Dynamic linking

[Guy Harris]

>	[asks for the possibility of dynamically linking functions
>	 into a running program under SunOS 4.1.1]
>
>You should get Wilson Ho's Dld, the dynamic link/unlink editor.

He can also just use the routines described in the DLOPEN(3X) manual
page if he's using SunOS 4.1[.x]; those routines are also in System V
Release 4. 

> 	Restrictions: 
> 	------------
> 
> 	  - I want the users to compile and link their functions with all
> 	    the necessary librairies (so they will be no unresolved external
> 	    when I'll dynamically link them).

If any of the libraries with which the user-defined module would be
linked are shared libraries with which the interpreter was linked,
that's not necessary; the dynamically-loaded code will get bound to the
libraries in question when it's loaded.  Otherwise, you'll have to link
the dynamically-loaded code with the library (with the disadvantage that
doing so will probably introduce non-position-independent code; that may
mean that two processes using the same dynamically-loaded code won't
share it all, if that matters, and may tickle some dynamic loader bugs).

There's also a bug in the 4.1[.x] dynamic loader (which I'm told is
absent from S5R4) such that it can't handle dynamically-loaded code if
it has any BSS.  A workaround is to:

	1) open "/dev/zero";

	2) dup that file descriptor;

	3) close the original file descriptor, leaving the dup'ed one
	   open;

as the first thing you do in "main()" (basically, the dynamic loader
assumes "/dev/zero" is still open from when it was used to load the
shared libraries; the above sequence re-opens it to what should be the
same file descriptor number).

A potential win of the "dlopen()" routines is portability; this doesn't
matter for SunOS 4.1[.x], as "dld" is supported on all the platforms on
which 4.1[.x] is supported, but may be a win for S5R4 - the dynamic
loader has to be ported by the vendor, so you don't have to port it
yourself to some new platform.  (I.e., it should be in S5R4 for e.g. MIPS
or the 88K; none of the platforms in the list of "dld" platforms have
those chips.)