the

4137

same way; i.e.@: escape sequences such as @samp{\a} are given the

4138

values they would have on the target machine.

4139

4140

The compiler evaluates a multi-character character constant a character

4141

at a time, shifting the previous value left by the number of bits per

4142

target character, and then or-ing in the bit-pattern of the new

4143

character truncated to the width of a target character. The final

4144

bit-pattern is given type @code{int}, and is therefore signed,

4145

regardless of whether single characters are signed or not (a slight

4146

change from versions 3.1 and earlier of GCC)@. If there are more

4147

characters in the constant than would fit in the target @code{int} the

4148

compiler issues a warning, and the excess leading characters are

4149

ignored.

4150

4151

For example, @code{'ab'} for a target with an 8-bit @code{char} would be

4152

interpreted as @w{@samp{(int) ((unsigned char) 'a' * 256 + (unsigned char)

4153

'b')}}, and @code{'\234a'} as @w{@samp{(int) ((unsigned char) '\234' *

4154

256 + (unsigned char) 'a')}}.

4155

4156

@item Source file inclusion.

4157

4158

For a discussion on how the preprocessor locates header files,

4159

@ref{Include Operation}.

4160

4161

@item Interpretation of the filename resulting from a macro-expanded

4162

@samp{#include} directive.

4163

4164

@xref{Computed Includes}.

4165

4166

@item Treatment of a @samp{#pragma} directive that after macro-expansion

4167

results in a standard pragma.

4168

4169

No macro expansion occurs on any @samp{#pragma} directive line, so the

4170

question does not arise.

4171

4172

Note that GCC does not yet implement any of the standard

4173

pragmas.

4174

4175

@end itemize

4176

4177

@node Implementation limits

4178

@section Implementation limits

4179

@cindex implementation limits

4180

4181

CPP has a small number of internal limits. This section lists the

4182

limits which the C standard requires to be no lower than some minimum,

4183

and all the others known. It is intended that there should be as few limits

4184

as possible. If you encounter an undocumented or inconvenient limit,

4185

please report that as a bug. @xref{Bugs, , Reporting Bugs, gcc, Using

4186

the GNU Compiler Collection (GCC)}.

4187

4188

Where we say something is limited @dfn{only by available memory}, that

4189

means that internal data structures impose no intrinsic limit, and space

4190

is allocated with @code{malloc} or equivalent. The actual limit will

4191

therefore depend on many things, such as the size of other things

4192

allocated by the compiler at the same time, the amount of memory

4193

consumed by other processes on the same computer, etc.

4194

4195

@itemize @bullet

4196

4197

@item Nesting levels of @samp{#include} files.

4198

4199

We impose an arbitrary limit of 200 levels, to avoid runaway recursion.

4200

The standard requires at least 15 levels.

4201

4202

@item Nesting levels of conditional inclusion.

4203

4204

The C standard mandates this be at least 63. CPP is limited only by

4205

available memory.

4206

4207

@item Levels of parenthesized expressions within a full expression.

4208

4209

The C standard requires this to be at least 63. In preprocessor

4210

conditional expressions, it is limited only by available memory.

4211

4212

@item Significant initial characters in an identifier or macro name.

4213

4214

The preprocessor treats all characters as significant. The C standard

4215

requires only that the first 63 be significant.

4216

4217

@item Number of macros simultaneously defined in a single translation unit.

4218

4219

The standard requires at least 4095 be possible. CPP is limited only

4220

by available memory.

4221

4222

@item Number of parameters in a macro definition and arguments in a macro call.

4223

4224

We allow @code{USHRT_MAX}, which is no smaller than 65,535. The minimum

4225

required by the standard is 127.

4226

4227

@item Number of characters on a logical source line.

4228

4229

The C standard requires a minimum of 4096 be permitted. CPP places

4230

no limits on this, but you may get incorrect column numbers reported in

4231

diagnostics for lines longer than 65,535 characters.

4232

4233

@item Maximum size of a source file.

4234

4235

The standard does not specify any lower limit on the maximum size of a

4236

source file. GNU cpp maps files into memory, so it is limited by the

4237

available address space. This is generally at least two gigabytes.

4238

Depending on the operating system, the size of physical memory may or

4239

may not be a limitation.

4240

4241

@end itemize

4242

4243

@node Obsolete Features

4244

@section Obsolete Features

4245

4246

CPP has some features which are present mainly for compatibility with

4247

older programs. We discourage their use in new code. In some cases,

4248

we plan to remove the feature in a future version of GCC@.

4249

4250

@subsection Assertions

4251

@cindex assertions

4252

4253

@dfn{Assertions} are a deprecated alternative to macros in writing

4254

conditionals to test what sort of computer or system the compiled

4255

program will run on. Assertions are usually predefined, but you can

4256

define them with preprocessing directives or command-line options.

4257

4258

Assertions were intended to provide a more systematic way to describe

4259

the compiler's target system and we added them for compatibility with

4260

existing compilers. In practice they are just as unpredictable as the

4261

system-specific predefined macros. In addition, they are not part of

4262

any standard, and only a few compilers support them.

4263

Therefore, the use of assertions is @strong{less} portable than the use

4264

of system-specific predefined macros. We recommend you do not use them at

4265

all.

4266

4267

@cindex predicates

4268

An assertion looks like this:

4269

4270

@smallexample

4271

#@var{predicate} (@var{answer})

4272

@end smallexample

4273

4274

@noindent

4275

@var{predicate} must be a single identifier. @var{answer} can be any

4276

sequence of tokens; all characters are significant except for leading

4277

and trailing whitespace, and differences in internal whitespace

4278

sequences are ignored. (This is similar to the rules governing macro

4279

redefinition.) Thus, @code{(x + y)} is different from @code{(x+y)} but

4280

equivalent to @code{@w{( x + y )}}. Parentheses do not nest inside an

4281

answer.

4282

4283

@cindex testing predicates

4284

To test an assertion, you write it in an @samp{#if}. For example, this

4285

conditional succeeds if either @code{vax} or @code{ns16000} has been

4286

asserted as an answer for @code{machine}.

4287

4288

@smallexample

4289

#if #machine (vax) || #machine (ns16000)

4290

@end smallexample

4291

4292

@noindent

4293

You can test whether @emph{any} answer is asserted for a predicate by

4294

omitting the answer in the conditional:

4295

4296

@smallexample

4297

#if #machine

4298

@end smallexample

4299

4300

@findex #assert

4301

Assertions are made with the @samp{#assert} directive. Its sole

4302

argument is the assertion to make, without the leading @samp{#} that

4303

identifies assertions in conditionals.

4304

4305

@smallexample

4306

#assert @var{predicate} (@var{answer})

4307

@end smallexample

4308

4309

@noindent

4310

You may make several assertions with the same predicate and different

4311

answers. Subsequent assertions do not override previous ones for the

4312

same predicate. All the answers for any given predicate are

4313

simultaneously true.

4314

4315

@cindex assertions, canceling

4316

@findex #unassert

4317

Assertions can be canceled with the @samp{#unassert} directive. It

4318

has the same syntax as @samp{#assert}. In that form it cancels only the

4319

answer which was specified on the @samp{#unassert} line; other answers

4320

for that predicate remain true. You can cancel an entire predicate by

4321

leaving out the answer:

4322

4323

@smallexample

4324

#unassert @var{predicate}

4325

@end smallexample

4326

4327

@noindent

4328

In either form, if no such assertion has been made, @samp{#unassert} has

4329

no effect.

4330

4331

You can also make or cancel assertions using command-line options.

4332

@xref{Invocation}.

4333

4334

@node Differences from previous versions

4335

@section Differences from previous versions

4336

@cindex differences from previous versions

4337

4338

This section details behavior which has changed from previous versions

4339

of CPP@. We do not plan to change it again in the near future, but

4340

we do not promise not to, either.

4341

4342

The ``previous versions'' discussed here are 2.95 and before. The

4343

behavior of GCC 3.0 is mostly the same as the behavior of the widely

4344

used 2.96 and 2.97 development snapshots. Where there are differences,

4345

they generally represent bugs in the snapshots.

4346

4347

@itemize @bullet

4348

4349

@item -I- deprecated

4350

4351

This option has been deprecated in 4.0. @option{-iquote} is meant to

4352

replace the need for this option.

4353

4354

@item Order of evaluation of @samp{#} and @samp{##} operators

4355

4356

The standard does not specify the order of evaluation of a chain of

4357

@samp{##} operators, nor whether @samp{#} is evaluated before, after, or

4358

at the same time as @samp{##}. You should therefore not write any code

4359

which depends on any specific ordering. It is possible to guarantee an

4360

ordering, if you need one, by suitable use of nested macros.

4361

4362

An example of where this might matter is pasting the arguments @samp{1},

4363

@samp{e} and @samp{-2}. This would be fine for left-to-right pasting,

4364

but right-to-left pasting would produce an invalid token @samp{e-2}.

4365

4366

GCC 3.0 evaluates @samp{#} and @samp{##} at the same time and strictly

4367

left to right. Older versions evaluated all @samp{#} operators first,

4368

then all @samp{##} operators, in an unreliable order.

4369

4370

@item The form of whitespace between tokens in preprocessor output

4371

4372

@xref{Preprocessor Output}, for the current textual format. This is

4373

also the format used by stringification. Normally, the preprocessor

4374

communicates tokens directly to the compiler's parser, and whitespace

4375

does not come up at all.

4376

4377

Older versions of GCC preserved all whitespace provided by the user and

4378

inserted lots more whitespace of their own, because they could not

4379

accurately predict when extra spaces were needed to prevent accidental

4380

token pasting.

4381

4382

@item Optional argument when invoking rest argument macros

4383

4384

As an extension, GCC permits you to omit the variable arguments entirely

4385

when you use a variable argument macro. This is forbidden by the 1999 C

4386

standard, and will provoke a pedantic warning with GCC 3.0. Previous

4387

versions accepted it silently.

4388

4389

@item @samp{##} swallowing preceding text in rest argument macros

4390

4391

Formerly, in a macro expansion, if @samp{##} appeared before a variable

4392

arguments parameter, and the set of tokens specified for that argument

4393

in the macro invocation was empty, previous versions of CPP would

4394

back up and remove the preceding sequence of non-whitespace characters

4395

(@strong{not} the preceding token). This extension is in direct

4396

conflict with the 1999 C standard and has been drastically pared back.

4397

4398

In the current version of the preprocessor, if @samp{##} appears between

4399

a comma and a variable arguments parameter, and the variable argument is

4400

omitted entirely, the comma will be removed from the expansion. If the

4401

variable argument is empty, or the token before @samp{##} is not a

4402

comma, then @samp{##} behaves as a normal token paste.

4403

4404

@item @samp{#line} and @samp{#include}

4405

4406

The @samp{#line} directive used to change GCC's notion of the

4407

``directory containing the current file'', used by @samp{#include} with

4408

a double-quoted header file name. In 3.0 and later, it does not.

4409

@xref{Line Control}, for further explanation.

4410

4411

@item Syntax of @samp{#line}

4412

4413

In GCC 2.95 and previous, the string constant argument to @samp{#line}

4414

was treated the same way as the argument to @samp{#include}: backslash

4415

escapes were not honored, and the string ended at the second @samp{"}.

4416

This is not compliant with the C standard. In GCC 3.0, an attempt was

4417

made to correct the behavior, so that the string was treated as a real

4418

string constant, but it turned out to be buggy. In 3.1, the bugs have

4419

been fixed. (We are not fixing the bugs in 3.0 because they affect

4420

relatively few people and the fix is quite invasive.)

4421

4422

@end itemize

4423

4424

@node Invocation

4425

@chapter Invocation

4426

@cindex invocation

4427

@cindex command line

4428

4429

Most often when you use the C preprocessor you will not have to invoke it

4430

explicitly: the C compiler will do so automatically. However, the

4431

preprocessor is sometimes useful on its own. All the options listed

4432

here are also acceptable to the C compiler and have the same meaning,

4433

except that the C compiler has different rules for specifying the output

4434

file.

4435

4436

@emph{Note:} Whether you use the preprocessor by way of @command{gcc}

4437

or @command{cpp}, the @dfn{compiler driver} is run first. This

4438

program's purpose is to translate your command into invocations of the

4439

programs that do the actual work. Their command-line interfaces are

4440

similar but not identical to the documented interface, and may change

4441

without notice.

4442

4443

@ignore

4444

@c man begin SYNOPSIS

4445

cpp [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]

4446

[@option{-I}@var{dir}@dots{}] [@option{-iquote}@var{dir}@dots{}]

4447

[@option{-W}@var{warn}@dots{}]

4448

[@option{-M}|@option{-MM}] [@option{-MG}] [@option{-MF} @var{filename}]

4449

[@option{-MP}] [@option{-MQ} @var{target}@dots{}]

4450

[@option{-MT} @var{target}@dots{}]

4451

[@option{-P}] [@option{-fno-working-directory}]

4452

[@option{-x} @var{language}] [@option{-std=}@var{standard}]

4453

@var{infile} @var{outfile}

4454

4455

Only the most useful options are listed here; see below for the remainder.

4456

@c man end

4457

@c man begin SEEALSO

4458

gpl(7), gfdl(7), fsf-funding(7),

4459

gcc(1), as(1), ld(1), and the Info entries for @file{cpp}, @file{gcc}, and

4460

@file{binutils}.

4461

@c man end

4462

@end ignore

4463

4464

@c man begin OPTIONS

4465

The C preprocessor expects two file names as arguments, @var{infile} and

4466

@var{outfile}. The preprocessor reads @var{infile} together with any

4467

other files it specifies with @samp{#include}. All the output generated

4468

by the combined input files is written in @var{outfile}.

4469

4470

Either @var{infile} or @var{outfile} may be @option{-}, which as

4471

@var{infile} means to read from standard input and as @var{outfile}

4472

means to write to standard output. Also, if either file is omitted, it

4473

means the same as if @option{-} had been specified for that file.

4474

4475

Unless otherwise noted, or the option ends in @samp{=}, all options

4476

which take an argument may have that argument appear either immediately

4477

after the option, or with a space between option and argument:

4478

@option{-Ifoo} and @option{-I foo} have the same effect.

4479

4480

@cindex grouping options

4481

@cindex options, grouping

4482

Many options have multi-letter names; therefore multiple single-letter

4483

options may @emph{not} be grouped: @option{-dM} is very different from

4484

@w{@samp{-d -M}}.

4485

4486

@cindex options

4487

@include cppopts.texi

4488

@c man end

4489

4490

@node Environment Variables

4491

@chapter Environment Variables

4492

@cindex environment variables

4493

@c man begin ENVIRONMENT

4494

4495

This section describes the environment variables that affect how CPP

4496

operates. You can use them to specify directories or prefixes to use

4497

when searching for include files, or to control dependency output.

4498

4499

Note that you can also specify places to search using options such as

4500

@option{-I}, and control dependency output with options like

4501

@option{-M} (@pxref{Invocation}). These take precedence over

4502

environment variables, which in turn take precedence over the

4503

configuration of GCC@.

4504

4505

@include cppenv.texi

4506

@c man end

4507

4508

@page

4509

@include fdl.texi

4510

4511

@page

4512

@node Index of Directives

4513

@unnumbered Index of Directives

4514

@printindex fn

4515

4516

@node Option Index

4517

@unnumbered Option Index

4518

@noindent

4519

CPP's command-line options and environment variables are indexed here

4520

without any initial @samp{-} or @samp{--}.

4521

@printindex op

4522

4523

@page

4524

@node Concept Index

4525

@unnumbered Concept Index

4526

@printindex cp

4527

4528

@bye