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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

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  25. <title>Function Attributes (Using the GNU Compiler Collection (GCC))</title>

  26. 

  27. <meta name="description" content="Function Attributes (Using the GNU Compiler Collection (GCC))">

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  34. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  35. <link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">

  36. <link href="Common-Function-Attributes.html#Common-Function-Attributes" rel="next" title="Common Function Attributes">

  37. <link href="Mixed-Declarations.html#Mixed-Declarations" rel="prev" title="Mixed Declarations">

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  67. </head>

  68. 

  69. <body lang="en">

  70. <a name="Function-Attributes"></a>

  71. <div class="header">

  72. <p>

  73. Next: <a href="Variable-Attributes.html#Variable-Attributes" accesskey="n" rel="next">Variable Attributes</a>, Previous: <a href="Mixed-Declarations.html#Mixed-Declarations" accesskey="p" rel="prev">Mixed Declarations</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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="Declaring-Attributes-of-Functions"></a>

  77. <h3 class="section">6.33 Declaring Attributes of Functions</h3>

  78. <a name="index-function-attributes"></a>

  79. <a name="index-declaring-attributes-of-functions"></a>

  80. <a name="index-volatile-applied-to-function"></a>

  81. <a name="index-const-applied-to-function"></a>

  82. 

  83. <p>In GNU C and C++, you can use function attributes to specify certain

  84. function properties that may help the compiler optimize calls or

  85. check code more carefully for correctness.  For example, you

  86. can use attributes to specify that a function never returns

  87. (<code>noreturn</code>), returns a value depending only on the values of

  88. its arguments (<code>const</code>), or has <code>printf</code>-style arguments

  89. (<code>format</code>).

  90. </p>

  91. <p>You can also use attributes to control memory placement, code

  92. generation options or call/return conventions within the function

  93. being annotated.  Many of these attributes are target-specific.  For

  94. example, many targets support attributes for defining interrupt

  95. handler functions, which typically must follow special register usage

  96. and return conventions.  Such attributes are described in the subsection

  97. for each target.  However, a considerable number of attributes are

  98. supported by most, if not all targets.  Those are described in

  99. the <a href="Common-Function-Attributes.html#Common-Function-Attributes">Common Function Attributes</a> section.

  100. </p>

  101. <p>Function attributes are introduced by the <code>__attribute__</code> keyword

  102. in the declaration of a function, followed by an attribute specification

  103. enclosed in double parentheses.  You can specify multiple attributes in

  104. a declaration by separating them by commas within the double parentheses

  105. or by immediately following one attribute specification with another.

  106. See <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>, for the exact rules on attribute syntax and

  107. placement.  Compatible attribute specifications on distinct declarations

  108. of the same function are merged.  An attribute specification that is not

  109. compatible with attributes already applied to a declaration of the same

  110. function is ignored with a warning.

  111. </p>

  112. <p>Some function attributes take one or more arguments that refer to

  113. the function&rsquo;s parameters by their positions within the function parameter

  114. list.  Such attribute arguments are referred to as <em>positional arguments</em>.

  115. Unless specified otherwise, positional arguments that specify properties

  116. of parameters with pointer types can also specify the same properties of

  117. the implicit C++ <code>this</code> argument in non-static member functions, and

  118. of parameters of reference to a pointer type.  For ordinary functions,

  119. position one refers to the first parameter on the list.  In C++ non-static

  120. member functions, position one refers to the implicit <code>this</code> pointer.

  121. The same restrictions and effects apply to function attributes used with

  122. ordinary functions or C++ member functions.

  123. </p>

  124. <p>GCC also supports attributes on

  125. variable declarations (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>),

  126. labels (see <a href="Label-Attributes.html#Label-Attributes">Label Attributes</a>),

  127. enumerators (see <a href="Enumerator-Attributes.html#Enumerator-Attributes">Enumerator Attributes</a>),

  128. statements (see <a href="Statement-Attributes.html#Statement-Attributes">Statement Attributes</a>),

  129. and types (see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>).

  130. </p>

  131. <p>There is some overlap between the purposes of attributes and pragmas

  132. (see <a href="Pragmas.html#Pragmas">Pragmas Accepted by GCC</a>).  It has been

  133. found convenient to use <code>__attribute__</code> to achieve a natural

  134. attachment of attributes to their corresponding declarations, whereas

  135. <code>#pragma</code> is of use for compatibility with other compilers

  136. or constructs that do not naturally form part of the grammar.

  137. </p>

  138. <p>In addition to the attributes documented here,

  139. GCC plugins may provide their own attributes.

  140. </p>

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

  142. <tr><td align="left" valign="top">&bull; <a href="Common-Function-Attributes.html#Common-Function-Attributes" accesskey="1">Common Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  143. </td></tr>

  144. <tr><td align="left" valign="top">&bull; <a href="AArch64-Function-Attributes.html#AArch64-Function-Attributes" accesskey="2">AArch64 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  145. </td></tr>

  146. <tr><td align="left" valign="top">&bull; <a href="AMD-GCN-Function-Attributes.html#AMD-GCN-Function-Attributes" accesskey="3">AMD GCN Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  147. </td></tr>

  148. <tr><td align="left" valign="top">&bull; <a href="ARC-Function-Attributes.html#ARC-Function-Attributes" accesskey="4">ARC Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  149. </td></tr>

  150. <tr><td align="left" valign="top">&bull; <a href="ARM-Function-Attributes.html#ARM-Function-Attributes" accesskey="5">ARM Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  151. </td></tr>

  152. <tr><td align="left" valign="top">&bull; <a href="AVR-Function-Attributes.html#AVR-Function-Attributes" accesskey="6">AVR Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  153. </td></tr>

  154. <tr><td align="left" valign="top">&bull; <a href="Blackfin-Function-Attributes.html#Blackfin-Function-Attributes" accesskey="7">Blackfin Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  155. </td></tr>

  156. <tr><td align="left" valign="top">&bull; <a href="BPF-Function-Attributes.html#BPF-Function-Attributes" accesskey="8">BPF Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  157. </td></tr>

  158. <tr><td align="left" valign="top">&bull; <a href="CR16-Function-Attributes.html#CR16-Function-Attributes" accesskey="9">CR16 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  159. </td></tr>

  160. <tr><td align="left" valign="top">&bull; <a href="C_002dSKY-Function-Attributes.html#C_002dSKY-Function-Attributes">C-SKY Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  161. </td></tr>

  162. <tr><td align="left" valign="top">&bull; <a href="Epiphany-Function-Attributes.html#Epiphany-Function-Attributes">Epiphany Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  163. </td></tr>

  164. <tr><td align="left" valign="top">&bull; <a href="H8_002f300-Function-Attributes.html#H8_002f300-Function-Attributes">H8/300 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  165. </td></tr>

  166. <tr><td align="left" valign="top">&bull; <a href="IA_002d64-Function-Attributes.html#IA_002d64-Function-Attributes">IA-64 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  167. </td></tr>

  168. <tr><td align="left" valign="top">&bull; <a href="M32C-Function-Attributes.html#M32C-Function-Attributes">M32C Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  169. </td></tr>

  170. <tr><td align="left" valign="top">&bull; <a href="M32R_002fD-Function-Attributes.html#M32R_002fD-Function-Attributes">M32R/D Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  171. </td></tr>

  172. <tr><td align="left" valign="top">&bull; <a href="m68k-Function-Attributes.html#m68k-Function-Attributes">m68k Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  173. </td></tr>

  174. <tr><td align="left" valign="top">&bull; <a href="MCORE-Function-Attributes.html#MCORE-Function-Attributes">MCORE Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  175. </td></tr>

  176. <tr><td align="left" valign="top">&bull; <a href="MeP-Function-Attributes.html#MeP-Function-Attributes">MeP Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  177. </td></tr>

  178. <tr><td align="left" valign="top">&bull; <a href="MicroBlaze-Function-Attributes.html#MicroBlaze-Function-Attributes">MicroBlaze Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  179. </td></tr>

  180. <tr><td align="left" valign="top">&bull; <a href="Microsoft-Windows-Function-Attributes.html#Microsoft-Windows-Function-Attributes">Microsoft Windows Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  181. </td></tr>

  182. <tr><td align="left" valign="top">&bull; <a href="MIPS-Function-Attributes.html#MIPS-Function-Attributes">MIPS Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  183. </td></tr>

  184. <tr><td align="left" valign="top">&bull; <a href="MSP430-Function-Attributes.html#MSP430-Function-Attributes">MSP430 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  185. </td></tr>

  186. <tr><td align="left" valign="top">&bull; <a href="NDS32-Function-Attributes.html#NDS32-Function-Attributes">NDS32 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  187. </td></tr>

  188. <tr><td align="left" valign="top">&bull; <a href="Nios-II-Function-Attributes.html#Nios-II-Function-Attributes">Nios II Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  189. </td></tr>

  190. <tr><td align="left" valign="top">&bull; <a href="Nvidia-PTX-Function-Attributes.html#Nvidia-PTX-Function-Attributes">Nvidia PTX Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  191. </td></tr>

  192. <tr><td align="left" valign="top">&bull; <a href="PowerPC-Function-Attributes.html#PowerPC-Function-Attributes">PowerPC Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  193. </td></tr>

  194. <tr><td align="left" valign="top">&bull; <a href="RISC_002dV-Function-Attributes.html#RISC_002dV-Function-Attributes">RISC-V Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  195. </td></tr>

  196. <tr><td align="left" valign="top">&bull; <a href="RL78-Function-Attributes.html#RL78-Function-Attributes">RL78 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  197. </td></tr>

  198. <tr><td align="left" valign="top">&bull; <a href="RX-Function-Attributes.html#RX-Function-Attributes">RX Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  199. </td></tr>

  200. <tr><td align="left" valign="top">&bull; <a href="S_002f390-Function-Attributes.html#S_002f390-Function-Attributes">S/390 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  201. </td></tr>

  202. <tr><td align="left" valign="top">&bull; <a href="SH-Function-Attributes.html#SH-Function-Attributes">SH Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  203. </td></tr>

  204. <tr><td align="left" valign="top">&bull; <a href="Symbian-OS-Function-Attributes.html#Symbian-OS-Function-Attributes">Symbian OS Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  205. </td></tr>

  206. <tr><td align="left" valign="top">&bull; <a href="V850-Function-Attributes.html#V850-Function-Attributes">V850 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  207. </td></tr>

  208. <tr><td align="left" valign="top">&bull; <a href="Visium-Function-Attributes.html#Visium-Function-Attributes">Visium Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  209. </td></tr>

  210. <tr><td align="left" valign="top">&bull; <a href="x86-Function-Attributes.html#x86-Function-Attributes">x86 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  211. </td></tr>

  212. <tr><td align="left" valign="top">&bull; <a href="Xstormy16-Function-Attributes.html#Xstormy16-Function-Attributes">Xstormy16 Function Attributes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  213. </td></tr>

  214. </table>

  215. 

  216. <hr>

  217. <div class="header">

  218. <p>

  219. Next: <a href="Variable-Attributes.html#Variable-Attributes" accesskey="n" rel="next">Variable Attributes</a>, Previous: <a href="Mixed-Declarations.html#Mixed-Declarations" accesskey="p" rel="prev">Mixed Declarations</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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>

  220. </div>

  221. 

  222. 

  223. 

  224. </body>

  225. </html>