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I have an executable with four shared libraries and the dependency tree look like this: Executable app does a dlopen of foo.so and bar.so. foo.so in turn links to fooHelper.so and bar.so links to barHelper.so.

Now, the issue is that fooHelper.so and barHelper.so have some of the same symbols. For instance, let us say we have a func with different implementations in fooHelper.so and barHelper.so. Is there a way to force foo.so to use fooHelper.so's implementation and bar.so to use barHelper.so's? What happens at present is that depending on the order of linking of the helpers, only one of the implementations of func is used by both foo.so and bar.so. This is because of the default Unix linkage model, if the definition of a symbol is already loaded, then any other definitions from shared libraries loaded subsequently are just discarded. Basically, func will be picked up from the helper library linked first. I need a way to explicitly specify the appropriate mapping without changing the source code of the shared libraries.

I'm working on Linux with g++ 4.4.

Rajkishan
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  • If you compile those libraries from source, you can look at http://stackoverflow.com/questions/6538501/linking-two-shared-libraries-with-some-of-the-same-symbols – ninjalj Dec 21 '12 at 09:29

2 Answers2

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Is there a way to force foo.so to use fooHelper.so's implementation and bar.so to use barHelper.so's?

Yes: that's what RTLD_LOCAL is for (when dlopening foo.so and bar.so).

RTLD_LOCAL
  This is the converse of RTLD_GLOBAL, and the default if neither flag
  is specified. Symbols defined in this library are not made available
  to resolve references in subsequently loaded libraries.
Employed Russian
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If both funcs happen to be in the same name-space, you're in a bit of trouble - if you are programming in C. The term to look for is "function overloading". There have been previous discussions on this topic, e.g. this one: function overloading in C

EDIT: http://litdream.blogspot.de/2007/03/dynamic-loading-using-dlopen-api-in-c.html

Community
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s-m-e
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  • Ah, should have made it explicit, I'm working with C++. Don't think overloading is relevant though, `func` has the same signature in both `fooHelper.so` and `barHelper.so`. I'll edit the question to make this clear. – Rajkishan Dec 05 '12 at 07:57