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<a name="Method-signatures"></a>
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<a name="Method-Signatures"></a>
<h4 class="subsection">8.3.3 Method Signatures</h4>

<p>This section documents the encoding of method types, which is rarely
needed to use Objective-C.  You should skip it at a first reading; the
runtime provides functions that will work on methods and can walk
through the list of parameters and interpret them for you.  These
functions are part of the public &ldquo;API&rdquo; and are the preferred way to
interact with method signatures from user code.
</p>
<p>But if you need to debug a problem with method signatures and need to
know how they are implemented (i.e., the &ldquo;ABI&rdquo;), read on.
</p>
<p>Methods have their &ldquo;signature&rdquo; encoded and made available to the
runtime.  The &ldquo;signature&rdquo; encodes all the information required to
dynamically build invocations of the method at runtime: return type
and arguments.
</p>
<p>The &ldquo;signature&rdquo; is a null-terminated string, composed of the following:
</p>
<ul>
<li> The return type, including type qualifiers.  For example, a method
returning <code>int</code> would have <code>i</code> here.

</li><li> The total size (in bytes) required to pass all the parameters.  This
includes the two hidden parameters (the object <code>self</code> and the
method selector <code>_cmd</code>).

</li><li> Each argument, with the type encoding, followed by the offset (in
bytes) of the argument in the list of parameters.

</li></ul>

<p>For example, a method with no arguments and returning <code>int</code> would
have the signature <code>i8@0:4</code> if the size of a pointer is 4.  The
signature is interpreted as follows: the <code>i</code> is the return type
(an <code>int</code>), the <code>8</code> is the total size of the parameters in
bytes (two pointers each of size 4), the <code>@0</code> is the first
parameter (an object at byte offset <code>0</code>) and <code>:4</code> is the
second parameter (a <code>SEL</code> at byte offset <code>4</code>).
</p>
<p>You can easily find more examples by running the &ldquo;strings&rdquo; program
on an Objective-C object file compiled by GCC.  You&rsquo;ll see a lot of
strings that look very much like <code>i8@0:4</code>.  They are signatures
of Objective-C methods.
</p>

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