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  25. <title>C Extensions (Using the GNU Compiler Collection (GCC))</title>

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  68. 

  69. <body lang="en">

  70. <a name="C-Extensions"></a>

  71. <div class="header">

  72. <p>

  73. Next: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="n" rel="next">C++ Extensions</a>, Previous: <a href="C_002b_002b-Implementation.html#C_002b_002b-Implementation" accesskey="p" rel="prev">C++ Implementation</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>

  74. </div>

  75. <hr>

  76. <a name="Extensions-to-the-C-Language-Family"></a>

  77. <h2 class="chapter">6 Extensions to the C Language Family</h2>

  78. <a name="index-extensions_002c-C-language"></a>

  79. <a name="index-C-language-extensions"></a>

  80. 

  81. <a name="index-pedantic-2"></a>

  82. <p>GNU C provides several language features not found in ISO standard C.

  83. (The <samp>-pedantic</samp> option directs GCC to print a warning message if

  84. any of these features is used.)  To test for the availability of these

  85. features in conditional compilation, check for a predefined macro

  86. <code>__GNUC__</code>, which is always defined under GCC.

  87. </p>

  88. <p>These extensions are available in C and Objective-C.  Most of them are

  89. also available in C++.  See <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions">Extensions to the

  90. C++ Language</a>, for extensions that apply <em>only</em> to C++.

  91. </p>

  92. <p>Some features that are in ISO C99 but not C90 or C++ are also, as

  93. extensions, accepted by GCC in C90 mode and in C++.

  94. </p>

  95. <table class="menu" border="0" cellspacing="0">

  96. <tr><td align="left" valign="top">&bull; <a href="Statement-Exprs.html#Statement-Exprs" accesskey="1">Statement Exprs</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Putting statements and declarations inside expressions.

  97. </td></tr>

  98. <tr><td align="left" valign="top">&bull; <a href="Local-Labels.html#Local-Labels" accesskey="2">Local Labels</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Labels local to a block.

  99. </td></tr>

  100. <tr><td align="left" valign="top">&bull; <a href="Labels-as-Values.html#Labels-as-Values" accesskey="3">Labels as Values</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Getting pointers to labels, and computed gotos.

  101. </td></tr>

  102. <tr><td align="left" valign="top">&bull; <a href="Nested-Functions.html#Nested-Functions" accesskey="4">Nested Functions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Nested function in GNU C.

  103. </td></tr>

  104. <tr><td align="left" valign="top">&bull; <a href="Nonlocal-Gotos.html#Nonlocal-Gotos" accesskey="5">Nonlocal Gotos</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Nonlocal gotos.

  105. </td></tr>

  106. <tr><td align="left" valign="top">&bull; <a href="Constructing-Calls.html#Constructing-Calls" accesskey="6">Constructing Calls</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Dispatching a call to another function.

  107. </td></tr>

  108. <tr><td align="left" valign="top">&bull; <a href="Typeof.html#Typeof" accesskey="7">Typeof</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top"><code>typeof</code>: referring to the type of an expression.

  109. </td></tr>

  110. <tr><td align="left" valign="top">&bull; <a href="Conditionals.html#Conditionals" accesskey="8">Conditionals</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Omitting the middle operand of a &lsquo;<samp>?:</samp>&rsquo; expression.

  111. </td></tr>

  112. <tr><td align="left" valign="top">&bull; <a href="_005f_005fint128.html#g_t_005f_005fint128" accesskey="9">__int128</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">128-bit integers&mdash;<code>__int128</code>.

  113. </td></tr>

  114. <tr><td align="left" valign="top">&bull; <a href="Long-Long.html#Long-Long">Long Long</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Double-word integers&mdash;<code>long long int</code>.

  115. </td></tr>

  116. <tr><td align="left" valign="top">&bull; <a href="Complex.html#Complex">Complex</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Data types for complex numbers.

  117. </td></tr>

  118. <tr><td align="left" valign="top">&bull; <a href="Floating-Types.html#Floating-Types">Floating Types</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Additional Floating Types.

  119. </td></tr>

  120. <tr><td align="left" valign="top">&bull; <a href="Half_002dPrecision.html#Half_002dPrecision">Half-Precision</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Half-Precision Floating Point.

  121. </td></tr>

  122. <tr><td align="left" valign="top">&bull; <a href="Decimal-Float.html#Decimal-Float">Decimal Float</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Decimal Floating Types.

  123. </td></tr>

  124. <tr><td align="left" valign="top">&bull; <a href="Hex-Floats.html#Hex-Floats">Hex Floats</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Hexadecimal floating-point constants.

  125. </td></tr>

  126. <tr><td align="left" valign="top">&bull; <a href="Fixed_002dPoint.html#Fixed_002dPoint">Fixed-Point</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Fixed-Point Types.

  127. </td></tr>

  128. <tr><td align="left" valign="top">&bull; <a href="Named-Address-Spaces.html#Named-Address-Spaces">Named Address Spaces</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Named address spaces.

  129. </td></tr>

  130. <tr><td align="left" valign="top">&bull; <a href="Zero-Length.html#Zero-Length">Zero Length</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Zero-length arrays.

  131. </td></tr>

  132. <tr><td align="left" valign="top">&bull; <a href="Empty-Structures.html#Empty-Structures">Empty Structures</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Structures with no members.

  133. </td></tr>

  134. <tr><td align="left" valign="top">&bull; <a href="Variable-Length.html#Variable-Length">Variable Length</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Arrays whose length is computed at run time.

  135. </td></tr>

  136. <tr><td align="left" valign="top">&bull; <a href="Variadic-Macros.html#Variadic-Macros">Variadic Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Macros with a variable number of arguments.

  137. </td></tr>

  138. <tr><td align="left" valign="top">&bull; <a href="Escaped-Newlines.html#Escaped-Newlines">Escaped Newlines</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Slightly looser rules for escaped newlines.

  139. </td></tr>

  140. <tr><td align="left" valign="top">&bull; <a href="Subscripting.html#Subscripting">Subscripting</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Any array can be subscripted, even if not an lvalue.

  141. </td></tr>

  142. <tr><td align="left" valign="top">&bull; <a href="Pointer-Arith.html#Pointer-Arith">Pointer Arith</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Arithmetic on <code>void</code>-pointers and function pointers.

  143. </td></tr>

  144. <tr><td align="left" valign="top">&bull; <a href="Variadic-Pointer-Args.html#Variadic-Pointer-Args">Variadic Pointer Args</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Pointer arguments to variadic functions.

  145. </td></tr>

  146. <tr><td align="left" valign="top">&bull; <a href="Pointers-to-Arrays.html#Pointers-to-Arrays">Pointers to Arrays</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Pointers to arrays with qualifiers work as expected.

  147. </td></tr>

  148. <tr><td align="left" valign="top">&bull; <a href="Initializers.html#Initializers">Initializers</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Non-constant initializers.

  149. </td></tr>

  150. <tr><td align="left" valign="top">&bull; <a href="Compound-Literals.html#Compound-Literals">Compound Literals</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Compound literals give structures, unions

  151.                         or arrays as values.

  152. </td></tr>

  153. <tr><td align="left" valign="top">&bull; <a href="Designated-Inits.html#Designated-Inits">Designated Inits</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Labeling elements of initializers.

  154. </td></tr>

  155. <tr><td align="left" valign="top">&bull; <a href="Case-Ranges.html#Case-Ranges">Case Ranges</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">&lsquo;case 1 ... 9&rsquo; and such.

  156. </td></tr>

  157. <tr><td align="left" valign="top">&bull; <a href="Cast-to-Union.html#Cast-to-Union">Cast to Union</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Casting to union type from any member of the union.

  158. </td></tr>

  159. <tr><td align="left" valign="top">&bull; <a href="Mixed-Declarations.html#Mixed-Declarations">Mixed Declarations</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Mixing declarations and code.

  160. </td></tr>

  161. <tr><td align="left" valign="top">&bull; <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Declaring that functions have no side effects,

  162.                         or that they can never return.

  163. </td></tr>

  164. <tr><td align="left" valign="top">&bull; <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes of variables.

  165. </td></tr>

  166. <tr><td align="left" valign="top">&bull; <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes of types.

  167. </td></tr>

  168. <tr><td align="left" valign="top">&bull; <a href="Label-Attributes.html#Label-Attributes">Label Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes on labels.

  169. </td></tr>

  170. <tr><td align="left" valign="top">&bull; <a href="Enumerator-Attributes.html#Enumerator-Attributes">Enumerator Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes on enumerators.

  171. </td></tr>

  172. <tr><td align="left" valign="top">&bull; <a href="Statement-Attributes.html#Statement-Attributes">Statement Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specifying attributes on statements.

  173. </td></tr>

  174. <tr><td align="left" valign="top">&bull; <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Formal syntax for attributes.

  175. </td></tr>

  176. <tr><td align="left" valign="top">&bull; <a href="Function-Prototypes.html#Function-Prototypes">Function Prototypes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Prototype declarations and old-style definitions.

  177. </td></tr>

  178. <tr><td align="left" valign="top">&bull; <a href="C_002b_002b-Comments.html#C_002b_002b-Comments">C++ Comments</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">C++ comments are recognized.

  179. </td></tr>

  180. <tr><td align="left" valign="top">&bull; <a href="Dollar-Signs.html#Dollar-Signs">Dollar Signs</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Dollar sign is allowed in identifiers.

  181. </td></tr>

  182. <tr><td align="left" valign="top">&bull; <a href="Character-Escapes.html#Character-Escapes">Character Escapes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">&lsquo;<samp>\e</samp>&rsquo; stands for the character <tt class="key">ESC</tt>.

  183. </td></tr>

  184. <tr><td align="left" valign="top">&bull; <a href="Alignment.html#Alignment">Alignment</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Determining the alignment of a function, type or variable.

  185. </td></tr>

  186. <tr><td align="left" valign="top">&bull; <a href="Inline.html#Inline">Inline</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Defining inline functions (as fast as macros).

  187. </td></tr>

  188. <tr><td align="left" valign="top">&bull; <a href="Volatiles.html#Volatiles">Volatiles</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">What constitutes an access to a volatile object.

  189. </td></tr>

  190. <tr><td align="left" valign="top">&bull; <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C">Using Assembly Language with C</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Instructions and extensions for interfacing C with assembler.

  191. </td></tr>

  192. <tr><td align="left" valign="top">&bull; <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top"><code>__const__</code>, <code>__asm__</code>, etc., for header files.

  193. </td></tr>

  194. <tr><td align="left" valign="top">&bull; <a href="Incomplete-Enums.html#Incomplete-Enums">Incomplete Enums</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top"><code>enum foo;</code>, with details to follow.

  195. </td></tr>

  196. <tr><td align="left" valign="top">&bull; <a href="Function-Names.html#Function-Names">Function Names</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Printable strings which are the name of the current

  197.                         function.

  198. </td></tr>

  199. <tr><td align="left" valign="top">&bull; <a href="Return-Address.html#Return-Address">Return Address</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Getting the return or frame address of a function.

  200. </td></tr>

  201. <tr><td align="left" valign="top">&bull; <a href="Vector-Extensions.html#Vector-Extensions">Vector Extensions</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Using vector instructions through built-in functions.

  202. </td></tr>

  203. <tr><td align="left" valign="top">&bull; <a href="Offsetof.html#Offsetof">Offsetof</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Special syntax for implementing <code>offsetof</code>.

  204. </td></tr>

  205. <tr><td align="left" valign="top">&bull; <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins">__sync Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Legacy built-in functions for atomic memory access.

  206. </td></tr>

  207. <tr><td align="left" valign="top">&bull; <a href="_005f_005fatomic-Builtins.html#g_t_005f_005fatomic-Builtins">__atomic Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Atomic built-in functions with memory model.

  208. </td></tr>

  209. <tr><td align="left" valign="top">&bull; <a href="Integer-Overflow-Builtins.html#Integer-Overflow-Builtins">Integer Overflow Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Built-in functions to perform arithmetics and

  210.                         arithmetic overflow checking.

  211. </td></tr>

  212. <tr><td align="left" valign="top">&bull; <a href="x86-specific-memory-model-extensions-for-transactional-memory.html#x86-specific-memory-model-extensions-for-transactional-memory">x86 specific memory model extensions for transactional memory</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">x86 memory models.

  213. </td></tr>

  214. <tr><td align="left" valign="top">&bull; <a href="Object-Size-Checking.html#Object-Size-Checking">Object Size Checking</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Built-in functions for limited buffer overflow

  215.                         checking.

  216. </td></tr>

  217. <tr><td align="left" valign="top">&bull; <a href="Other-Builtins.html#Other-Builtins">Other Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Other built-in functions.

  218. </td></tr>

  219. <tr><td align="left" valign="top">&bull; <a href="Target-Builtins.html#Target-Builtins">Target Builtins</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Built-in functions specific to particular targets.

  220. </td></tr>

  221. <tr><td align="left" valign="top">&bull; <a href="Target-Format-Checks.html#Target-Format-Checks">Target Format Checks</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Format checks specific to particular targets.

  222. </td></tr>

  223. <tr><td align="left" valign="top">&bull; <a href="Pragmas.html#Pragmas">Pragmas</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Pragmas accepted by GCC.

  224. </td></tr>

  225. <tr><td align="left" valign="top">&bull; <a href="Unnamed-Fields.html#Unnamed-Fields">Unnamed Fields</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Unnamed struct/union fields within structs/unions.

  226. </td></tr>

  227. <tr><td align="left" valign="top">&bull; <a href="Thread_002dLocal.html#Thread_002dLocal">Thread-Local</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Per-thread variables.

  228. </td></tr>

  229. <tr><td align="left" valign="top">&bull; <a href="Binary-constants.html#Binary-constants">Binary constants</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Binary constants using the &lsquo;<samp>0b</samp>&rsquo; prefix.

  230. </td></tr>

  231. </table>

  232. 

  233. <hr>

  234. <div class="header">

  235. <p>

  236. Next: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="n" rel="next">C++ Extensions</a>, Previous: <a href="C_002b_002b-Implementation.html#C_002b_002b-Implementation" accesskey="p" rel="prev">C++ Implementation</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>

  237. </div>

  238. 

  239. 

  240. 

  241. </body>

  242. </html>