GNU-ld behaviour does not match native linker behaviour

[Nick Clifton <nickc at cygnus dot com>]

Hi Guys,

  It has been brought to my attention that GNU-ld does not handle
  commons in libraries in the same way as SUN's or HP's linkers handle
  them.   Here is an example:

GNU-ld  (eg Linux)

   % cat main.c
   int common_sym;
   int main (void) { return printf ("%d\n\n", common_sym); }
 
   % gcc main.c
   % a.out
   0
 
   % cat define.c
   int common_sym = 3;
   int foo (void) { return 0; }
 
   % gcc -c define.c
   % ar r libtest.a define.o

   % gcc main.c define.o
   % a.out
   3
 
   % gcc main.c libtest.a
   % a.out
   0
 
   % gcc -Wl,-ufoo main.c libtest.a
   % a.out
   3

Note how linking with a library which contains a defintion of the
contents of a common symbol has no effect unless there is some other
reason to link in the archive element which contains the common
defintion.

However on Solaris 7 things are slightly different:

  % /opt/SUNWspro/bin/cc main.c
  % a.out
  0

  % /opt/SUNWspro/bin/cc -c define.c 
  % /usr/ccs/bin/ar r libtest.a define.o

  % /opt/SUNWspro/bin/cc main.c define.o
  % a.out
  3

  % /opt/SUNWspro/bin/cc main.c libtest.a
  % a.out
  3

  % /opt/SUNWspro/bin/cc -Wl,-ufoo main.c libtest.a
  % a.out
  3

So now the common symbol gets defined from the library even if there
is no other reason to link in that particular library element.  The
same thing happens using HP-UX 11.0's native linker.

So - which linker is doing the right thing ?

I have a patch to change the behaviour of GNU-ld, although it is
rather cumbersome, but I do not know if I should even submit it, since
I am not at all sure that GNU-ld is doing the wrong thing.

Note - this handling of commons is particularly important when linking
together FORTRAN libraries, since the definition of the contents of a
common symbol can often be found in a seperate object file to the
defintion of the use of the common symbol.

Cheers
	Nick