libraries may hide porting problems

[wsmith at m.cs.uiuc.edu]

This problem with the BSD linker can cause portability problems for
programs that use programmer created libraries.

Description:  
	In the BSD C compiler may not complain of multiply defined functions
	while in other C compilers will fail to link the same program.
	If the only data needed by the linker from a module is a variable
	in BSS, multiply defined functions in that same module will be 
	ignored.

Repeat-By:  
	Create two files, one.c two.c

one.c:
	extern int a;
	main()
	{
		printf("%d\n",a);
	}
	four() { }
two.c:
	int a;
	four() { }

If the program is compiled with "cc -o aprog one.c two.c", _four is
reported as multiply defined.  However, if the program is compiled with 
"cc -c one.c two.c
 ar r alib.a two.o
 ranlib alib.a
 cc -o aprog one.o alib.a" no error will be reported (under BSD).  

With other compilers on other operating systems, _four will be multiply
defined in both cases.  However, if the first line of two.c is changed to
"int a = 7;", the BSD C compiler will report _four as multiply defined in 
both cases.  Linking with a library should be equivalent to linking the
individual modules in the library that are needed.  In this problematic
case, the equivalence is broken.

Fix: ???

Bill Smith	wsmith at cs.uiuc.edu		uiucdcs!wsmith