projects / OpenSPARC-T2-SAM / blame
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| 16 | <title>4.2 Differences Between Unix and Windows </title> | |
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| 32 | <td align="center" width="100%">Extending and Embedding the Python Interpreter</td> | |
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| 52 | ||
| 53 | <H1><A NAME="SECTION006200000000000000000"></A><A NAME="dynamic-linking"></A> | |
| 54 | <BR> | |
| 55 | 4.2 Differences Between <span class="Unix">Unix</span> and Windows | |
| 56 | ||
| 57 | </H1> | |
| 58 | ||
| 59 | <P> | |
| 60 | <span class="Unix">Unix</span> and Windows use completely different paradigms for run-time | |
| 61 | loading of code. Before you try to build a module that can be | |
| 62 | dynamically loaded, be aware of how your system works. | |
| 63 | ||
| 64 | <P> | |
| 65 | In <span class="Unix">Unix</span>, a shared object (<span class="file">.so</span>) file contains code to be used by the | |
| 66 | program, and also the names of functions and data that it expects to | |
| 67 | find in the program. When the file is joined to the program, all | |
| 68 | references to those functions and data in the file's code are changed | |
| 69 | to point to the actual locations in the program where the functions | |
| 70 | and data are placed in memory. This is basically a link operation. | |
| 71 | ||
| 72 | <P> | |
| 73 | In Windows, a dynamic-link library (<span class="file">.dll</span>) file has no dangling | |
| 74 | references. Instead, an access to functions or data goes through a | |
| 75 | lookup table. So the DLL code does not have to be fixed up at runtime | |
| 76 | to refer to the program's memory; instead, the code already uses the | |
| 77 | DLL's lookup table, and the lookup table is modified at runtime to | |
| 78 | point to the functions and data. | |
| 79 | ||
| 80 | <P> | |
| 81 | In <span class="Unix">Unix</span>, there is only one type of library file (<span class="file">.a</span>) which | |
| 82 | contains code from several object files (<span class="file">.o</span>). During the link | |
| 83 | step to create a shared object file (<span class="file">.so</span>), the linker may find | |
| 84 | that it doesn't know where an identifier is defined. The linker will | |
| 85 | look for it in the object files in the libraries; if it finds it, it | |
| 86 | will include all the code from that object file. | |
| 87 | ||
| 88 | <P> | |
| 89 | In Windows, there are two types of library, a static library and an | |
| 90 | import library (both called <span class="file">.lib</span>). A static library is like a | |
| 91 | <span class="Unix">Unix</span> <span class="file">.a</span> file; it contains code to be included as necessary. | |
| 92 | An import library is basically used only to reassure the linker that a | |
| 93 | certain identifier is legal, and will be present in the program when | |
| 94 | the DLL is loaded. So the linker uses the information from the | |
| 95 | import library to build the lookup table for using identifiers that | |
| 96 | are not included in the DLL. When an application or a DLL is linked, | |
| 97 | an import library may be generated, which will need to be used for all | |
| 98 | future DLLs that depend on the symbols in the application or DLL. | |
| 99 | ||
| 100 | <P> | |
| 101 | Suppose you are building two dynamic-load modules, B and C, which should | |
| 102 | share another block of code A. On <span class="Unix">Unix</span>, you would <em>not</em> pass | |
| 103 | <span class="file">A.a</span> to the linker for <span class="file">B.so</span> and <span class="file">C.so</span>; that would | |
| 104 | cause it to be included twice, so that B and C would each have their | |
| 105 | own copy. In Windows, building <span class="file">A.dll</span> will also build | |
| 106 | <span class="file">A.lib</span>. You <em>do</em> pass <span class="file">A.lib</span> to the linker for B and | |
| 107 | C. <span class="file">A.lib</span> does not contain code; it just contains information | |
| 108 | which will be used at runtime to access A's code. | |
| 109 | ||
| 110 | <P> | |
| 111 | In Windows, using an import library is sort of like using "<tt class="samp">import | |
| 112 | spam</tt>"; it gives you access to spam's names, but does not create a | |
| 113 | separate copy. On <span class="Unix">Unix</span>, linking with a library is more like | |
| 114 | "<tt class="samp">from spam import *</tt>"; it does create a separate copy. | |
| 115 | ||
| 116 | <P> | |
| 117 | ||
| 118 | <DIV CLASS="navigation"> | |
| 119 | <div class='online-navigation'> | |
| 120 | <p></p><hr /> | |
| 121 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> | |
| 122 | <tr> | |
| 123 | <td class='online-navigation'><a rel="prev" title="4.1 A Cookbook Approach" | |
| 124 | href="win-cookbook.html"><img src='../icons/previous.png' | |
| 125 | border='0' height='32' alt='Previous Page' width='32' /></A></td> | |
| 126 | <td class='online-navigation'><a rel="parent" title="4. Building C and" | |
| 127 | href="building-on-windows.html"><img src='../icons/up.png' | |
| 128 | border='0' height='32' alt='Up One Level' width='32' /></A></td> | |
| 129 | <td class='online-navigation'><a rel="next" title="4.3 Using DLLs in" | |
| 130 | href="win-dlls.html"><img src='../icons/next.png' | |
| 131 | border='0' height='32' alt='Next Page' width='32' /></A></td> | |
| 132 | <td align="center" width="100%">Extending and Embedding the Python Interpreter</td> | |
| 133 | <td class='online-navigation'><a rel="contents" title="Table of Contents" | |
| 134 | href="contents.html"><img src='../icons/contents.png' | |
| 135 | border='0' height='32' alt='Contents' width='32' /></A></td> | |
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| 139 | border='0' height='32' alt='' width='32' /></td> | |
| 140 | </tr></table> | |
| 141 | <div class='online-navigation'> | |
| 142 | <b class="navlabel">Previous:</b> | |
| 143 | <a class="sectref" rel="prev" href="win-cookbook.html">4.1 A Cookbook Approach</A> | |
| 144 | <b class="navlabel">Up:</b> | |
| 145 | <a class="sectref" rel="parent" href="building-on-windows.html">4. Building C and</A> | |
| 146 | <b class="navlabel">Next:</b> | |
| 147 | <a class="sectref" rel="next" href="win-dlls.html">4.3 Using DLLs in</A> | |
| 148 | </div> | |
| 149 | </div> | |
| 150 | <hr /> | |
| 151 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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