[TUHS] History of symbol preemption

[Paul Winalski <paul.winalski@gmail.com>]

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.