linux g++ linking 64 bit shared library code to static libraries

Question

Context: I can create a shared object library which is linked to a static library without any problems on 32bit linux. When I attempt the same build on 64bit linux, I see this linker error:

• relocation R_X86_64_32S against `a local symbol' can not be used when making a shared object; recompile with -fPIC

This error is quite common on the web. The solution is to compile the statically linked library with position independent code (-fPIC).

What I do not understand is why this is not required for the 32bit build. Can anyone help out?

Answer 1

Hi -

"Position Independent Code" is always required if your object module will be used in a shared library. It's highly platform dependent, and it incurs some overhead.

The reason you have to specify it explicitly on amd64, but not x386, is simply that it happens to be the default for x86, but not amd64.

Note, too, the difference between "-fpic" and "-fPIC":

-fpic
    Generate position-independent code (PIC) suitable for use in a 
    shared library, if supported for the target machine. Such code 
    accesses all constant addresses through a global offset table 
    (GOT). The dynamic loader resolves the GOT entries when the pro-
    gram starts (the dynamic loader is not part of GCC; it is part
    of the operating system). If the GOT size for the linked execu-
    table exceeds a machine-specific maximum size, you get an error 
    message from the linker indicating that -fpic does not work; in 
    that case, recompile with -fPIC instead. (These maximums are 8k 
    on the SPARC and 32k on the m68k and RS/6000. The 386 has no 
    such limit.)

    Position-independent code requires special support, and there
    fore works only on certain machines. For the 386, GCC supports 
    PIC for System V but not for the Sun 386i. Code generated for 
    the IBM RS/6000 is always position-independent.

    When this flag is set, the macros __pic__ and __PIC__ are defined to 1.

-fPIC
    If supported for the target machine, emit position-independent 
    code, suitable for dynamic linking and avoiding any limit on the 
    size of the global offset table. This option makes a difference 
    on the m68k, PowerPC and SPARC.

    Position-independent code requires special support, and therefore 
    works only on certain machines.

    When this flag is set, the macros __pic__ and __PIC__ are defined to 2. 

Answer 2

Ok the answer is described in detail here: http://www.technovelty.org/code/c/amd64-pic.html.

The basic gist of the explanation is that the i386 architecture implicitly dereferences the frame pointer for each function (explained on the last paragraph of the linked page). This process incurs some extra overhead so in the new 64-bit architectures, this dereferencing overhead was eliminated as an optimization.

The consequence of this optimization from a linking perspective was that unless 64-bit code is explicitly compiled as position independent code, it will produce code that is hard-coded with offsets for its execution context.

This is an imperfect explanation of the content in the linked page but it suffices for my purposes.