Undefined external symbol in shared library

I recently ran nm -m -p -g on the System.B.dylib library from the iOS SDK4.3 and was surprised to find a lot of symbols marked (undefined) (external). Why and when would an undefined symbol be marked external? I can understand a undefined external symbol marked lazy or weak but these aren't. Many of the pthread_xxx functions fal开发者_JAVA百科l in this category. When I link with this library however, all symbols are resolved. The pthread_xxx symbols are defined in one of the libraries in the \usr\lib\system folder so I am assume they are satisfied from there. How does that work during linking?

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It's been a while since I was an nm and ld C-coding ninja, but I think this only means that there are other libraries this one links against.

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Usually this is how dynamic linking works. If you were to nm a static archive of System.B, you would not have observed this behavior. The System.B.dylib on it's own would not do much; unless you make it as part of an ensemble set of dynamic and static libraries whose functions it makes use of. If you now try to compile your final binary BUT omit the library path '/usr/lib/system' then you linker will cry foul and exit with an error telling you that it cannot find a reference to pthread_XXX() (using your above example). During the final assembling of the binary, it needs to make sure it knows the location of each and every function used.

HTH