C++ name mangling in a so

Question

Here's what i did:

I changed a .h file from

SomeObj* getCacheObj( int i = 0 );

to

SomeObj* getCacheObj( int i );
SomeObj* getCacheObj();

I recompiled the code (no problems), the changes went to somelib.so (one of many so files). I then replaced the old so on the equipment with this one and got the folowing error when loading the so: undefined symbol: _ZN13KeypathHelper11getCacheObjEv

Now the strange part is that I've been told this class is only used in this so file (How can I make sure?). I am not that experienced and not sure how to investigate. Any suggestions are welcome.

Update

This particular problem was caused because another so file was using the KeypathHelper class and I only replaced the one containing it. The way I found out which other so needed to be updated was by greping all so's for KeypathHelper.

Answer 1

The _ZN13KeypathHelper11getCacheObjEv symbol is a mangled name for KeypathHelper::getCacheObj() (you can easily translate using c++filt, for example). Given that you have only added a method and whatever is loading the shared object cannot find it makes me think that you either haven't updated the shared object or forgot to provide a definition for KeypathHelper::getCacheObj() (in other words — implement the method).

In order to investigate, you have to see what is failing to resolve the symbol. Usually, developers have a sense for it. Say, if a binary XXX cannot load library YYY due to unresolved symbol, then XXX is using it and it does not appear to be in YYY (or anywhere else for that matter). If there is no sense for that, one can resort to reading ld.so (8) manual page and debug the dynamic linker by using available means like defining LD_DEBUG.

Also, @PlasmaHH has asked a very good question. If the only change you made was to the header file, then you must know that a single function/method with a default value for a parameter is not the same as as two functions/methods where one has a parameter and one does not.

As for your second question about how to make sure that symbol in a shared object is not being used outside — you have to change the symbol visibility so that nobody from the outside is able to link/resolve/use the symbol. For example, see GCC Visibility.

Hope it helps. Good Luck!