Re: difference between 32 bit and 64 bit .so

[Ian Lance Taylor <iant at google dot com>]

Alan Modra <amodra@gmail.com> writes:

>> I am wondering why this stops working on 64 bit machines?
>
> You'd sacrifice speed in executables if the compiler was forced to
> generate code that could be used in shared libs.

Another way to answer this is to observe that by default x86_64 uses the
small memory model.  This means that non-PIC code uses 32-bit PC
relative references for everything.  This means that in cases where the
main executable overrides a symbol defined in a non-PIC shared library,
the dynamic linker will only be able to resolve the reloc if the
executable and the shared library have addresses within 32 bits.  Since
the address space is 64 bits on x86_64, there is no way to guarantee
that.

I actually think that in general non-PIC shared libraries are useful,
because they shift the PIC performance penalty to startup.  That is a
poor choice in general, but a good choice for a long running program.
Unfortunately there is no way to make it work on x86_64.

Ian