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<title>Using the GNU Compiler Collection (GCC)</title>
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<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 if
any of these features is used.) To test for the availability of these
features 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 are
also 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, as
extensions, 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 statement-expression.
<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="Lvalues.html#Lvalues">Lvalues</a>: Using <code>?:</code>, <code>,</code> and casts in lvalues.
<li><a accesskey="8" href="Conditionals.html#Conditionals">Conditionals</a>: Omitting the middle operand of a <code>?:</code> expression.
<li><a accesskey="9" href="Long-Long.html#Long%20Long">Long Long</a>: Double-word integers---<code>long long int</code>.
<li><a 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="Multi-line-Strings.html#Multi-line%20Strings">Multi-line Strings</a>: String literals with embedded 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>
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