How can I link with a directory which might not exist while running?

I'm writing a library for Linux which has some functionality which relies on the a shared library libfoo.so.

I'开发者_StackOverflow中文版m trying to case where libfoo.so doesn't exist into consideration. I have a well defined behavior for such a case, but I don't know how to properly implement it, linkage wise.

Currently, my library is shipped compiled against libfoo.so, when a client tries to compile it's code against it on an environment which doesn't include libfoo.so he gets linkage errors.

My question is, how can I build my library in such a way that it would compile even if libfoo.so doesn't exist, but behave differently. The only solution I was able to come up with myself is to branch from it a version which doesn't support foo, but there must be a better way...

Thanks in advance

A follow up due to the given answer: It seems that not linking against libfoo but rather dynamically loading it with dlopen solves the problem, but it requires me to manually export all the symbols, and is limited in scope... Is there any "less painful" to achieve that?

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There are two different problems that kind of fit the description, depending on whether the presence of the library is to be detected at compile time or at runtime.

At compile time you will have to use some tool to detect whether the library is present and modify the build scripts to pass that information down to the code (think defines).

At runtime, you can avoid linking against the library, and rather dynamically load it. The code should handle failures to locate/load the library and fall back to the alternative version.