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28 KiB
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<head>
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<title>C Dialect Options (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="C Dialect Options (Using the GNU Compiler Collection (GCC))">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Invoking-GCC.html#Invoking-GCC" rel="up" title="Invoking GCC">
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<link href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options" rel="next" title="C++ Dialect Options">
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<link href="Invoking-G_002b_002b.html#Invoking-G_002b_002b" rel="prev" title="Invoking G++">
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</head>
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<body lang="en">
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<a name="C-Dialect-Options"></a>
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<div class="header">
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<p>
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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> [<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>
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</div>
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<hr>
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<a name="Options-Controlling-C-Dialect"></a>
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<h3 class="section">3.4 Options Controlling C Dialect</h3>
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<a name="index-dialect-options"></a>
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<a name="index-language-dialect-options"></a>
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<a name="index-options_002c-dialect"></a>
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<p>The following options control the dialect of C (or languages derived
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from C, such as C++, Objective-C and Objective-C++) that the compiler
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accepts:
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</p>
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<dl compact="compact">
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<dd><a name="index-ANSI-support"></a>
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<a name="index-ISO-support"></a>
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</dd>
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<dt><code>-ansi</code></dt>
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<dd><a name="index-ansi-1"></a>
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<p>In C mode, this is equivalent to <samp>-std=c90</samp>. In C++ mode, it is
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equivalent to <samp>-std=c++98</samp>.
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</p>
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<p>This turns off certain features of GCC that are incompatible with ISO
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C90 (when compiling C code), or of standard C++ (when compiling C++ code),
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such as the <code>asm</code> and <code>typeof</code> keywords, and
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predefined macros such as <code>unix</code> and <code>vax</code> that identify the
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type of system you are using. It also enables the undesirable and
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rarely used ISO trigraph feature. For the C compiler,
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it disables recognition of C++ style ‘<samp>//</samp>’ comments as well as
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the <code>inline</code> keyword.
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</p>
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<p>The alternate keywords <code>__asm__</code>, <code>__extension__</code>,
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<code>__inline__</code> and <code>__typeof__</code> continue to work despite
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<samp>-ansi</samp>. You would not want to use them in an ISO C program, of
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course, but it is useful to put them in header files that might be included
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in compilations done with <samp>-ansi</samp>. Alternate predefined macros
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such as <code>__unix__</code> and <code>__vax__</code> are also available, with or
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without <samp>-ansi</samp>.
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</p>
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<p>The <samp>-ansi</samp> option does not cause non-ISO programs to be
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rejected gratuitously. For that, <samp>-Wpedantic</samp> is required in
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addition to <samp>-ansi</samp>. See <a href="Warning-Options.html#Warning-Options">Warning Options</a>.
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</p>
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<p>The macro <code>__STRICT_ANSI__</code> is predefined when the <samp>-ansi</samp>
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option is used. Some header files may notice this macro and refrain
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from declaring certain functions or defining certain macros that the
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ISO standard doesn’t call for; this is to avoid interfering with any
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programs that might use these names for other things.
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</p>
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<p>Functions that are normally built in but do not have semantics
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defined by ISO C (such as <code>alloca</code> and <code>ffs</code>) are not built-in
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functions when <samp>-ansi</samp> is used. See <a href="Other-Builtins.html#Other-Builtins">Other
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built-in functions provided by GCC</a>, for details of the functions
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affected.
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</p>
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</dd>
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<dt><code>-std=</code></dt>
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<dd><a name="index-std-1"></a>
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<p>Determine the language standard. See <a href="Standards.html#Standards">Language Standards
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Supported by GCC</a>, for details of these standard versions. This option
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is currently only supported when compiling C or C++.
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</p>
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<p>The compiler can accept several base standards, such as ‘<samp>c90</samp>’ or
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‘<samp>c++98</samp>’, and GNU dialects of those standards, such as
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‘<samp>gnu90</samp>’ or ‘<samp>gnu++98</samp>’. When a base standard is specified, the
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compiler accepts all programs following that standard plus those
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using GNU extensions that do not contradict it. For example,
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<samp>-std=c90</samp> turns off certain features of GCC that are
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incompatible with ISO C90, such as the <code>asm</code> and <code>typeof</code>
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keywords, but not other GNU extensions that do not have a meaning in
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ISO C90, such as omitting the middle term of a <code>?:</code>
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expression. On the other hand, when a GNU dialect of a standard is
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specified, all features supported by the compiler are enabled, even when
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those features change the meaning of the base standard. As a result, some
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strict-conforming programs may be rejected. The particular standard
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is used by <samp>-Wpedantic</samp> to identify which features are GNU
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extensions given that version of the standard. For example
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<samp>-std=gnu90 -Wpedantic</samp> warns about C++ style ‘<samp>//</samp>’
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comments, while <samp>-std=gnu99 -Wpedantic</samp> does not.
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</p>
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<p>A value for this option must be provided; possible values are
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</p>
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<dl compact="compact">
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<dt>‘<samp>c90</samp>’</dt>
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<dt>‘<samp>c89</samp>’</dt>
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<dt>‘<samp>iso9899:1990</samp>’</dt>
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<dd><p>Support all ISO C90 programs (certain GNU extensions that conflict
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with ISO C90 are disabled). Same as <samp>-ansi</samp> for C code.
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</p>
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</dd>
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<dt>‘<samp>iso9899:199409</samp>’</dt>
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<dd><p>ISO C90 as modified in amendment 1.
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</p>
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</dd>
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<dt>‘<samp>c99</samp>’</dt>
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<dt>‘<samp>c9x</samp>’</dt>
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<dt>‘<samp>iso9899:1999</samp>’</dt>
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<dt>‘<samp>iso9899:199x</samp>’</dt>
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<dd><p>ISO C99. This standard is substantially completely supported, modulo
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bugs and floating-point issues
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(mainly but not entirely relating to optional C99 features from
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Annexes F and G). See
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<a href="http://gcc.gnu.org/c99status.html">http://gcc.gnu.org/c99status.html</a><!-- /@w --> for more information. The
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names ‘<samp>c9x</samp>’ and ‘<samp>iso9899:199x</samp>’ are deprecated.
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</p>
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</dd>
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<dt>‘<samp>c11</samp>’</dt>
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<dt>‘<samp>c1x</samp>’</dt>
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<dt>‘<samp>iso9899:2011</samp>’</dt>
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<dd><p>ISO C11, the 2011 revision of the ISO C standard. This standard is
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substantially completely supported, modulo bugs, floating-point issues
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(mainly but not entirely relating to optional C11 features from
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Annexes F and G) and the optional Annexes K (Bounds-checking
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interfaces) and L (Analyzability). The name ‘<samp>c1x</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>c17</samp>’</dt>
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<dt>‘<samp>c18</samp>’</dt>
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<dt>‘<samp>iso9899:2017</samp>’</dt>
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<dt>‘<samp>iso9899:2018</samp>’</dt>
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<dd><p>ISO C17, the 2017 revision of the ISO C standard (expected to be
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published in 2018). This standard is
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same as C11 except for corrections of defects (all of which are also
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applied with <samp>-std=c11</samp>) and a new value of
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<code>__STDC_VERSION__</code>, and so is supported to the same extent as C11.
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</p>
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</dd>
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<dt>‘<samp>gnu90</samp>’</dt>
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<dt>‘<samp>gnu89</samp>’</dt>
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<dd><p>GNU dialect of ISO C90 (including some C99 features).
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</p>
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</dd>
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<dt>‘<samp>gnu99</samp>’</dt>
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<dt>‘<samp>gnu9x</samp>’</dt>
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<dd><p>GNU dialect of ISO C99. The name ‘<samp>gnu9x</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>gnu11</samp>’</dt>
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<dt>‘<samp>gnu1x</samp>’</dt>
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<dd><p>GNU dialect of ISO C11.
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The name ‘<samp>gnu1x</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>gnu17</samp>’</dt>
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<dt>‘<samp>gnu18</samp>’</dt>
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<dd><p>GNU dialect of ISO C17. This is the default for C code.
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</p>
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</dd>
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<dt>‘<samp>c++98</samp>’</dt>
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<dt>‘<samp>c++03</samp>’</dt>
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<dd><p>The 1998 ISO C++ standard plus the 2003 technical corrigendum and some
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additional defect reports. Same as <samp>-ansi</samp> for C++ code.
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</p>
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</dd>
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<dt>‘<samp>gnu++98</samp>’</dt>
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<dt>‘<samp>gnu++03</samp>’</dt>
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<dd><p>GNU dialect of <samp>-std=c++98</samp>.
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</p>
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</dd>
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<dt>‘<samp>c++11</samp>’</dt>
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<dt>‘<samp>c++0x</samp>’</dt>
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<dd><p>The 2011 ISO C++ standard plus amendments.
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The name ‘<samp>c++0x</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>gnu++11</samp>’</dt>
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<dt>‘<samp>gnu++0x</samp>’</dt>
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<dd><p>GNU dialect of <samp>-std=c++11</samp>.
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The name ‘<samp>gnu++0x</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>c++14</samp>’</dt>
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<dt>‘<samp>c++1y</samp>’</dt>
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<dd><p>The 2014 ISO C++ standard plus amendments.
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The name ‘<samp>c++1y</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>gnu++14</samp>’</dt>
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<dt>‘<samp>gnu++1y</samp>’</dt>
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<dd><p>GNU dialect of <samp>-std=c++14</samp>.
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This is the default for C++ code.
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The name ‘<samp>gnu++1y</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>c++17</samp>’</dt>
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<dt>‘<samp>c++1z</samp>’</dt>
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<dd><p>The 2017 ISO C++ standard plus amendments.
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The name ‘<samp>c++1z</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>gnu++17</samp>’</dt>
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<dt>‘<samp>gnu++1z</samp>’</dt>
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<dd><p>GNU dialect of <samp>-std=c++17</samp>.
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The name ‘<samp>gnu++1z</samp>’ is deprecated.
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</p>
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</dd>
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<dt>‘<samp>c++2a</samp>’</dt>
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<dd><p>The next revision of the ISO C++ standard, tentatively planned for
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2020. Support is highly experimental, and will almost certainly
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change in incompatible ways in future releases.
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</p>
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</dd>
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<dt>‘<samp>gnu++2a</samp>’</dt>
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<dd><p>GNU dialect of <samp>-std=c++2a</samp>. Support is highly experimental,
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and will almost certainly change in incompatible ways in future
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releases.
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</p></dd>
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</dl>
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</dd>
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<dt><code>-fgnu89-inline</code></dt>
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<dd><a name="index-fgnu89_002dinline"></a>
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<p>The option <samp>-fgnu89-inline</samp> tells GCC to use the traditional
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GNU semantics for <code>inline</code> functions when in C99 mode.
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See <a href="Inline.html#Inline">An Inline Function is As Fast As a Macro</a>.
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Using this option is roughly equivalent to adding the
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<code>gnu_inline</code> function attribute to all inline functions
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(see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>).
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</p>
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<p>The option <samp>-fno-gnu89-inline</samp> explicitly tells GCC to use the
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C99 semantics for <code>inline</code> when in C99 or gnu99 mode (i.e., it
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specifies the default behavior).
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This option is not supported in <samp>-std=c90</samp> or
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<samp>-std=gnu90</samp> mode.
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</p>
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<p>The preprocessor macros <code>__GNUC_GNU_INLINE__</code> and
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<code>__GNUC_STDC_INLINE__</code> may be used to check which semantics are
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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
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Macros</a> in <cite>The C Preprocessor</cite>.
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</p>
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</dd>
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<dt><code>-fpermitted-flt-eval-methods=<var>style</var></code></dt>
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<dd><a name="index-fpermitted_002dflt_002deval_002dmethods"></a>
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<a name="index-fpermitted_002dflt_002deval_002dmethods_003dc11"></a>
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<a name="index-fpermitted_002dflt_002deval_002dmethods_003dts_002d18661_002d3"></a>
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<p>ISO/IEC TS 18661-3 defines new permissible values for
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<code>FLT_EVAL_METHOD</code> that indicate that operations and constants with
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a semantic type that is an interchange or extended format should be
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evaluated to the precision and range of that type. These new values are
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a superset of those permitted under C99/C11, which does not specify the
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meaning of other positive values of <code>FLT_EVAL_METHOD</code>. As such, code
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conforming to C11 may not have been written expecting the possibility of
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the new values.
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</p>
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<p><samp>-fpermitted-flt-eval-methods</samp> specifies whether the compiler
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should allow only the values of <code>FLT_EVAL_METHOD</code> specified in C99/C11,
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or the extended set of values specified in ISO/IEC TS 18661-3.
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</p>
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<p><var>style</var> is either <code>c11</code> or <code>ts-18661-3</code> as appropriate.
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</p>
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<p>The default when in a standards compliant mode (<samp>-std=c11</samp> or similar)
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is <samp>-fpermitted-flt-eval-methods=c11</samp>. The default when in a GNU
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dialect (<samp>-std=gnu11</samp> or similar) is
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<samp>-fpermitted-flt-eval-methods=ts-18661-3</samp>.
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</p>
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</dd>
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<dt><code>-aux-info <var>filename</var></code></dt>
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<dd><a name="index-aux_002dinfo"></a>
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<p>Output to the given filename prototyped declarations for all functions
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declared and/or defined in a translation unit, including those in header
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files. This option is silently ignored in any language other than C.
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</p>
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<p>Besides declarations, the file indicates, in comments, the origin of
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each declaration (source file and line), whether the declaration was
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implicit, prototyped or unprototyped (‘<samp>I</samp>’, ‘<samp>N</samp>’ for new or
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‘<samp>O</samp>’ for old, respectively, in the first character after the line
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number and the colon), and whether it came from a declaration or a
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definition (‘<samp>C</samp>’ or ‘<samp>F</samp>’, respectively, in the following
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character). In the case of function definitions, a K&R-style list of
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arguments followed by their declarations is also provided, inside
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comments, after the declaration.
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</p>
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</dd>
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<dt><code>-fallow-parameterless-variadic-functions</code></dt>
|
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<dd><a name="index-fallow_002dparameterless_002dvariadic_002dfunctions"></a>
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<p>Accept variadic functions without named parameters.
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</p>
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<p>Although it is possible to define such a function, this is not very
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useful as it is not possible to read the arguments. This is only
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supported for C as this construct is allowed by C++.
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</p>
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</dd>
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<dt><code>-fno-asm</code></dt>
|
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<dd><a name="index-fno_002dasm"></a>
|
|
<p>Do not recognize <code>asm</code>, <code>inline</code> or <code>typeof</code> as a
|
|
keyword, so that code can use these words as identifiers. You can use
|
|
the keywords <code>__asm__</code>, <code>__inline__</code> and <code>__typeof__</code>
|
|
instead. <samp>-ansi</samp> implies <samp>-fno-asm</samp>.
|
|
</p>
|
|
<p>In C++, this switch only affects the <code>typeof</code> keyword, since
|
|
<code>asm</code> and <code>inline</code> are standard keywords. You may want to
|
|
use the <samp>-fno-gnu-keywords</samp> flag instead, which has the same
|
|
effect. In C99 mode (<samp>-std=c99</samp> or <samp>-std=gnu99</samp>), this
|
|
switch only affects the <code>asm</code> and <code>typeof</code> keywords, since
|
|
<code>inline</code> is a standard keyword in ISO C99.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fno-builtin</code></dt>
|
|
<dt><code>-fno-builtin-<var>function</var></code></dt>
|
|
<dd><a name="index-fno_002dbuiltin"></a>
|
|
<a name="index-built_002din-functions"></a>
|
|
<p>Don’t recognize built-in functions that do not begin with
|
|
‘<samp>__builtin_</samp>’ as prefix. See <a href="Other-Builtins.html#Other-Builtins">Other built-in
|
|
functions provided by GCC</a>, for details of the functions affected,
|
|
including those which are not built-in functions when <samp>-ansi</samp> or
|
|
<samp>-std</samp> options for strict ISO C conformance are used because they
|
|
do not have an ISO standard meaning.
|
|
</p>
|
|
<p>GCC normally generates special code to handle certain built-in functions
|
|
more efficiently; for instance, calls to <code>alloca</code> may become single
|
|
instructions which adjust the stack directly, and calls to <code>memcpy</code>
|
|
may become inline copy loops. The resulting code is often both smaller
|
|
and faster, but since the function calls no longer appear as such, you
|
|
cannot set a breakpoint on those calls, nor can you change the behavior
|
|
of the functions by linking with a different library. In addition,
|
|
when a function is recognized as a built-in function, GCC may use
|
|
information about that function to warn about problems with calls to
|
|
that function, or to generate more efficient code, even if the
|
|
resulting code still contains calls to that function. For example,
|
|
warnings are given with <samp>-Wformat</samp> for bad calls to
|
|
<code>printf</code> when <code>printf</code> is built in and <code>strlen</code> is
|
|
known not to modify global memory.
|
|
</p>
|
|
<p>With the <samp>-fno-builtin-<var>function</var></samp> option
|
|
only the built-in function <var>function</var> is
|
|
disabled. <var>function</var> must not begin with ‘<samp>__builtin_</samp>’. If a
|
|
function is named that is not built-in in this version of GCC, this
|
|
option is ignored. There is no corresponding
|
|
<samp>-fbuiltin-<var>function</var></samp> option; if you wish to enable
|
|
built-in functions selectively when using <samp>-fno-builtin</samp> or
|
|
<samp>-ffreestanding</samp>, you may define macros such as:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define abs(n) __builtin_abs ((n))
|
|
#define strcpy(d, s) __builtin_strcpy ((d), (s))
|
|
</pre></div>
|
|
|
|
</dd>
|
|
<dt><code>-fgimple</code></dt>
|
|
<dd><a name="index-fgimple"></a>
|
|
|
|
<p>Enable parsing of function definitions marked with <code>__GIMPLE</code>.
|
|
This is an experimental feature that allows unit testing of GIMPLE
|
|
passes.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fhosted</code></dt>
|
|
<dd><a name="index-fhosted"></a>
|
|
<a name="index-hosted-environment-1"></a>
|
|
|
|
<p>Assert that compilation targets a hosted environment. This implies
|
|
<samp>-fbuiltin</samp>. A hosted environment is one in which the
|
|
entire standard library is available, and in which <code>main</code> has a return
|
|
type of <code>int</code>. Examples are nearly everything except a kernel.
|
|
This is equivalent to <samp>-fno-freestanding</samp>.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-ffreestanding</code></dt>
|
|
<dd><a name="index-ffreestanding-1"></a>
|
|
<a name="index-hosted-environment-2"></a>
|
|
|
|
<p>Assert that compilation targets a freestanding environment. This
|
|
implies <samp>-fno-builtin</samp>. A freestanding environment
|
|
is one in which the standard library may not exist, and program startup may
|
|
not necessarily be at <code>main</code>. The most obvious example is an OS kernel.
|
|
This is equivalent to <samp>-fno-hosted</samp>.
|
|
</p>
|
|
<p>See <a href="Standards.html#Standards">Language Standards Supported by GCC</a>, for details of
|
|
freestanding and hosted environments.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fopenacc</code></dt>
|
|
<dd><a name="index-fopenacc"></a>
|
|
<a name="index-OpenACC-accelerator-programming"></a>
|
|
<p>Enable handling of OpenACC directives <code>#pragma acc</code> in C/C++ and
|
|
<code>!$acc</code> in Fortran. When <samp>-fopenacc</samp> is specified, the
|
|
compiler generates accelerated code according to the OpenACC Application
|
|
Programming Interface v2.0 <a href="https://www.openacc.org">https://www.openacc.org</a><!-- /@w -->. This option
|
|
implies <samp>-pthread</samp>, and thus is only supported on targets that
|
|
have support for <samp>-pthread</samp>.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fopenacc-dim=<var>geom</var></code></dt>
|
|
<dd><a name="index-fopenacc_002ddim"></a>
|
|
<a name="index-OpenACC-accelerator-programming-1"></a>
|
|
<p>Specify default compute dimensions for parallel offload regions that do
|
|
not explicitly specify. The <var>geom</var> value is a triple of
|
|
’:’-separated sizes, in order ’gang’, ’worker’ and, ’vector’. A size
|
|
can be omitted, to use a target-specific default value.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fopenmp</code></dt>
|
|
<dd><a name="index-fopenmp"></a>
|
|
<a name="index-OpenMP-parallel"></a>
|
|
<p>Enable handling of OpenMP directives <code>#pragma omp</code> in C/C++ and
|
|
<code>!$omp</code> in Fortran. When <samp>-fopenmp</samp> is specified, the
|
|
compiler generates parallel code according to the OpenMP Application
|
|
Program Interface v4.5 <a href="http://www.openmp.org/">http://www.openmp.org/</a><!-- /@w -->. This option
|
|
implies <samp>-pthread</samp>, and thus is only supported on targets that
|
|
have support for <samp>-pthread</samp>. <samp>-fopenmp</samp> implies
|
|
<samp>-fopenmp-simd</samp>.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fopenmp-simd</code></dt>
|
|
<dd><a name="index-fopenmp_002dsimd"></a>
|
|
<a name="index-OpenMP-SIMD"></a>
|
|
<a name="index-SIMD"></a>
|
|
<p>Enable handling of OpenMP’s SIMD directives with <code>#pragma omp</code>
|
|
in C/C++ and <code>!$omp</code> in Fortran. Other OpenMP directives
|
|
are ignored.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fgnu-tm</code></dt>
|
|
<dd><a name="index-fgnu_002dtm"></a>
|
|
<p>When the option <samp>-fgnu-tm</samp> is specified, the compiler
|
|
generates code for the Linux variant of Intel’s current Transactional
|
|
Memory ABI specification document (Revision 1.1, May 6 2009). This is
|
|
an experimental feature whose interface may change in future versions
|
|
of GCC, as the official specification changes. Please note that not
|
|
all architectures are supported for this feature.
|
|
</p>
|
|
<p>For more information on GCC’s support for transactional memory,
|
|
See <a href="../libitm/Enabling-libitm.html#Enabling-libitm">The GNU Transactional Memory Library</a> in <cite>GNU
|
|
Transactional Memory Library</cite>.
|
|
</p>
|
|
<p>Note that the transactional memory feature is not supported with
|
|
non-call exceptions (<samp>-fnon-call-exceptions</samp>).
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fms-extensions</code></dt>
|
|
<dd><a name="index-fms_002dextensions"></a>
|
|
<p>Accept some non-standard constructs used in Microsoft header files.
|
|
</p>
|
|
<p>In C++ code, this allows member names in structures to be similar
|
|
to previous types declarations.
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">typedef int UOW;
|
|
struct ABC {
|
|
UOW UOW;
|
|
};
|
|
</pre></div>
|
|
|
|
<p>Some cases of unnamed fields in structures and unions are only
|
|
accepted with this option. See <a href="Unnamed-Fields.html#Unnamed-Fields">Unnamed struct/union
|
|
fields within structs/unions</a>, for details.
|
|
</p>
|
|
<p>Note that this option is off for all targets but x86
|
|
targets using ms-abi.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fplan9-extensions</code></dt>
|
|
<dd><a name="index-fplan9_002dextensions"></a>
|
|
<p>Accept some non-standard constructs used in Plan 9 code.
|
|
</p>
|
|
<p>This enables <samp>-fms-extensions</samp>, permits passing pointers to
|
|
structures with anonymous fields to functions that expect pointers to
|
|
elements of the type of the field, and permits referring to anonymous
|
|
fields declared using a typedef. See <a href="Unnamed-Fields.html#Unnamed-Fields">Unnamed
|
|
struct/union fields within structs/unions</a>, for details. This is only
|
|
supported for C, not C++.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fcond-mismatch</code></dt>
|
|
<dd><a name="index-fcond_002dmismatch"></a>
|
|
<p>Allow conditional expressions with mismatched types in the second and
|
|
third arguments. The value of such an expression is void. This option
|
|
is not supported for C++.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-flax-vector-conversions</code></dt>
|
|
<dd><a name="index-flax_002dvector_002dconversions"></a>
|
|
<p>Allow implicit conversions between vectors with differing numbers of
|
|
elements and/or incompatible element types. This option should not be
|
|
used for new code.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-funsigned-char</code></dt>
|
|
<dd><a name="index-funsigned_002dchar"></a>
|
|
<p>Let the type <code>char</code> be unsigned, like <code>unsigned char</code>.
|
|
</p>
|
|
<p>Each kind of machine has a default for what <code>char</code> should
|
|
be. It is either like <code>unsigned char</code> by default or like
|
|
<code>signed char</code> by default.
|
|
</p>
|
|
<p>Ideally, a portable program should always use <code>signed char</code> or
|
|
<code>unsigned char</code> when it depends on the signedness of an object.
|
|
But many programs have been written to use plain <code>char</code> and
|
|
expect it to be signed, or expect it to be unsigned, depending on the
|
|
machines they were written for. This option, and its inverse, let you
|
|
make such a program work with the opposite default.
|
|
</p>
|
|
<p>The type <code>char</code> is always a distinct type from each of
|
|
<code>signed char</code> or <code>unsigned char</code>, even though its behavior
|
|
is always just like one of those two.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fsigned-char</code></dt>
|
|
<dd><a name="index-fsigned_002dchar"></a>
|
|
<p>Let the type <code>char</code> be signed, like <code>signed char</code>.
|
|
</p>
|
|
<p>Note that this is equivalent to <samp>-fno-unsigned-char</samp>, which is
|
|
the negative form of <samp>-funsigned-char</samp>. Likewise, the option
|
|
<samp>-fno-signed-char</samp> is equivalent to <samp>-funsigned-char</samp>.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fsigned-bitfields</code></dt>
|
|
<dt><code>-funsigned-bitfields</code></dt>
|
|
<dt><code>-fno-signed-bitfields</code></dt>
|
|
<dt><code>-fno-unsigned-bitfields</code></dt>
|
|
<dd><a name="index-fsigned_002dbitfields"></a>
|
|
<a name="index-funsigned_002dbitfields"></a>
|
|
<a name="index-fno_002dsigned_002dbitfields"></a>
|
|
<a name="index-fno_002dunsigned_002dbitfields"></a>
|
|
<p>These options control whether a bit-field is signed or unsigned, when the
|
|
declaration does not use either <code>signed</code> or <code>unsigned</code>. By
|
|
default, such a bit-field is signed, because this is consistent: the
|
|
basic integer types such as <code>int</code> are signed types.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-fsso-struct=<var>endianness</var></code></dt>
|
|
<dd><a name="index-fsso_002dstruct"></a>
|
|
<p>Set the default scalar storage order of structures and unions to the
|
|
specified endianness. The accepted values are ‘<samp>big-endian</samp>’,
|
|
‘<samp>little-endian</samp>’ and ‘<samp>native</samp>’ for the native endianness of
|
|
the target (the default). This option is not supported for C++.
|
|
</p>
|
|
<p><strong>Warning:</strong> the <samp>-fsso-struct</samp> switch causes GCC to generate
|
|
code that is not binary compatible with code generated without it if the
|
|
specified endianness is not the native endianness of the target.
|
|
</p></dd>
|
|
</dl>
|
|
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
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> [<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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|