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* [TUHS] History of symbol preemption
@ 2020-01-13 19:58 Paul Winalski
  2020-01-13 20:46 ` Rico Pajarola
                   ` (2 more replies)
  0 siblings, 3 replies; 15+ messages in thread
From: Paul Winalski @ 2020-01-13 19:58 UTC (permalink / raw)
  To: The Eunuchs Hysterical Society

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.

^ permalink raw reply	[flat|nested] 15+ messages in thread

end of thread, other threads:[~2020-01-15 16:41 UTC | newest]

Thread overview: 15+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-01-13 19:58 [TUHS] History of symbol preemption Paul Winalski
2020-01-13 20:46 ` Rico Pajarola
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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