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  68. 

  69. <body lang="en">

  70. <a name="C-Dialect-Options"></a>

  71. <div class="header">

  72. <p>

  73. Next: <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options" accesskey="n" rel="next">C++ Dialect Options</a>, Previous: <a href="Invoking-G_002b_002b.html#Invoking-G_002b_002b" accesskey="p" rel="prev">Invoking G++</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</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="Options-Controlling-C-Dialect"></a>

  77. <h3 class="section">3.4 Options Controlling C Dialect</h3>

  78. <a name="index-dialect-options"></a>

  79. <a name="index-language-dialect-options"></a>

  80. <a name="index-options_002c-dialect"></a>

  81. 

  82. <p>The following options control the dialect of C (or languages derived

  83. from C, such as C++, Objective-C and Objective-C++) that the compiler

  84. accepts:

  85. </p>

  86. <dl compact="compact">

  87. <dd><a name="index-ANSI-support"></a>

  88. <a name="index-ISO-support"></a>

  89. </dd>

  90. <dt><code>-ansi</code></dt>

  91. <dd><a name="index-ansi-1"></a>

  92. <p>In C mode, this is equivalent to <samp>-std=c90</samp>. In C++ mode, it is

  93. equivalent to <samp>-std=c++98</samp>.

  94. </p>

  95. <p>This turns off certain features of GCC that are incompatible with ISO

  96. C90 (when compiling C code), or of standard C++ (when compiling C++ code),

  97. such as the <code>asm</code> and <code>typeof</code> keywords, and

  98. predefined macros such as <code>unix</code> and <code>vax</code> that identify the

  99. type of system you are using.  It also enables the undesirable and

  100. rarely used ISO trigraph feature.  For the C compiler,

  101. it disables recognition of C++ style &lsquo;<samp>//</samp>&rsquo; comments as well as

  102. the <code>inline</code> keyword.

  103. </p>

  104. <p>The alternate keywords <code>__asm__</code>, <code>__extension__</code>,

  105. <code>__inline__</code> and <code>__typeof__</code> continue to work despite

  106. <samp>-ansi</samp>.  You would not want to use them in an ISO C program, of

  107. course, but it is useful to put them in header files that might be included

  108. in compilations done with <samp>-ansi</samp>.  Alternate predefined macros

  109. such as <code>__unix__</code> and <code>__vax__</code> are also available, with or

  110. without <samp>-ansi</samp>.

  111. </p>

  112. <p>The <samp>-ansi</samp> option does not cause non-ISO programs to be

  113. rejected gratuitously.  For that, <samp>-Wpedantic</samp> is required in

  114. addition to <samp>-ansi</samp>.  See <a href="Warning-Options.html#Warning-Options">Warning Options</a>.

  115. </p>

  116. <p>The macro <code>__STRICT_ANSI__</code> is predefined when the <samp>-ansi</samp>

  117. option is used.  Some header files may notice this macro and refrain

  118. from declaring certain functions or defining certain macros that the

  119. ISO standard doesn&rsquo;t call for; this is to avoid interfering with any

  120. programs that might use these names for other things.

  121. </p>

  122. <p>Functions that are normally built in but do not have semantics

  123. defined by ISO C (such as <code>alloca</code> and <code>ffs</code>) are not built-in

  124. functions when <samp>-ansi</samp> is used.  See <a href="Other-Builtins.html#Other-Builtins">Other

  125. built-in functions provided by GCC</a>, for details of the functions

  126. affected.

  127. </p>

  128. </dd>

  129. <dt><code>-std=</code></dt>

  130. <dd><a name="index-std-1"></a>

  131. <p>Determine the language standard. See <a href="Standards.html#Standards">Language Standards

  132. Supported by GCC</a>, for details of these standard versions.  This option

  133. is currently only supported when compiling C or C++.

  134. </p>

  135. <p>The compiler can accept several base standards, such as &lsquo;<samp>c90</samp>&rsquo; or

  136. &lsquo;<samp>c++98</samp>&rsquo;, and GNU dialects of those standards, such as

  137. &lsquo;<samp>gnu90</samp>&rsquo; or &lsquo;<samp>gnu++98</samp>&rsquo;.  When a base standard is specified, the

  138. compiler accepts all programs following that standard plus those

  139. using GNU extensions that do not contradict it.  For example,

  140. <samp>-std=c90</samp> turns off certain features of GCC that are

  141. incompatible with ISO C90, such as the <code>asm</code> and <code>typeof</code>

  142. keywords, but not other GNU extensions that do not have a meaning in

  143. ISO C90, such as omitting the middle term of a <code>?:</code>

  144. expression. On the other hand, when a GNU dialect of a standard is

  145. specified, all features supported by the compiler are enabled, even when

  146. those features change the meaning of the base standard.  As a result, some

  147. strict-conforming programs may be rejected.  The particular standard

  148. is used by <samp>-Wpedantic</samp> to identify which features are GNU

  149. extensions given that version of the standard. For example

  150. <samp>-std=gnu90 -Wpedantic</samp> warns about C++ style &lsquo;<samp>//</samp>&rsquo;

  151. comments, while <samp>-std=gnu99 -Wpedantic</samp> does not.

  152. </p>

  153. <p>A value for this option must be provided; possible values are

  154. </p>

  155. <dl compact="compact">

  156. <dt>&lsquo;<samp>c90</samp>&rsquo;</dt>

  157. <dt>&lsquo;<samp>c89</samp>&rsquo;</dt>

  158. <dt>&lsquo;<samp>iso9899:1990</samp>&rsquo;</dt>

  159. <dd><p>Support all ISO C90 programs (certain GNU extensions that conflict

  160. with ISO C90 are disabled). Same as <samp>-ansi</samp> for C code.

  161. </p>

  162. </dd>

  163. <dt>&lsquo;<samp>iso9899:199409</samp>&rsquo;</dt>

  164. <dd><p>ISO C90 as modified in amendment 1.

  165. </p>

  166. </dd>

  167. <dt>&lsquo;<samp>c99</samp>&rsquo;</dt>

  168. <dt>&lsquo;<samp>c9x</samp>&rsquo;</dt>

  169. <dt>&lsquo;<samp>iso9899:1999</samp>&rsquo;</dt>

  170. <dt>&lsquo;<samp>iso9899:199x</samp>&rsquo;</dt>

  171. <dd><p>ISO C99.  This standard is substantially completely supported, modulo

  172. bugs and floating-point issues

  173. (mainly but not entirely relating to optional C99 features from

  174. Annexes F and G).  See

  175. <a href="http://gcc.gnu.org/c99status.html">http://gcc.gnu.org/c99status.html</a><!-- /@w --> for more information.  The

  176. names &lsquo;<samp>c9x</samp>&rsquo; and &lsquo;<samp>iso9899:199x</samp>&rsquo; are deprecated.

  177. </p>

  178. </dd>

  179. <dt>&lsquo;<samp>c11</samp>&rsquo;</dt>

  180. <dt>&lsquo;<samp>c1x</samp>&rsquo;</dt>

  181. <dt>&lsquo;<samp>iso9899:2011</samp>&rsquo;</dt>

  182. <dd><p>ISO C11, the 2011 revision of the ISO C standard.  This standard is

  183. substantially completely supported, modulo bugs, floating-point issues

  184. (mainly but not entirely relating to optional C11 features from

  185. Annexes F and G) and the optional Annexes K (Bounds-checking

  186. interfaces) and L (Analyzability).  The name &lsquo;<samp>c1x</samp>&rsquo; is deprecated.

  187. </p>

  188. </dd>

  189. <dt>&lsquo;<samp>c17</samp>&rsquo;</dt>

  190. <dt>&lsquo;<samp>c18</samp>&rsquo;</dt>

  191. <dt>&lsquo;<samp>iso9899:2017</samp>&rsquo;</dt>

  192. <dt>&lsquo;<samp>iso9899:2018</samp>&rsquo;</dt>

  193. <dd><p>ISO C17, the 2017 revision of the ISO C standard

  194. (published in 2018).  This standard is

  195. same as C11 except for corrections of defects (all of which are also

  196. applied with <samp>-std=c11</samp>) and a new value of

  197. <code>__STDC_VERSION__</code>, and so is supported to the same extent as C11.

  198. </p>

  199. </dd>

  200. <dt>&lsquo;<samp>c2x</samp>&rsquo;</dt>

  201. <dd><p>The next version of the ISO C standard, still under development.  The

  202. support for this version is experimental and incomplete.

  203. </p>

  204. </dd>

  205. <dt>&lsquo;<samp>gnu90</samp>&rsquo;</dt>

  206. <dt>&lsquo;<samp>gnu89</samp>&rsquo;</dt>

  207. <dd><p>GNU dialect of ISO C90 (including some C99 features).

  208. </p>

  209. </dd>

  210. <dt>&lsquo;<samp>gnu99</samp>&rsquo;</dt>

  211. <dt>&lsquo;<samp>gnu9x</samp>&rsquo;</dt>

  212. <dd><p>GNU dialect of ISO C99.  The name &lsquo;<samp>gnu9x</samp>&rsquo; is deprecated.

  213. </p>

  214. </dd>

  215. <dt>&lsquo;<samp>gnu11</samp>&rsquo;</dt>

  216. <dt>&lsquo;<samp>gnu1x</samp>&rsquo;</dt>

  217. <dd><p>GNU dialect of ISO C11.

  218. The name &lsquo;<samp>gnu1x</samp>&rsquo; is deprecated.

  219. </p>

  220. </dd>

  221. <dt>&lsquo;<samp>gnu17</samp>&rsquo;</dt>

  222. <dt>&lsquo;<samp>gnu18</samp>&rsquo;</dt>

  223. <dd><p>GNU dialect of ISO C17.  This is the default for C code.

  224. </p>

  225. </dd>

  226. <dt>&lsquo;<samp>gnu2x</samp>&rsquo;</dt>

  227. <dd><p>The next version of the ISO C standard, still under development, plus

  228. GNU extensions.  The support for this version is experimental and

  229. incomplete.

  230. </p>

  231. </dd>

  232. <dt>&lsquo;<samp>c++98</samp>&rsquo;</dt>

  233. <dt>&lsquo;<samp>c++03</samp>&rsquo;</dt>

  234. <dd><p>The 1998 ISO C++ standard plus the 2003 technical corrigendum and some

  235. additional defect reports. Same as <samp>-ansi</samp> for C++ code.

  236. </p>

  237. </dd>

  238. <dt>&lsquo;<samp>gnu++98</samp>&rsquo;</dt>

  239. <dt>&lsquo;<samp>gnu++03</samp>&rsquo;</dt>

  240. <dd><p>GNU dialect of <samp>-std=c++98</samp>.

  241. </p>

  242. </dd>

  243. <dt>&lsquo;<samp>c++11</samp>&rsquo;</dt>

  244. <dt>&lsquo;<samp>c++0x</samp>&rsquo;</dt>

  245. <dd><p>The 2011 ISO C++ standard plus amendments.

  246. The name &lsquo;<samp>c++0x</samp>&rsquo; is deprecated.

  247. </p>

  248. </dd>

  249. <dt>&lsquo;<samp>gnu++11</samp>&rsquo;</dt>

  250. <dt>&lsquo;<samp>gnu++0x</samp>&rsquo;</dt>

  251. <dd><p>GNU dialect of <samp>-std=c++11</samp>.

  252. The name &lsquo;<samp>gnu++0x</samp>&rsquo; is deprecated.

  253. </p>

  254. </dd>

  255. <dt>&lsquo;<samp>c++14</samp>&rsquo;</dt>

  256. <dt>&lsquo;<samp>c++1y</samp>&rsquo;</dt>

  257. <dd><p>The 2014 ISO C++ standard plus amendments.

  258. The name &lsquo;<samp>c++1y</samp>&rsquo; is deprecated.

  259. </p>

  260. </dd>

  261. <dt>&lsquo;<samp>gnu++14</samp>&rsquo;</dt>

  262. <dt>&lsquo;<samp>gnu++1y</samp>&rsquo;</dt>

  263. <dd><p>GNU dialect of <samp>-std=c++14</samp>.

  264. This is the default for C++ code.

  265. The name &lsquo;<samp>gnu++1y</samp>&rsquo; is deprecated.

  266. </p>

  267. </dd>

  268. <dt>&lsquo;<samp>c++17</samp>&rsquo;</dt>

  269. <dt>&lsquo;<samp>c++1z</samp>&rsquo;</dt>

  270. <dd><p>The 2017 ISO C++ standard plus amendments.

  271. The name &lsquo;<samp>c++1z</samp>&rsquo; is deprecated.

  272. </p>

  273. </dd>

  274. <dt>&lsquo;<samp>gnu++17</samp>&rsquo;</dt>

  275. <dt>&lsquo;<samp>gnu++1z</samp>&rsquo;</dt>

  276. <dd><p>GNU dialect of <samp>-std=c++17</samp>.

  277. The name &lsquo;<samp>gnu++1z</samp>&rsquo; is deprecated.

  278. </p>

  279. </dd>

  280. <dt>&lsquo;<samp>c++20</samp>&rsquo;</dt>

  281. <dt>&lsquo;<samp>c++2a</samp>&rsquo;</dt>

  282. <dd><p>The next revision of the ISO C++ standard, planned for

  283. 2020.  Support is highly experimental, and will almost certainly

  284. change in incompatible ways in future releases.

  285. </p>

  286. </dd>

  287. <dt>&lsquo;<samp>gnu++20</samp>&rsquo;</dt>

  288. <dt>&lsquo;<samp>gnu++2a</samp>&rsquo;</dt>

  289. <dd><p>GNU dialect of <samp>-std=c++20</samp>.  Support is highly experimental,

  290. and will almost certainly change in incompatible ways in future

  291. releases.

  292. </p></dd>

  293. </dl>

  294. 

  295. </dd>

  296. <dt><code>-fgnu89-inline</code></dt>

  297. <dd><a name="index-fgnu89_002dinline"></a>

  298. <p>The option <samp>-fgnu89-inline</samp> tells GCC to use the traditional

  299. GNU semantics for <code>inline</code> functions when in C99 mode.

  300. See <a href="Inline.html#Inline">An Inline Function is As Fast As a Macro</a>.

  301. Using this option is roughly equivalent to adding the

  302. <code>gnu_inline</code> function attribute to all inline functions

  303. (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>).

  304. </p>

  305. <p>The option <samp>-fno-gnu89-inline</samp> explicitly tells GCC to use the

  306. C99 semantics for <code>inline</code> when in C99 or gnu99 mode (i.e., it

  307. specifies the default behavior).

  308. This option is not supported in <samp>-std=c90</samp> or

  309. <samp>-std=gnu90</samp> mode.

  310. </p>

  311. <p>The preprocessor macros <code>__GNUC_GNU_INLINE__</code> and

  312. <code>__GNUC_STDC_INLINE__</code> may be used to check which semantics are

  313. in effect for <code>inline</code> functions.  See <a href="http://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html#Common-Predefined-Macros">Common Predefined

  314. Macros</a> in <cite>The C Preprocessor</cite>.

  315. </p>

  316. </dd>

  317. <dt><code>-fpermitted-flt-eval-methods=<var>style</var></code></dt>

  318. <dd><a name="index-fpermitted_002dflt_002deval_002dmethods"></a>

  319. <a name="index-fpermitted_002dflt_002deval_002dmethods_003dc11"></a>

  320. <a name="index-fpermitted_002dflt_002deval_002dmethods_003dts_002d18661_002d3"></a>

  321. <p>ISO/IEC TS 18661-3 defines new permissible values for

  322. <code>FLT_EVAL_METHOD</code> that indicate that operations and constants with

  323. a semantic type that is an interchange or extended format should be

  324. evaluated to the precision and range of that type.  These new values are

  325. a superset of those permitted under C99/C11, which does not specify the

  326. meaning of other positive values of <code>FLT_EVAL_METHOD</code>.  As such, code

  327. conforming to C11 may not have been written expecting the possibility of

  328. the new values.

  329. </p>

  330. <p><samp>-fpermitted-flt-eval-methods</samp> specifies whether the compiler

  331. should allow only the values of <code>FLT_EVAL_METHOD</code> specified in C99/C11,

  332. or the extended set of values specified in ISO/IEC TS 18661-3.

  333. </p>

  334. <p><var>style</var> is either <code>c11</code> or <code>ts-18661-3</code> as appropriate.

  335. </p>

  336. <p>The default when in a standards compliant mode (<samp>-std=c11</samp> or similar)

  337. is <samp>-fpermitted-flt-eval-methods=c11</samp>.  The default when in a GNU

  338. dialect (<samp>-std=gnu11</samp> or similar) is

  339. <samp>-fpermitted-flt-eval-methods=ts-18661-3</samp>.

  340. </p>

  341. </dd>

  342. <dt><code>-aux-info <var>filename</var></code></dt>

  343. <dd><a name="index-aux_002dinfo"></a>

  344. <p>Output to the given filename prototyped declarations for all functions

  345. declared and/or defined in a translation unit, including those in header

  346. files.  This option is silently ignored in any language other than C.

  347. </p>

  348. <p>Besides declarations, the file indicates, in comments, the origin of

  349. each declaration (source file and line), whether the declaration was

  350. implicit, prototyped or unprototyped (&lsquo;<samp>I</samp>&rsquo;, &lsquo;<samp>N</samp>&rsquo; for new or

  351. &lsquo;<samp>O</samp>&rsquo; for old, respectively, in the first character after the line

  352. number and the colon), and whether it came from a declaration or a

  353. definition (&lsquo;<samp>C</samp>&rsquo; or &lsquo;<samp>F</samp>&rsquo;, respectively, in the following

  354. character).  In the case of function definitions, a K&amp;R-style list of

  355. arguments followed by their declarations is also provided, inside

  356. comments, after the declaration.

  357. </p>

  358. </dd>

  359. <dt><code>-fallow-parameterless-variadic-functions</code></dt>

  360. <dd><a name="index-fallow_002dparameterless_002dvariadic_002dfunctions"></a>

  361. <p>Accept variadic functions without named parameters.

  362. </p>

  363. <p>Although it is possible to define such a function, this is not very

  364. useful as it is not possible to read the arguments.  This is only

  365. supported for C as this construct is allowed by C++.

  366. </p>

  367. </dd>

  368. <dt><code>-fno-asm</code></dt>

  369. <dd><a name="index-fno_002dasm"></a>

  370. <a name="index-fasm"></a>

  371. <p>Do not recognize <code>asm</code>, <code>inline</code> or <code>typeof</code> as a

  372. keyword, so that code can use these words as identifiers.  You can use

  373. the keywords <code>__asm__</code>, <code>__inline__</code> and <code>__typeof__</code>

  374. instead.  <samp>-ansi</samp> implies <samp>-fno-asm</samp>.

  375. </p>

  376. <p>In C++, this switch only affects the <code>typeof</code> keyword, since

  377. <code>asm</code> and <code>inline</code> are standard keywords.  You may want to

  378. use the <samp>-fno-gnu-keywords</samp> flag instead, which has the same

  379. effect.  In C99 mode (<samp>-std=c99</samp> or <samp>-std=gnu99</samp>), this

  380. switch only affects the <code>asm</code> and <code>typeof</code> keywords, since

  381. <code>inline</code> is a standard keyword in ISO C99.

  382. </p>

  383. </dd>

  384. <dt><code>-fno-builtin</code></dt>

  385. <dt><code>-fno-builtin-<var>function</var></code></dt>

  386. <dd><a name="index-fno_002dbuiltin"></a>

  387. <a name="index-fbuiltin"></a>

  388. <a name="index-built_002din-functions"></a>

  389. <p>Don&rsquo;t recognize built-in functions that do not begin with

  390. &lsquo;<samp>__builtin_</samp>&rsquo; as prefix.  See <a href="Other-Builtins.html#Other-Builtins">Other built-in

  391. functions provided by GCC</a>, for details of the functions affected,

  392. including those which are not built-in functions when <samp>-ansi</samp> or

  393. <samp>-std</samp> options for strict ISO C conformance are used because they

  394. do not have an ISO standard meaning.

  395. </p>

  396. <p>GCC normally generates special code to handle certain built-in functions

  397. more efficiently; for instance, calls to <code>alloca</code> may become single

  398. instructions which adjust the stack directly, and calls to <code>memcpy</code>

  399. may become inline copy loops.  The resulting code is often both smaller

  400. and faster, but since the function calls no longer appear as such, you

  401. cannot set a breakpoint on those calls, nor can you change the behavior

  402. of the functions by linking with a different library.  In addition,

  403. when a function is recognized as a built-in function, GCC may use

  404. information about that function to warn about problems with calls to

  405. that function, or to generate more efficient code, even if the

  406. resulting code still contains calls to that function.  For example,

  407. warnings are given with <samp>-Wformat</samp> for bad calls to

  408. <code>printf</code> when <code>printf</code> is built in and <code>strlen</code> is

  409. known not to modify global memory.

  410. </p>

  411. <p>With the <samp>-fno-builtin-<var>function</var></samp> option

  412. only the built-in function <var>function</var> is

  413. disabled.  <var>function</var> must not begin with &lsquo;<samp>__builtin_</samp>&rsquo;.  If a

  414. function is named that is not built-in in this version of GCC, this

  415. option is ignored.  There is no corresponding

  416. <samp>-fbuiltin-<var>function</var></samp> option; if you wish to enable

  417. built-in functions selectively when using <samp>-fno-builtin</samp> or

  418. <samp>-ffreestanding</samp>, you may define macros such as:

  419. </p>

  420. <div class="smallexample">

  421. <pre class="smallexample">#define abs(n)          __builtin_abs ((n))

  422. #define strcpy(d, s)    __builtin_strcpy ((d), (s))

  423. </pre></div>

  424. 

  425. </dd>

  426. <dt><code>-fgimple</code></dt>

  427. <dd><a name="index-fgimple"></a>

  428. 

  429. <p>Enable parsing of function definitions marked with <code>__GIMPLE</code>.

  430. This is an experimental feature that allows unit testing of GIMPLE

  431. passes.

  432. </p>

  433. </dd>

  434. <dt><code>-fhosted</code></dt>

  435. <dd><a name="index-fhosted"></a>

  436. <a name="index-hosted-environment-1"></a>

  437. 

  438. <p>Assert that compilation targets a hosted environment.  This implies

  439. <samp>-fbuiltin</samp>.  A hosted environment is one in which the

  440. entire standard library is available, and in which <code>main</code> has a return

  441. type of <code>int</code>.  Examples are nearly everything except a kernel.

  442. This is equivalent to <samp>-fno-freestanding</samp>.

  443. </p>

  444. </dd>

  445. <dt><code>-ffreestanding</code></dt>

  446. <dd><a name="index-ffreestanding-1"></a>

  447. <a name="index-hosted-environment-2"></a>

  448. 

  449. <p>Assert that compilation targets a freestanding environment.  This

  450. implies <samp>-fno-builtin</samp>.  A freestanding environment

  451. is one in which the standard library may not exist, and program startup may

  452. not necessarily be at <code>main</code>.  The most obvious example is an OS kernel.

  453. This is equivalent to <samp>-fno-hosted</samp>.

  454. </p>

  455. <p>See <a href="Standards.html#Standards">Language Standards Supported by GCC</a>, for details of

  456. freestanding and hosted environments.

  457. </p>

  458. </dd>

  459. <dt><code>-fopenacc</code></dt>

  460. <dd><a name="index-fopenacc"></a>

  461. <a name="index-OpenACC-accelerator-programming"></a>

  462. <p>Enable handling of OpenACC directives <code>#pragma acc</code> in C/C++ and

  463. <code>!$acc</code> in Fortran.  When <samp>-fopenacc</samp> is specified, the

  464. compiler generates accelerated code according to the OpenACC Application

  465. Programming Interface v2.6 <a href="https://www.openacc.org">https://www.openacc.org</a><!-- /@w -->.  This option

  466. implies <samp>-pthread</samp>, and thus is only supported on targets that

  467. have support for <samp>-pthread</samp>.

  468. </p>

  469. </dd>

  470. <dt><code>-fopenacc-dim=<var>geom</var></code></dt>

  471. <dd><a name="index-fopenacc_002ddim"></a>

  472. <a name="index-OpenACC-accelerator-programming-1"></a>

  473. <p>Specify default compute dimensions for parallel offload regions that do

  474. not explicitly specify.  The <var>geom</var> value is a triple of

  475. &rsquo;:&rsquo;-separated sizes, in order &rsquo;gang&rsquo;, &rsquo;worker&rsquo; and, &rsquo;vector&rsquo;.  A size

  476. can be omitted, to use a target-specific default value.

  477. </p>

  478. </dd>

  479. <dt><code>-fopenmp</code></dt>

  480. <dd><a name="index-fopenmp"></a>

  481. <a name="index-OpenMP-parallel"></a>

  482. <p>Enable handling of OpenMP directives <code>#pragma omp</code> in C/C++ and

  483. <code>!$omp</code> in Fortran.  When <samp>-fopenmp</samp> is specified, the

  484. compiler generates parallel code according to the OpenMP Application

  485. Program Interface v4.5 <a href="https://www.openmp.org">https://www.openmp.org</a><!-- /@w -->.  This option

  486. implies <samp>-pthread</samp>, and thus is only supported on targets that

  487. have support for <samp>-pthread</samp>. <samp>-fopenmp</samp> implies

  488. <samp>-fopenmp-simd</samp>.

  489. </p>

  490. </dd>

  491. <dt><code>-fopenmp-simd</code></dt>

  492. <dd><a name="index-fopenmp_002dsimd"></a>

  493. <a name="index-OpenMP-SIMD"></a>

  494. <a name="index-SIMD"></a>

  495. <p>Enable handling of OpenMP&rsquo;s SIMD directives with <code>#pragma omp</code>

  496. in C/C++ and <code>!$omp</code> in Fortran. Other OpenMP directives

  497. are ignored.

  498. </p>

  499. </dd>

  500. <dt><code>-fgnu-tm</code></dt>

  501. <dd><a name="index-fgnu_002dtm"></a>

  502. <p>When the option <samp>-fgnu-tm</samp> is specified, the compiler

  503. generates code for the Linux variant of Intel&rsquo;s current Transactional

  504. Memory ABI specification document (Revision 1.1, May 6 2009).  This is

  505. an experimental feature whose interface may change in future versions

  506. of GCC, as the official specification changes.  Please note that not

  507. all architectures are supported for this feature.

  508. </p>

  509. <p>For more information on GCC&rsquo;s support for transactional memory,

  510. See <a href="../libitm/Enabling-libitm.html#Enabling-libitm">The GNU Transactional Memory Library</a> in <cite>GNU

  511. Transactional Memory Library</cite>.

  512. </p>

  513. <p>Note that the transactional memory feature is not supported with

  514. non-call exceptions (<samp>-fnon-call-exceptions</samp>).

  515. </p>

  516. </dd>

  517. <dt><code>-fms-extensions</code></dt>

  518. <dd><a name="index-fms_002dextensions"></a>

  519. <p>Accept some non-standard constructs used in Microsoft header files.

  520. </p>

  521. <p>In C++ code, this allows member names in structures to be similar

  522. to previous types declarations.

  523. </p>

  524. <div class="smallexample">

  525. <pre class="smallexample">typedef int UOW;

  526. struct ABC {

  527.   UOW UOW;

  528. };

  529. </pre></div>

  530. 

  531. <p>Some cases of unnamed fields in structures and unions are only

  532. accepted with this option.  See <a href="Unnamed-Fields.html#Unnamed-Fields">Unnamed struct/union

  533. fields within structs/unions</a>, for details.

  534. </p>

  535. <p>Note that this option is off for all targets except for x86

  536. targets using ms-abi.

  537. </p>

  538. </dd>

  539. <dt><code>-fplan9-extensions</code></dt>

  540. <dd><a name="index-fplan9_002dextensions"></a>

  541. <p>Accept some non-standard constructs used in Plan 9 code.

  542. </p>

  543. <p>This enables <samp>-fms-extensions</samp>, permits passing pointers to

  544. structures with anonymous fields to functions that expect pointers to

  545. elements of the type of the field, and permits referring to anonymous

  546. fields declared using a typedef.  See <a href="Unnamed-Fields.html#Unnamed-Fields">Unnamed

  547. struct/union fields within structs/unions</a>, for details.  This is only

  548. supported for C, not C++.

  549. </p>

  550. </dd>

  551. <dt><code>-fcond-mismatch</code></dt>

  552. <dd><a name="index-fcond_002dmismatch"></a>

  553. <p>Allow conditional expressions with mismatched types in the second and

  554. third arguments.  The value of such an expression is void.  This option

  555. is not supported for C++.

  556. </p>

  557. </dd>

  558. <dt><code>-flax-vector-conversions</code></dt>

  559. <dd><a name="index-flax_002dvector_002dconversions"></a>

  560. <p>Allow implicit conversions between vectors with differing numbers of

  561. elements and/or incompatible element types.  This option should not be

  562. used for new code.

  563. </p>

  564. </dd>

  565. <dt><code>-funsigned-char</code></dt>

  566. <dd><a name="index-funsigned_002dchar"></a>

  567. <p>Let the type <code>char</code> be unsigned, like <code>unsigned char</code>.

  568. </p>

  569. <p>Each kind of machine has a default for what <code>char</code> should

  570. be.  It is either like <code>unsigned char</code> by default or like

  571. <code>signed char</code> by default.

  572. </p>

  573. <p>Ideally, a portable program should always use <code>signed char</code> or

  574. <code>unsigned char</code> when it depends on the signedness of an object.

  575. But many programs have been written to use plain <code>char</code> and

  576. expect it to be signed, or expect it to be unsigned, depending on the

  577. machines they were written for.  This option, and its inverse, let you

  578. make such a program work with the opposite default.

  579. </p>

  580. <p>The type <code>char</code> is always a distinct type from each of

  581. <code>signed char</code> or <code>unsigned char</code>, even though its behavior

  582. is always just like one of those two.

  583. </p>

  584. </dd>

  585. <dt><code>-fsigned-char</code></dt>

  586. <dd><a name="index-fsigned_002dchar"></a>

  587. <p>Let the type <code>char</code> be signed, like <code>signed char</code>.

  588. </p>

  589. <p>Note that this is equivalent to <samp>-fno-unsigned-char</samp>, which is

  590. the negative form of <samp>-funsigned-char</samp>.  Likewise, the option

  591. <samp>-fno-signed-char</samp> is equivalent to <samp>-funsigned-char</samp>.

  592. </p>

  593. </dd>

  594. <dt><code>-fsigned-bitfields</code></dt>

  595. <dt><code>-funsigned-bitfields</code></dt>

  596. <dt><code>-fno-signed-bitfields</code></dt>

  597. <dt><code>-fno-unsigned-bitfields</code></dt>

  598. <dd><a name="index-fsigned_002dbitfields"></a>

  599. <a name="index-funsigned_002dbitfields"></a>

  600. <a name="index-fno_002dsigned_002dbitfields"></a>

  601. <a name="index-fno_002dunsigned_002dbitfields"></a>

  602. <p>These options control whether a bit-field is signed or unsigned, when the

  603. declaration does not use either <code>signed</code> or <code>unsigned</code>.  By

  604. default, such a bit-field is signed, because this is consistent: the

  605. basic integer types such as <code>int</code> are signed types.

  606. </p>

  607. </dd>

  608. <dt><code>-fsso-struct=<var>endianness</var></code></dt>

  609. <dd><a name="index-fsso_002dstruct"></a>

  610. <p>Set the default scalar storage order of structures and unions to the

  611. specified endianness.  The accepted values are &lsquo;<samp>big-endian</samp>&rsquo;,

  612. &lsquo;<samp>little-endian</samp>&rsquo; and &lsquo;<samp>native</samp>&rsquo; for the native endianness of

  613. the target (the default).  This option is not supported for C++.

  614. </p>

  615. <p><strong>Warning:</strong> the <samp>-fsso-struct</samp> switch causes GCC to generate

  616. code that is not binary compatible with code generated without it if the

  617. specified endianness is not the native endianness of the target.

  618. </p></dd>

  619. </dl>

  620. 

  621. <hr>

  622. <div class="header">

  623. <p>

  624. Next: <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options" accesskey="n" rel="next">C++ Dialect Options</a>, Previous: <a href="Invoking-G_002b_002b.html#Invoking-G_002b_002b" accesskey="p" rel="prev">Invoking G++</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</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>

  625. </div>

  626. 

  627. 

  628. 

  629. </body>

  630. </html>