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From: Rico Pajarola <rp@servium.ch>
To: Paul Winalski <paul.winalski@gmail.com>
Cc: The Eunuchs Hysterical Society <tuhs@tuhs.org>
Subject: Re: [TUHS] History of symbol preemption
Date: Mon, 13 Jan 2020 12:46:52 -0800	[thread overview]
Message-ID: <CACwAiQ=QTRc6OJrGDQ_Jw50sAPUCaTqQq3mJVLJ3GN0K4U=YBw@mail.gmail.com> (raw)
In-Reply-To: <CABH=_VTOhJ57t-iWkcV9KYoSmDjy-6-6-sUTVYz-nJDTrprHjw@mail.gmail.com>

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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@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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  reply	other threads:[~2020-01-13 20:47 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-13 19:58 Paul Winalski
2020-01-13 20:46 ` Rico Pajarola [this message]
2020-01-13 21:04   ` Clem Cole
2020-01-13 21:40     ` Rico Pajarola
2020-01-13 21:44       ` Paul Winalski
2020-01-13 21:45         ` Rico Pajarola
2020-01-13 22:20           ` Larry McVoy
2020-01-13 21:42     ` Paul Winalski
2020-01-13 22:53       ` Henry Bent
2020-01-14  0:31         ` Clem Cole
2020-01-14  2:53 ` Rob Gingell
2020-01-14 19:21 ` [TUHS] two AIX items [was " Charles H Sauer
2020-01-14 20:31   ` Clem Cole
2020-01-14 23:22   ` Kevin Bowling
2020-01-15 16:41   ` Paul Winalski

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