visus
/
toolchain-gcc-arm-embedded
Watch 1
Star 0
Fork 0
Code Issues 0 Pull Requests 0 Releases 0 Wiki Activity
You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
2 Commits
1 Branch
Tree: 898a39b5ab
Branches Tags
${ item.name }
Create branch ${ searchTerm }
from '898a39b5ab'
${ noResults }
toolchain-gcc-arm-embedded/share/doc/gcc-arm-none-eabi/html/gcc/Forwarding-hook.html

143 lines
7.3KB
Raw Blame History 

  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

  2. <html>

  3. <!-- Copyright (C) 1988-2020 Free Software Foundation, Inc.

  4. 

  5. Permission is granted to copy, distribute and/or modify this document

  6. under the terms of the GNU Free Documentation License, Version 1.3 or

  7. any later version published by the Free Software Foundation; with the

  8. Invariant Sections being "Funding Free Software", the Front-Cover

  9. Texts being (a) (see below), and with the Back-Cover Texts being (b)

  10. (see below).  A copy of the license is included in the section entitled

  11. "GNU Free Documentation License".

  12. 

  13. (a) The FSF's Front-Cover Text is:

  14. 

  15. A GNU Manual

  16. 

  17. (b) The FSF's Back-Cover Text is:

  18. 

  19. You have freedom to copy and modify this GNU Manual, like GNU

  20.      software.  Copies published by the Free Software Foundation raise

  21.      funds for GNU development. -->

  22. <!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->

  23. <head>

  24. <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

  25. <title>Forwarding hook (Using the GNU Compiler Collection (GCC))</title>

  26. 

  27. <meta name="description" content="Forwarding hook (Using the GNU Compiler Collection (GCC))">

  28. <meta name="keywords" content="Forwarding hook (Using the GNU Compiler Collection (GCC))">

  29. <meta name="resource-type" content="document">

  30. <meta name="distribution" content="global">

  31. <meta name="Generator" content="makeinfo">

  32. <link href="index.html#Top" rel="start" title="Top">

  33. <link href="Option-Index.html#Option-Index" rel="index" title="Option Index">

  34. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  35. <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">

  36. <link href="Compatibility.html#Compatibility" rel="next" title="Compatibility">

  37. <link href="Dynamically-registering-methods.html#Dynamically-registering-methods" rel="prev" title="Dynamically registering methods">

  38. <style type="text/css">

  39. <!--

  40. a.summary-letter {text-decoration: none}

  41. blockquote.indentedblock {margin-right: 0em}

  42. blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}

  43. blockquote.smallquotation {font-size: smaller}

  44. div.display {margin-left: 3.2em}

  45. div.example {margin-left: 3.2em}

  46. div.lisp {margin-left: 3.2em}

  47. div.smalldisplay {margin-left: 3.2em}

  48. div.smallexample {margin-left: 3.2em}

  49. div.smalllisp {margin-left: 3.2em}

  50. kbd {font-style: oblique}

  51. pre.display {font-family: inherit}

  52. pre.format {font-family: inherit}

  53. pre.menu-comment {font-family: serif}

  54. pre.menu-preformatted {font-family: serif}

  55. pre.smalldisplay {font-family: inherit; font-size: smaller}

  56. pre.smallexample {font-size: smaller}

  57. pre.smallformat {font-family: inherit; font-size: smaller}

  58. pre.smalllisp {font-size: smaller}

  59. span.nolinebreak {white-space: nowrap}

  60. span.roman {font-family: initial; font-weight: normal}

  61. span.sansserif {font-family: sans-serif; font-weight: normal}

  62. ul.no-bullet {list-style: none}

  63. -->

  64. </style>

  65. 

  66. 

  67. </head>

  68. 

  69. <body lang="en">

  70. <a name="Forwarding-hook"></a>

  71. <div class="header">

  72. <p>

  73. 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> &nbsp; [<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>

  74. </div>

  75. <hr>

  76. <a name="Forwarding-Hook"></a>

  77. <h4 class="subsection">8.10.2 Forwarding Hook</h4>

  78. 

  79. <p>The GNU Objective-C runtime provides a hook, called

  80. <code>__objc_msg_forward2</code>, which is called by

  81. <code>objc_msg_lookup()</code> when it cannot find a method implementation in

  82. the runtime tables and after calling <code>+resolveInstanceMethod:</code>

  83. and <code>+resolveClassMethod:</code> has been attempted and did not succeed

  84. in dynamically registering the method.

  85. </p>

  86. <p>To configure the hook, you set the global variable

  87. <code>__objc_msg_forward2</code> to a function with the same argument and

  88. return types of <code>objc_msg_lookup()</code>.  When

  89. <code>objc_msg_lookup()</code> cannot find a method implementation, it

  90. invokes the hook function you provided to get a method implementation

  91. to return.  So, in practice <code>__objc_msg_forward2</code> allows you to

  92. extend <code>objc_msg_lookup()</code> by adding some custom code that is

  93. called to do a further lookup when no standard method implementation

  94. can be found using the normal lookup.

  95. </p>

  96. <p>This hook is generally reserved for &ldquo;Foundation&rdquo; libraries such as

  97. GNUstep Base, which use it to implement their high-level method

  98. forwarding API, typically based around the <code>forwardInvocation:</code>

  99. method.  So, unless you are implementing your own &ldquo;Foundation&rdquo;

  100. library, you should not set this hook.

  101. </p>

  102. <p>In a typical forwarding implementation, the <code>__objc_msg_forward2</code>

  103. hook function determines the argument and return type of the method

  104. that is being looked up, and then creates a function that takes these

  105. arguments and has that return type, and returns it to the caller.

  106. Creating this function is non-trivial and is typically performed using

  107. a dedicated library such as <code>libffi</code>.

  108. </p>

  109. <p>The forwarding method implementation thus created is returned by

  110. <code>objc_msg_lookup()</code> and is executed as if it was a normal method

  111. implementation.  When the forwarding method implementation is called,

  112. it is usually expected to pack all arguments into some sort of object

  113. (typically, an <code>NSInvocation</code> in a &ldquo;Foundation&rdquo; library), and

  114. hand it over to the programmer (<code>forwardInvocation:</code>) who is then

  115. allowed to manipulate the method invocation using a high-level API

  116. provided by the &ldquo;Foundation&rdquo; library.  For example, the programmer

  117. may want to examine the method invocation arguments and name and

  118. potentially change them before forwarding the method invocation to one

  119. or more local objects (<code>performInvocation:</code>) or even to remote

  120. objects (by using Distributed Objects or some other mechanism).  When

  121. all this completes, the return value is passed back and must be

  122. returned correctly to the original caller.

  123. </p>

  124. <p>Note that the GNU Objective-C runtime currently provides no support

  125. for method forwarding or method invocations other than the

  126. <code>__objc_msg_forward2</code> hook.

  127. </p>

  128. <p>If the forwarding hook does not exist or returns <code>NULL</code>, the

  129. runtime currently attempts forwarding using an older, deprecated API,

  130. and if that fails, it aborts the program.  In future versions of the

  131. GNU Objective-C runtime, the runtime will immediately abort.

  132. </p>

  133. <hr>

  134. <div class="header">

  135. <p>

  136. 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> &nbsp; [<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>

  137. </div>

  138. 

  139. 

  140. 

  141. </body>

  142. </html>