Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!cs.utexas.edu!pp!cadillac!beihl
From: beihl@cadillac.CAD.MCC.COM (Gary Beihl)
Newsgroups: comp.sys.apollo
Subject: Dynamic loading summarized
Message-ID: <685@cadillac.CAD.MCC.COM>
Date: 3 Jan 89 15:20:46 GMT
Organization: MCC VLSI-CAD Program, Austin, TX
Lines: 40

Thanks to those who responded to my recent inquiry regarding
dynamic loading.  What follows is a summary of what I've
learned.  It is indeed possible to dynamically load .o files
under the Apollo version of BSD Unix (SR10.0).  There are a
few gotchas however, which keep us from using it in our
application.

First, the .o file to be loaded has to be compiled with the
"-pic" (position-independent code) option.  This can be done
under BSD by specifying -W0,-pic to /bin/cc.  I believe some
performance penalty is paid when using this option due to 
an extra level of indirection.  Second, use the loader_$load()
call to access the system loader.  With the proper setting
of options to this function, one can lookup symbols and execute
functions in the new code.

The rub comes when the new object tries to reference symbols 
in the original executable.  You can't (by default).  It was
suggested to me that I redefine main() to be a stub which 
dynamically loads the entire program, thus making all symbols
known.  Unfortunately, this means everything must be compiled
with -W0,-pic. The loader kept telling me there was some part
of my main program requiring linking (I assume this meant not
compiled with -pic).  The `file' command does not distinguish
-pic from normal code.  Perhaps some of the system libraries
were being complained about.  I *did* compile all the application
code with -pic.

It seems to me that a better solution to this problem is to
have an incremental option on the linker.  The BSD interface
does not provide one.  Perhaps it's possible under AEGIS. I don't
think so.  Incremental linking eliminates the need for position
independent code, since the code can be custom-relocated to
the virtual address specified by the loading application. 




-- 
Gary Beihl  (beihl@mcc.com)