Question

I was recently building a certain shared library (ELF) targeting x86-64 architecture, like this:

g++ -o binary.so -shared --no-undefined ... -lfoo -lbar

This failed with the following error:

relocation R_X86_64_32 against `a local symbol’ can not be used when making a shared object; recompile with -fPIC

Of course, it means I need to rebuild it as position-independent code, so it’s suitable for linking into a shared library.

But this works perfectly well on x86 with exactly the same build arguments. So the question is, how is relocation on x86 different from x86-64 and why don’t I need to compile with -fPIC on the former?

Answer 1

I have found a nice and detailed explanation, which boils down to:

1. x86-64 uses IP-relative offset to load global data, x86-32 cannot, so it dereferences a global offset.
2. IP-relative offset does not work for shared libraries, because global symbols can be overridden, so x86-64 breaks down when not built with PIC.
3. If x86-64 built with PIC, the IP-relative offset dereference now yields a pointer to GOT entry, which is then dereferenced.
4. x86-32, however, already uses a dereference of a global offset, so it is turned into GOT entry directly.