Re: [TUHS] History of symbol preemption

@ 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(a)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(a)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. >> >