Mac OSX中的@executable_path, @load_path和@rpath的理解

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　　本文转载自：https://wincent.com/wiki/@executable_path,_@load_path_and_@rpath。个人觉得写的很不错，简洁明了。
Absolute paths
　　Useful for frameworks installed in shared locations. Example:

• Install path: /Library/Frameworks/Foo.framework/Versions/A/Foo
  

@executable_path
　　Useful for frameworks embedded inside applications, because it allows you to specify the location of the framework relative to the application's executable:

• Install path: @executable_path/../Frameworks/Foo.framework/Versions/A/Foo
  
• Application location: /Applications/Foo.app
  
• Executable path: /Applications/Foo.app/Contents/MacOS
  
• Framework location: /Applications/Foo.app/Contents/Frameworks/Foo.framework
  
• Linker puts all this together to figure out that the framework binary can be found at:/Applications/Foo.app/Contents/MacOS/../Frameworks/Foo.framework/Versions/A/Foo
  

@loader_path
　　Available from Mac OS X 10.4 Tiger onwards; useful for frameworks embedded inside plug-ins, because it allows you to specify the location of the framework relative to the plug-in's code (remember, plug-ins may not actually know where they are going to be installed, relative to the application, so knowing @executable_path doesn't help us in this case):

• Install path: @loader_path/../Frameworks/Foo.framework/Versions/A/Foo
  
• Application location: /Applications/Foo.app
  
• Plug-in location: /Library/Application Support/Foo/Plug-Ins/Bar.bundle
  
• Executable path: /Applications/Foo.app/Contents/MacOS
  
• Loader path: /Library/Application Support/Foo/Plug-Ins/Bar.bundle/Contents/MacOS
  
• Framework location: /Library/Application Support/Foo/Plug-Ins/Bar.bundle/Contents/Frameworks/Foo.framework
  
• Linker puts all this together to figure out that the framework binary can be found at:/Library/Application Support/Foo/Plug-Ins/Bar.bundle/Contents/MacOS/../Frameworks/Foo.framework/Versions/A/Foo
  

　　Note that if the "loader" is an application rather than a plug-in, the @loader_path ends up being equivalent to @executable_path.
@rpath
　　New in Mac OS X 10.5 Leopard is @rpath. Key points:

• @rpath instructs the dynamic linker to search a list of paths in order to locate the framework
  
• critically, this list is embedded in the loading application
  
• this means that a single framework with @rpath/Foo.framework/Versions/A/Foo can be made to work in a number of different ways; that is, you are effectively no longer limited by the choice of specifying your "install path" using either @executable_path or @loader_path
  
• the down side: you now have to pass additional linker flags when building the host application (eg. -rpath @executable_path/../Frameworks or/Library/Frameworks; note that specifying both will cause the dynamic linker to try looking in both locations)
  

Sources

• Nice overview: http://www.mikeash.com/pyblog/friday-qa-2009-11-06-linking-and-install-names.html
  
• Docs for using PFiddlesoft Frameworks (I've never used these, but the manual itself makes some nice general points about using frameworks): http://pfiddlesoft.com/frameworks/downloads/Assistive_Application_Programming_Guide.pdf