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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below). A copy of the license isincluded in the section entitled "GNU Free Documentation License". <p>(a) The FSF's Front-Cover Text is: <p>A GNU Manual <p>(b) The FSF's Back-Cover Text is: <p>You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller }--></style></head><body><div class="node"><p>Node: <a name="C%20Extensions">C Extensions</a>,Next: <a rel="next" accesskey="n" href="C---Extensions.html#C++%20Extensions">C++ Extensions</a>,Previous: <a rel="previous" accesskey="p" href="C-Implementation.html#C%20Implementation">C Implementation</a>,Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a><hr><br></div><h2 class="chapter">Extensions to the C Language Family</h2><p>GNU C provides several language features not found in ISO standard C. (The <code>-pedantic</code> option directs GCC to print a warning message ifany of these features is used.) To test for the availability of thesefeatures in conditional compilation, check for a predefined macro<code>__GNUC__</code>, which is always defined under GCC. <p>These extensions are available in C and Objective-C. Most of them arealso available in C++. See <a href="C---Extensions.html#C++%20Extensions">Extensions to the C++ Language</a>, for extensions that apply <em>only</em> to C++. <p>Some features that are in ISO C99 but not C89 or C++ are also, asextensions, accepted by GCC in C89 mode and in C++.<ul class="menu"><li><a accesskey="1" href="Statement-Exprs.html#Statement%20Exprs">Statement Exprs</a>: Putting statements and declarations inside expressions. <li><a accesskey="2" href="Local-Labels.html#Local%20Labels">Local Labels</a>: Labels local to a block. <li><a accesskey="3" href="Labels-as-Values.html#Labels%20as%20Values">Labels as Values</a>: Getting pointers to labels, and computed gotos. <li><a accesskey="4" href="Nested-Functions.html#Nested%20Functions">Nested Functions</a>: As in Algol and Pascal, lexical scoping of functions. <li><a accesskey="5" href="Constructing-Calls.html#Constructing%20Calls">Constructing Calls</a>: Dispatching a call to another function. <li><a accesskey="6" href="Typeof.html#Typeof">Typeof</a>: <code>typeof</code>: referring to the type of an expression. <li><a accesskey="7" href="Conditionals.html#Conditionals">Conditionals</a>: Omitting the middle operand of a <code>?:</code> expression. <li><a accesskey="8" href="Long-Long.html#Long%20Long">Long Long</a>: Double-word integers---<code>long long int</code>. <li><a accesskey="9" href="Complex.html#Complex">Complex</a>: Data types for complex numbers. <li><a href="Hex-Floats.html#Hex%20Floats">Hex Floats</a>: Hexadecimal floating-point constants. <li><a href="Zero-Length.html#Zero%20Length">Zero Length</a>: Zero-length arrays. <li><a href="Variable-Length.html#Variable%20Length">Variable Length</a>: Arrays whose length is computed at run time. <li><a href="Empty-Structures.html#Empty%20Structures">Empty Structures</a>: Structures with no members. <li><a href="Variadic-Macros.html#Variadic%20Macros">Variadic Macros</a>: Macros with a variable number of arguments. <li><a href="Escaped-Newlines.html#Escaped%20Newlines">Escaped Newlines</a>: Slightly looser rules for escaped newlines. <li><a href="Subscripting.html#Subscripting">Subscripting</a>: Any array can be subscripted, even if not an lvalue. <li><a href="Pointer-Arith.html#Pointer%20Arith">Pointer Arith</a>: Arithmetic on <code>void</code>-pointers and function pointers. <li><a href="Initializers.html#Initializers">Initializers</a>: Non-constant initializers. <li><a href="Compound-Literals.html#Compound%20Literals">Compound Literals</a>: Compound literals give structures, unions or arrays as values. <li><a href="Designated-Inits.html#Designated%20Inits">Designated Inits</a>: Labeling elements of initializers. <li><a href="Cast-to-Union.html#Cast%20to%20Union">Cast to Union</a>: Casting to union type from any member of the union. <li><a href="Case-Ranges.html#Case%20Ranges">Case Ranges</a>: `case 1 ... 9' and such. <li><a href="Mixed-Declarations.html#Mixed%20Declarations">Mixed Declarations</a>: Mixing declarations and code. <li><a href="Function-Attributes.html#Function%20Attributes">Function Attributes</a>: Declaring that functions have no side effects, or that they can never return. <li><a href="Attribute-Syntax.html#Attribute%20Syntax">Attribute Syntax</a>: Formal syntax for attributes. <li><a href="Function-Prototypes.html#Function%20Prototypes">Function Prototypes</a>: Prototype declarations and old-style definitions. <li><a href="C---Comments.html#C++%20Comments">C++ Comments</a>: C++ comments are recognized. <li><a href="Dollar-Signs.html#Dollar%20Signs">Dollar Signs</a>: Dollar sign is allowed in identifiers. <li><a href="Character-Escapes.html#Character%20Escapes">Character Escapes</a>: <code>\e</code> stands for the character <ESC>. <li><a href="Variable-Attributes.html#Variable%20Attributes">Variable Attributes</a>: Specifying attributes of variables. <li><a href="Type-Attributes.html#Type%20Attributes">Type Attributes</a>: Specifying attributes of types. <li><a href="Alignment.html#Alignment">Alignment</a>: Inquiring about the alignment of a type or variable. <li><a href="Inline.html#Inline">Inline</a>: Defining inline functions (as fast as macros). <li><a href="Extended-Asm.html#Extended%20Asm">Extended Asm</a>: Assembler instructions with C expressions as operands. (With them you can define ``built-in'' functions.) <li><a href="Constraints.html#Constraints">Constraints</a>: Constraints for asm operands<li><a href="Asm-Labels.html#Asm%20Labels">Asm Labels</a>: Specifying the assembler name to use for a C symbol. <li><a href="Explicit-Reg-Vars.html#Explicit%20Reg%20Vars">Explicit Reg Vars</a>: Defining variables residing in specified registers. <li><a href="Alternate-Keywords.html#Alternate%20Keywords">Alternate Keywords</a>: <code>__const__</code>, <code>__asm__</code>, etc., for header files. <li><a href="Incomplete-Enums.html#Incomplete%20Enums">Incomplete Enums</a>: <code>enum foo;</code>, with details to follow. <li><a href="Function-Names.html#Function%20Names">Function Names</a>: Printable strings which are the name of the current function. <li><a href="Return-Address.html#Return%20Address">Return Address</a>: Getting the return or frame address of a function. <li><a href="Vector-Extensions.html#Vector%20Extensions">Vector Extensions</a>: Using vector instructions through built-in functions. <li><a href="Other-Builtins.html#Other%20Builtins">Other Builtins</a>: Other built-in functions. <li><a href="Target-Builtins.html#Target%20Builtins">Target Builtins</a>: Built-in functions specific to particular targets. <li><a href="Pragmas.html#Pragmas">Pragmas</a>: Pragmas accepted by GCC. <li><a href="Unnamed-Fields.html#Unnamed%20Fields">Unnamed Fields</a>: Unnamed struct/union fields within structs/unions. <li><a href="Thread-Local.html#Thread-Local">Thread-Local</a>: Per-thread variables. </ul> </body></html>⌨️ 快捷键说明
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