diff options
Diffstat (limited to 'share/doc/gcc/Forwarding-hook.html')
| -rw-r--r-- | share/doc/gcc/Forwarding-hook.html | 145 |
1 files changed, 145 insertions, 0 deletions
diff --git a/share/doc/gcc/Forwarding-hook.html b/share/doc/gcc/Forwarding-hook.html new file mode 100644 index 0000000..e52e43a --- /dev/null +++ b/share/doc/gcc/Forwarding-hook.html @@ -0,0 +1,145 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 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 1.3 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. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Using the GNU Compiler Collection (GCC): Forwarding hook</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Forwarding hook"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Forwarding hook"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Indices.html#Indices" rel="index" title="Indices"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<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"> +<link href="Compatibility.html#Compatibility" rel="next" title="Compatibility"> +<link href="Dynamically-registering-methods.html#Dynamically-registering-methods" rel="previous" title="Dynamically registering methods"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en_US" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Forwarding-hook"></a> +<div class="header"> +<p> +Previous: <a href="Dynamically-registering-methods.html#Dynamically-registering-methods" accesskey="p" rel="previous">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="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Forwarding-Hook"></a> +<h4 class="subsection">8.10.2 Forwarding Hook</h4> + +<p>The GNU Objective-C runtime provides a hook, called +<code>__objc_msg_forward2</code>, which is called by +<code>objc_msg_lookup()</code> when it cannot find a method implementation in +the runtime tables and after calling <code>+resolveInstanceMethod:</code> +and <code>+resolveClassMethod:</code> has been attempted and did not succeed +in dynamically registering the method. +</p> +<p>To configure the hook, you set the global variable +<code>__objc_msg_forward2</code> to a function with the same argument and +return types of <code>objc_msg_lookup()</code>. When +<code>objc_msg_lookup()</code> cannot find a method implementation, it +invokes the hook function you provided to get a method implementation +to return. So, in practice <code>__objc_msg_forward2</code> allows you to +extend <code>objc_msg_lookup()</code> by adding some custom code that is +called to do a further lookup when no standard method implementation +can be found using the normal lookup. +</p> +<p>This hook is generally reserved for “Foundation” libraries such as +GNUstep Base, which use it to implement their high-level method +forwarding API, typically based around the <code>forwardInvocation:</code> +method. So, unless you are implementing your own “Foundation” +library, you should not set this hook. +</p> +<p>In a typical forwarding implementation, the <code>__objc_msg_forward2</code> +hook function determines the argument and return type of the method +that is being looked up, and then creates a function that takes these +arguments and has that return type, and returns it to the caller. +Creating this function is non-trivial and is typically performed using +a dedicated library such as <code>libffi</code>. +</p> +<p>The forwarding method implementation thus created is returned by +<code>objc_msg_lookup()</code> and is executed as if it was a normal method +implementation. When the forwarding method implementation is called, +it is usually expected to pack all arguments into some sort of object +(typically, an <code>NSInvocation</code> in a “Foundation” library), and +hand it over to the programmer (<code>forwardInvocation:</code>) who is then +allowed to manipulate the method invocation using a high-level API +provided by the “Foundation” library. For example, the programmer +may want to examine the method invocation arguments and name and +potentially change them before forwarding the method invocation to one +or more local objects (<code>performInvocation:</code>) or even to remote +objects (by using Distributed Objects or some other mechanism). When +all this completes, the return value is passed back and must be +returned correctly to the original caller. +</p> +<p>Note that the GNU Objective-C runtime currently provides no support +for method forwarding or method invocations other than the +<code>__objc_msg_forward2</code> hook. +</p> +<p>If the forwarding hook does not exist or returns <code>NULL</code>, the +runtime currently attempts forwarding using an older, deprecated API, +and if that fails, it aborts the program. In future versions of the +GNU Objective-C runtime, the runtime will immediately abort. +</p> +<hr> +<div class="header"> +<p> +Previous: <a href="Dynamically-registering-methods.html#Dynamically-registering-methods" accesskey="p" rel="previous">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="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |