[TUHS] History of symbol preemption

[Clem Cole]

@ Rico I'm failing sure ELF came from AT&T Summit, not Sun.
@ Steve Johnson were you the manager when was created or were you folks
still using COFF?

Anyway... There were issues with COFF WRT being
architecture-independent and supporting dynamic loading well.  Steve Rago
would also be a good person to ask if you want some of the details.  At one
point there was a COFF2 document, but it may have been System Vx licenses
only.   Also, one of the issues was that AT&T had officially tied up COFF
as a proprietary format -- all part of the 'consider it standard' trying to
force their lunch down all the other UNIX systems throat which was not
having it.   As a result, CMU's MachO was about to become the default
format (OSF and Apple were already using it for that reason), and Unix
International stepped in and convinced AT&T to released the ELF documents
(I was on the UI technical board at that point).  I'm not sure how/why OSF
decided to back off, maybe because after ELF became public it got supported
by GCC.

Now my memory is a little hazy... I think OSF/1-386 used MachO originally,
but I've forgotten.   Switching the kernel to use ELF was one of the
differences between OSF1 and Tru64 IIRC.

On Mon, Jan 13, 2020 at 3:47 PM Rico Pajarola <rp at servium.ch> wrote:

> This seems to have originated with SunOS 4. I believe a good proxy for
> finding anything that inherited from or was inspired by this is a linker
> that recognizes LD_PRELOAD. I wonder if there are other independent
> implementations in the Unix space.
>
>
> On Mon, Jan 13, 2020 at 11:59 AM Paul Winalski <paul.winalski at gmail.com>
> wrote:
>
>> The Executable and Linkable Format (ELF) is the modern standard for
>> object files in Unix and Unix-like OSes (e.g., Linux), and even for
>> OpenVMS.  LInux, AIX and probably other implementations of ELF have a
>> feature in the runtime loader called symbol preemption.  When loading
>> a shared library, the runtime loader examines the library's symbol
>> table.  If there is a global symbol with default visibility, and a
>> value for that symbol has already been loaded, all references to the
>> symbol in the library being loaded are rebound to the existing
>> definition.  The existing value thus preempts the definition in the
>> library.
>>
>> I'm curious about the history of symbol preemption.  It does not exist
>> in other implementations of shared libraries, such as IBM OS/370 and
>> its descendants, OpenVMS, and Microsoft Windows NT.  ELF apparently
>> was designed in the mid-1990s.  I have found a copy of the System V
>> Application Binary Interface from April 2001 that describes symbol
>> preemption in the section on the ELF symbol table.
>>
>> When was symbol preemption when loading shared objects first
>> implemented in Unix?  Are there versions of Unix that don't do symbol
>> preemption?
>>
>> -Paul W.
>>
>
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