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| 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 | |
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