| <html lang="en"> |
| <head> |
| <title>Structures unions enumerations and bit-fields implementation - Using the GNU Compiler Collection (GCC)</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Using the GNU Compiler Collection (GCC)"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="C-Implementation.html#C-Implementation" title="C Implementation"> |
| <link rel="prev" href="Hints-implementation.html#Hints-implementation" title="Hints implementation"> |
| <link rel="next" href="Qualifiers-implementation.html#Qualifiers-implementation" title="Qualifiers implementation"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.2 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Funding Free Software'', the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| ``GNU Free Documentation License''. |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development.--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Structures-unions-enumerations-and-bit-fields-implementation"></a> |
| <a name="Structures-unions-enumerations-and-bit_002dfields-implementation"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Qualifiers-implementation.html#Qualifiers-implementation">Qualifiers implementation</a>, |
| Previous: <a rel="previous" accesskey="p" href="Hints-implementation.html#Hints-implementation">Hints implementation</a>, |
| Up: <a rel="up" accesskey="u" href="C-Implementation.html#C-Implementation">C Implementation</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">4.9 Structures, unions, enumerations, and bit-fields</h3> |
| |
| <ul> |
| <li><cite>A member of a union object is accessed using a member of a |
| different type (C90 6.3.2.3).</cite> |
| |
| <p>The relevant bytes of the representation of the object are treated as |
| an object of the type used for the access. See <a href="Type_002dpunning.html#Type_002dpunning">Type-punning</a>. This |
| may be a trap representation. |
| |
| <li><cite>Whether a “plain” </cite><code>int</code><cite> bit-field is treated as a |
| </cite><code>signed int</code><cite> bit-field or as an </cite><code>unsigned int</code><cite> bit-field |
| (C90 6.5.2, C90 6.5.2.1, C99 6.7.2, C99 6.7.2.1).</cite> |
| |
| <p><a name="index-funsigned_002dbitfields-2174"></a>By default it is treated as <code>signed int</code> but this may be changed |
| by the <samp><span class="option">-funsigned-bitfields</span></samp> option. |
| |
| <li><cite>Allowable bit-field types other than </cite><code>_Bool</code><cite>, </cite><code>signed int</code><cite>, |
| and </cite><code>unsigned int</code><cite> (C99 6.7.2.1).</cite> |
| |
| <p>No other types are permitted in strictly conforming mode. |
| <!-- Would it be better to restrict the pedwarn for other types to C90 --> |
| <!-- mode and document the other types for C99 mode? --> |
| |
| <li><cite>Whether a bit-field can straddle a storage-unit boundary (C90 |
| 6.5.2.1, C99 6.7.2.1).</cite> |
| |
| <p>Determined by ABI. |
| |
| <li><cite>The order of allocation of bit-fields within a unit (C90 |
| 6.5.2.1, C99 6.7.2.1).</cite> |
| |
| <p>Determined by ABI. |
| |
| <li><cite>The alignment of non-bit-field members of structures (C90 |
| 6.5.2.1, C99 6.7.2.1).</cite> |
| |
| <p>Determined by ABI. |
| |
| <li><cite>The integer type compatible with each enumerated type (C90 |
| 6.5.2.2, C99 6.7.2.2).</cite> |
| |
| <p><a name="index-fshort_002denums-2175"></a>Normally, the type is <code>unsigned int</code> if there are no negative |
| values in the enumeration, otherwise <code>int</code>. If |
| <samp><span class="option">-fshort-enums</span></samp> is specified, then if there are negative values |
| it is the first of <code>signed char</code>, <code>short</code> and <code>int</code> |
| that can represent all the values, otherwise it is the first of |
| <code>unsigned char</code>, <code>unsigned short</code> and <code>unsigned int</code> |
| that can represent all the values. |
| <!-- On a few unusual targets with 64-bit int, this doesn't agree with --> |
| <!-- the code and one of the types accessed via mode attributes (which --> |
| <!-- are not currently considered extended integer types) may be used. --> |
| <!-- If these types are made extended integer types, it would still be --> |
| <!-- the case that -fshort-enums stops the implementation from --> |
| <!-- conforming to C90 on those targets. --> |
| |
| <p>On some targets, <samp><span class="option">-fshort-enums</span></samp> is the default; this is |
| determined by the ABI. |
| |
| </ul> |
| |
| </body></html> |
| |
