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143 lines
7.3 KiB
HTML
143 lines
7.3 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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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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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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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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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Forwarding hook (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Forwarding hook (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Forwarding hook (Using the GNU Compiler Collection (GCC))">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="Messaging-with-the-GNU-Objective_002dC-runtime.html#Messaging-with-the-GNU-Objective_002dC-runtime" rel="up" title="Messaging with the GNU Objective-C runtime">
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<link href="Compatibility.html#Compatibility" rel="next" title="Compatibility">
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<link href="Dynamically-registering-methods.html#Dynamically-registering-methods" rel="prev" title="Dynamically registering methods">
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</style>
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</head>
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<body lang="en">
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<a name="Forwarding-hook"></a>
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<div class="header">
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<p>
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Previous: <a href="Dynamically-registering-methods.html#Dynamically-registering-methods" accesskey="p" rel="prev">Dynamically registering methods</a>, Up: <a href="Messaging-with-the-GNU-Objective_002dC-runtime.html#Messaging-with-the-GNU-Objective_002dC-runtime" accesskey="u" rel="up">Messaging with the GNU Objective-C runtime</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="Forwarding-Hook"></a>
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<h4 class="subsection">8.10.2 Forwarding Hook</h4>
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<p>The GNU Objective-C runtime provides a hook, called
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<code>__objc_msg_forward2</code>, which is called by
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<code>objc_msg_lookup()</code> when it cannot find a method implementation in
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the runtime tables and after calling <code>+resolveInstanceMethod:</code>
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and <code>+resolveClassMethod:</code> has been attempted and did not succeed
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in dynamically registering the method.
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</p>
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<p>To configure the hook, you set the global variable
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<code>__objc_msg_forward2</code> to a function with the same argument and
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return types of <code>objc_msg_lookup()</code>. When
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<code>objc_msg_lookup()</code> can not find a method implementation, it
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invokes the hook function you provided to get a method implementation
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to return. So, in practice <code>__objc_msg_forward2</code> allows you to
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extend <code>objc_msg_lookup()</code> by adding some custom code that is
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called to do a further lookup when no standard method implementation
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can be found using the normal lookup.
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</p>
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<p>This hook is generally reserved for “Foundation” libraries such as
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GNUstep Base, which use it to implement their high-level method
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forwarding API, typically based around the <code>forwardInvocation:</code>
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method. So, unless you are implementing your own “Foundation”
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library, you should not set this hook.
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</p>
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<p>In a typical forwarding implementation, the <code>__objc_msg_forward2</code>
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hook function determines the argument and return type of the method
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that is being looked up, and then creates a function that takes these
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arguments and has that return type, and returns it to the caller.
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Creating this function is non-trivial and is typically performed using
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a dedicated library such as <code>libffi</code>.
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</p>
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<p>The forwarding method implementation thus created is returned by
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<code>objc_msg_lookup()</code> and is executed as if it was a normal method
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implementation. When the forwarding method implementation is called,
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it is usually expected to pack all arguments into some sort of object
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(typically, an <code>NSInvocation</code> in a “Foundation” library), and
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hand it over to the programmer (<code>forwardInvocation:</code>) who is then
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allowed to manipulate the method invocation using a high-level API
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provided by the “Foundation” library. For example, the programmer
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may want to examine the method invocation arguments and name and
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potentially change them before forwarding the method invocation to one
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or more local objects (<code>performInvocation:</code>) or even to remote
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objects (by using Distributed Objects or some other mechanism). When
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all this completes, the return value is passed back and must be
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returned correctly to the original caller.
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</p>
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<p>Note that the GNU Objective-C runtime currently provides no support
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for method forwarding or method invocations other than the
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<code>__objc_msg_forward2</code> hook.
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</p>
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<p>If the forwarding hook does not exist or returns <code>NULL</code>, the
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runtime currently attempts forwarding using an older, deprecated API,
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and if that fails, it aborts the program. In future versions of the
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GNU Objective-C runtime, the runtime will immediately abort.
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</p>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Dynamically-registering-methods.html#Dynamically-registering-methods" accesskey="p" rel="prev">Dynamically registering methods</a>, Up: <a href="Messaging-with-the-GNU-Objective_002dC-runtime.html#Messaging-with-the-GNU-Objective_002dC-runtime" accesskey="u" rel="up">Messaging with the GNU Objective-C runtime</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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</body>
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</html>
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