[vtkusers] compilation with -fPIC necessary?

[Obada Mahdi]

Hi Sebastiaan,

I'm a bit confused now concerning where which flag is missing :)

Does compiling shared VTK libraries on the EMT64 really lack a "-fPIC",
or is it just the "libmpich.a" thing (which obviously was compiled
without "-fPIC"), or both?

On Thu, 16 Mar 2006, Sebastiaan wrote:
> Recompiling with -fPIC does not work: I get the same error. Recompiling mpich 
> with -fPIC and then VTK with -fPIC seems to compiles clean (but I do not know 
> if VTK is build correctly).

Since the linker seems to complain about "libmpich.a" only, it seems
that "-fPIC" was already present in the first place, and I suppose that
"libmpich.a" is the only problem.

On Fri, 17 Mar 2006, Sebastiaan wrote:
> I have also read about -fPIC is almost always needed on 64 bit (don't know 
> why it is not needed on 32 bit). I also read that -fPIC can break packages. 
> Since most of VTK compiles clean without -fPIC, it seems overkill to compile 
> all of VTK with this flag.

VTK itself does not require PIC, building real shared libraries does.
It is normal to use "-fPIC" when building shared libraries.

Code compiled with "-fPIC" uses a different calling convention, which,
for example, can break some inline assembly constructs.  Linking a
PIC-compiled static library to a non-PIC application can also cause
problems (your PIC-version of "libmpich.a" could break other programs
that will be linked against it).  However, there is nothing to worry
about when building shared libraries; after all, they are meant to be
place-independent (that is what makes them shareable).

> Why isn't this flag automatically added to the compiler for those shared
> libraries, if it is compiled on a 32 bit system?

I was not aware that this is possible, thanks for pointing that out.
Actually, it should be added.  From what I have just read, it seems that
Linux's IA32 dynamic linker supports not place-independent.  However,
even in that case, such libraries can be dynamically loaded, but not be
shared among different processes.


I am very sorry if I am just causing more confusion -- hope you get it
to work, though.


Regards

Obada