| <html lang="en"> |
| <head> |
| <title>Type encoding - 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="Objective_002dC.html#Objective_002dC" title="Objective-C"> |
| <link rel="prev" href="Executing-code-before-main.html#Executing-code-before-main" title="Executing code before main"> |
| <link rel="next" href="Garbage-Collection.html#Garbage-Collection" title="Garbage Collection"> |
| <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="Type-encoding"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Garbage-Collection.html#Garbage-Collection">Garbage Collection</a>, |
| Previous: <a rel="previous" accesskey="p" href="Executing-code-before-main.html#Executing-code-before-main">Executing code before main</a>, |
| Up: <a rel="up" accesskey="u" href="Objective_002dC.html#Objective_002dC">Objective-C</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">8.2 Type encoding</h3> |
| |
| <p>The Objective-C compiler generates type encodings for all the |
| types. These type encodings are used at runtime to find out information |
| about selectors and methods and about objects and classes. |
| |
| <p>The types are encoded in the following way: |
| |
| <!-- @sp 1 --> |
| <p><table summary=""><tr align="left"><td valign="top" width="25%"><code>_Bool</code> |
| </td><td valign="top" width="75%"><code>B</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>char</code> |
| </td><td valign="top" width="75%"><code>c</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>unsigned char</code> |
| </td><td valign="top" width="75%"><code>C</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>short</code> |
| </td><td valign="top" width="75%"><code>s</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>unsigned short</code> |
| </td><td valign="top" width="75%"><code>S</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>int</code> |
| </td><td valign="top" width="75%"><code>i</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>unsigned int</code> |
| </td><td valign="top" width="75%"><code>I</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>long</code> |
| </td><td valign="top" width="75%"><code>l</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>unsigned long</code> |
| </td><td valign="top" width="75%"><code>L</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>long long</code> |
| </td><td valign="top" width="75%"><code>q</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>unsigned long long</code> |
| </td><td valign="top" width="75%"><code>Q</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>float</code> |
| </td><td valign="top" width="75%"><code>f</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>double</code> |
| </td><td valign="top" width="75%"><code>d</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>void</code> |
| </td><td valign="top" width="75%"><code>v</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>id</code> |
| </td><td valign="top" width="75%"><code>@</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>Class</code> |
| </td><td valign="top" width="75%"><code>#</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>SEL</code> |
| </td><td valign="top" width="75%"><code>:</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>char*</code> |
| </td><td valign="top" width="75%"><code>*</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%">unknown type |
| </td><td valign="top" width="75%"><code>?</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%">Complex types |
| </td><td valign="top" width="75%"><code>j</code> followed by the inner type. For example <code>_Complex double</code> is encoded as "jd". |
| <br></td></tr><tr align="left"><td valign="top" width="25%">bit-fields |
| </td><td valign="top" width="75%"><code>b</code> followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below) |
| <br></td></tr></table> |
| |
| <!-- @sp 1 --> |
| <p>The encoding of bit-fields has changed to allow bit-fields to be properly |
| handled by the runtime functions that compute sizes and alignments of |
| types that contain bit-fields. The previous encoding contained only the |
| size of the bit-field. Using only this information it is not possible to |
| reliably compute the size occupied by the bit-field. This is very |
| important in the presence of the Boehm's garbage collector because the |
| objects are allocated using the typed memory facility available in this |
| collector. The typed memory allocation requires information about where |
| the pointers are located inside the object. |
| |
| <p>The position in the bit-field is the position, counting in bits, of the |
| bit closest to the beginning of the structure. |
| |
| <p>The non-atomic types are encoded as follows: |
| |
| <!-- @sp 1 --> |
| <p><table summary=""><tr align="left"><td valign="top" width="20%">pointers |
| </td><td valign="top" width="80%">‘<samp><span class="samp">^</span></samp>’ followed by the pointed type. |
| <br></td></tr><tr align="left"><td valign="top" width="20%">arrays |
| </td><td valign="top" width="80%">‘<samp><span class="samp">[</span></samp>’ followed by the number of elements in the array followed by the type of the elements followed by ‘<samp><span class="samp">]</span></samp>’ |
| <br></td></tr><tr align="left"><td valign="top" width="20%">structures |
| </td><td valign="top" width="80%">‘<samp><span class="samp">{</span></samp>’ followed by the name of the structure (or ‘<samp><span class="samp">?</span></samp>’ if the structure is unnamed), the ‘<samp><span class="samp">=</span></samp>’ sign, the type of the members and by ‘<samp><span class="samp">}</span></samp>’ |
| <br></td></tr><tr align="left"><td valign="top" width="20%">unions |
| </td><td valign="top" width="80%">‘<samp><span class="samp">(</span></samp>’ followed by the name of the structure (or ‘<samp><span class="samp">?</span></samp>’ if the union is unnamed), the ‘<samp><span class="samp">=</span></samp>’ sign, the type of the members followed by ‘<samp><span class="samp">)</span></samp>’ |
| <br></td></tr></table> |
| |
| <p>Here are some types and their encodings, as they are generated by the |
| compiler on an i386 machine: |
| |
| <pre class="sp"> |
| |
| </pre> |
| <p><table summary=""><tr align="left"><td valign="top" width="25%">Objective-C type |
| </td><td valign="top" width="75%">Compiler encoding |
| <br></td></tr><tr align="left"><td valign="top" width="25%"> |
| <pre class="smallexample"> int a[10]; |
| </pre> |
| <p></td><td valign="top" width="75%"><code>[10i]</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"> |
| <pre class="smallexample"> struct { |
| int i; |
| float f[3]; |
| int a:3; |
| int b:2; |
| char c; |
| } |
| </pre> |
| <p></td><td valign="top" width="75%"><code>{?=i[3f]b128i3b131i2c}</code> |
| <br></td></tr></table> |
| |
| <pre class="sp"> |
| |
| </pre> |
| |
| In addition to the types the compiler also encodes the type |
| specifiers. The table below describes the encoding of the current |
| Objective-C type specifiers: |
| |
| <pre class="sp"> |
| |
| </pre> |
| <p><table summary=""><tr align="left"><td valign="top" width="25%">Specifier |
| </td><td valign="top" width="75%">Encoding |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>const</code> |
| </td><td valign="top" width="75%"><code>r</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>in</code> |
| </td><td valign="top" width="75%"><code>n</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>inout</code> |
| </td><td valign="top" width="75%"><code>N</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>out</code> |
| </td><td valign="top" width="75%"><code>o</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>bycopy</code> |
| </td><td valign="top" width="75%"><code>O</code> |
| <br></td></tr><tr align="left"><td valign="top" width="25%"><code>oneway</code> |
| </td><td valign="top" width="75%"><code>V</code> |
| <br></td></tr></table> |
| |
| <pre class="sp"> |
| |
| </pre> |
| |
| The type specifiers are encoded just before the type. Unlike types |
| however, the type specifiers are only encoded when they appear in method |
| argument types. |
| |
| </body></html> |
| |
