gcc's fvisibility at compile time or link time

I am trying to limit the ABI of a shared library using the gcc's fvisibility feature. However I am confused what is the correct way to do it.

My makefile organizes the build process in two stages. At the first step all .cpp files are built to object files using some gcc options. Then all the object files are linked together using another set of gcc and ld options. From what I have read fvisibility is relevant to the second step. However this contradicts with the results I observer. If I add fvisibility=hidden to the compile time options the result is as expected, nm -D reporting a much smaller set of exported symbols. On the contrary if I add it to the link time options it does not seem to affect the build.

While looking for an explanation I have compared the object files produced with and without fvisibility. The difference seems to be in the addresses of the symbols inside the object file. However I am not aware how that difference in addresses carries the me开发者_如何学Gossage to the linker so that it is able to hide the symbols in one of the cases and expose them in the other.

Could anyone please explain to me that. Thank you for your time.

Compile time, as the visibility is placed in the object (.o) files, and then used by the linker when creating the complete executable/shared object. When using it at link time, but not compile time, it will have no effect, as the visibility in the object files is still default. There's also no need to use it at link time at all I've found.

In the case of how the visibility is stored, the different symbols are probably in different segments, and they get their visibility from the options of the segment.

You may find http://gcc.gnu.org/wiki/Visibility to be helpful