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| 38 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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| 39 | <a name="bbv2.faq.dll-path"></a>Why are the <code class="computeroutput">dll-path</code> and |
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| 40 | <code class="computeroutput">hardcode-dll-paths</code> properties useful? |
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| 41 | </h2></div></div></div> |
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| 42 | <p>(This entry is specific to Unix system.)Before answering the |
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| 43 | questions, let's recall a few points about shared libraries. Shared |
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| 44 | libraries can be used by several applications, or other libraries, |
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| 45 | without phisycally including the library in the application. This can |
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| 46 | greatly decrease the total size of applications. It's also possible to |
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| 47 | upgrade a shared library when the application is already |
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| 48 | installed. Finally, shared linking can be faster. |
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| 49 | </p> |
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| 50 | <p>However, the shared library must be found when the application is |
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| 51 | started. The dynamic linker will search in a system-defined list of |
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| 52 | paths, load the library and resolve the symbols. Which means that you |
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| 53 | should either change the system-defined list, given by the |
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| 54 | <code class="envar">LD_LIBRARY_PATH</code> environment variable, or install the |
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| 55 | libraries to a system location. This can be inconvenient when |
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| 56 | developing, since the libraries are not yet ready to be installed, and |
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| 57 | cluttering system paths is undesirable. Luckily, on Unix there's another |
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| 58 | way. |
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| 59 | </p> |
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| 60 | <p>An executable can include a list of additional library paths, which |
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| 61 | will be searched before system paths. This is excellent for development, |
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| 62 | because the build system knows the paths to all libraries and can include |
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| 63 | them in executables. That's done when the <code class="computeroutput">hardcode-dll-paths</code> |
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| 64 | feature has the <code class="literal">true</code> value, which is the |
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| 65 | default. When the executables should be installed, the story is |
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| 66 | different. |
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| 67 | </p> |
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| 68 | <p> |
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| 69 | Obviously, installed executable should not hardcode paths to your |
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| 70 | development tree. (The <code class="computeroutput">stage</code> rule explicitly disables the |
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| 71 | <code class="computeroutput">hardcode-dll-paths</code> feature for that reason.) However, you |
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| 72 | can use the <code class="computeroutput">dll-path</code> feature to add explicit paths |
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| 73 | manually. For example: |
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| 74 | </p> |
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| 75 | <pre class="programlisting"> |
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| 76 | stage installed : application : <dll-path>/usr/lib/snake |
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| 77 | <location>/usr/bin ; |
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| 78 | </pre> |
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| 79 | <p> |
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| 80 | will allow the application to find libraries placed to |
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| 81 | <code class="filename">/usr/lib/snake</code>. |
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| 82 | </p> |
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| 83 | <p>If you install libraries to a nonstandard location and add an |
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| 84 | explicit path, you get more control over libraries which will be used. A |
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| 85 | library of the same name in a system location will not be inadvertently |
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| 86 | used. If you install libraries to a system location and do not add any |
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| 87 | paths, the system administrator will have more control. Each library can |
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| 88 | be individually upgraded, and all applications will use the new library. |
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| 89 | </p> |
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| 90 | <p>Which approach is best depends on your situation. If the libraries |
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| 91 | are relatively standalone and can be used by third party applications, |
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| 92 | they should be installed in the system location. If you have lots of |
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| 93 | libraries which can be used only by your application, it makes sense to |
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| 94 | install it to a nonstandard directory and add an explicit path, like the |
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| 95 | example above shows. Please also note that guidelines for different |
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| 96 | systems differ in this respect. The Debian guidelines prohibit any |
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| 97 | additional search paths, and Solaris guidelines suggest that they should |
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| 98 | always be used. |
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| 99 | </p> |
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