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<title>Using the GNU Compiler Collection (GCC): Method signatures</title>
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<a name="Method-signatures"></a>
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<p>
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Previous: <a href="_0040encode.html#g_t_0040encode" accesskey="p" rel="prev">@encode</a>, Up: <a href="Type-encoding.html#Type-encoding" accesskey="u" rel="up">Type encoding</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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<a name="Method-Signatures"></a>
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<h4 class="subsection">8.3.3 Method Signatures</h4>
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<p>This section documents the encoding of method types, which is rarely
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needed to use Objective-C. You should skip it at a first reading; the
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runtime provides functions that will work on methods and can walk
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through the list of parameters and interpret them for you. These
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functions are part of the public “API” and are the preferred way to
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interact with method signatures from user code.
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</p>
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<p>But if you need to debug a problem with method signatures and need to
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know how they are implemented (i.e., the “ABI”), read on.
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</p>
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<p>Methods have their “signature” encoded and made available to the
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runtime. The “signature” encodes all the information required to
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dynamically build invocations of the method at runtime: return type
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and arguments.
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</p>
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<p>The “signature” is a null-terminated string, composed of the following:
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</p>
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<ul>
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<li> The return type, including type qualifiers. For example, a method
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returning <code>int</code> would have <code>i</code> here.
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</li><li> The total size (in bytes) required to pass all the parameters. This
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includes the two hidden parameters (the object <code>self</code> and the
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method selector <code>_cmd</code>).
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</li><li> Each argument, with the type encoding, followed by the offset (in
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bytes) of the argument in the list of parameters.
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</li></ul>
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<p>For example, a method with no arguments and returning <code>int</code> would
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have the signature <code>i8@0:4</code> if the size of a pointer is 4. The
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signature is interpreted as follows: the <code>i</code> is the return type
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(an <code>int</code>), the <code>8</code> is the total size of the parameters in
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bytes (two pointers each of size 4), the <code>@0</code> is the first
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parameter (an object at byte offset <code>0</code>) and <code>:4</code> is the
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second parameter (a <code>SEL</code> at byte offset <code>4</code>).
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</p>
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<p>You can easily find more examples by running the “strings” program
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on an Objective-C object file compiled by GCC. You’ll see a lot of
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strings that look very much like <code>i8@0:4</code>. They are signatures
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of Objective-C methods.
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</p>
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Previous: <a href="_0040encode.html#g_t_0040encode" accesskey="p" rel="prev">@encode</a>, Up: <a href="Type-encoding.html#Type-encoding" accesskey="u" rel="up">Type encoding</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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