Re: [boost] Build system/GCC questions

From: Greg Chicares (chicares_at_[hidden])
Date: 2001-06-05 00:09:59

David Abrahams wrote:
> 
> I am trying to understand how best to support shared libraries
> in the build system. Several people have mentioned to me that
> under GCC, DLL source files should be compiled with -fPIC.

Not on mingw, where a DLL is built this way:

ftp://ftp.nanotech.wisc.edu/pub/khan/gnu-win32/mingw32/snapshots/gcc-2.95.2-1/README

    Building a DLL: 
      $ gcc -c foo.c
      $ gcc -shared -Wl,--implib,libfoo.a -o foo.dll foo.o

I just seached the mingw mailing list archives and found
no evidence of anyone using -fPIC.

According to the gcc online manual

  http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_2.html#SEC2

  -shared
     Produce a shared object which can then be linked with
     other objects to form an executable. Not all systems
     support this option. You must also specify `-fpic' or
     `-fPIC' on some systems when you specify this option. 

  -fpic 

     Position-independent code requires special support,
     and therefore 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.

It appears that -fpic and -fPIC may be different, but I
can't say for sure. I've never experimented with this
so take what follows with a grain of salt--I'm just
reporting what I gleaned from a web search.

From the cygwin mailing list

  http://www.cygwin.com/ml/cygwin/1999-06/msg00329.html

  > To recap, DO NOT use -fPIC/pic with egcs-1.1 if you
  > don't want bad code.
  >
  That was the sollution! Thank you very much.

I found a patch, dated several months before that message,
that would have caused -fpic to be ignored, but I guess
it wasn't applied.

Also, I found a message on a BeOS list:

  http://www.bespecific.com/dialog/bedevtalk/archive/981102/00000061.htm

that suggests -fpic has a 10% speed penalty, although
the followups dispute it.