Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!ames!pasteur!ucbvax!decwrl!megatest!djones
From: djones@megatest.UUCP (Dave Jones)
Newsgroups: comp.lang.misc
Subject: Re: Call by string (was: B&D)
Message-ID: <1316@goofy.megatest.UUCP>
Date: 31 Jan 89 00:42:39 GMT
References: <403@rpi.edu>
Organization: Megatest Corporation, San Jose, Ca
Lines: 41

From article <403@rpi.edu>, by jefu@pawl.rpi.edu (Jeffrey Putnam):
... 
> I have been following this discussion for a while and just realized this
> morning that it is possible that everyone has been asking the wrong question.
> (Parenthetically, i worked out a scheme whereby a C program compiled with
> debugging stuff on could read its symbol table and then jump to the 
> address of the appropriate subroutine.  Cute.)  
> 


Under BSD4.2  and Sun3, it is not even necessary to compile with
debugging stuff.  Just don't strip out the symbol-table.


> The question is, "why do you want to do this?"

...


The reason might be that the procedure you want to jump to was
compiled after the main program was compiled, and was not, and could
not be, known to the main program at the time of its compilation.

I'll give you a real world example. Well, it's real world if you include
manufacturing and testing computer parts as "real world".

Let's suppose you have engineers all over the world who come up with
designs for ASIC parts. Each of these part-specifications generates
a procedure which tests the part. A program monitors loading chips
or wafers by robot, interacting with the operator, etc., and it keeps
timing calibration and other data concerning the tester itself. The
program has to be able to call any of these part-specific procedures
which had not even been written when the main program was written. But
putting each part-specific procedure in its own process would be 
prohibitively expensive without some mechanism for shared data: It would
take too long to initialize it with the calibration data, etc.,
and it would require too much memory, even with today's multimegabyte
rams.

So the program needs to read the part-specific procedure into memory,
and then branch to it.