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From: kdarling@hobbes.catt.ncsu.edu (Kevin Darling)
Newsgroups: comp.os.os9
Subject: Re: Can't execute "J" - Error #000: 216
Message-ID: <1991Jun18.235439.6486@ncsu.edu>
Date: 18 Jun 91 23:54:39 GMT
References: <1991Jun14.202821.20308@unhd.unh.edu> <1991Jun15.001249.5963@ncsu.edu> <1991Jun18.033225.12476@mmm.serc.3m.com> <1991Jun18.184533.1648@urz.unibas.ch>
Sender: news@ncsu.edu (USENET News System)
Organization: North Carolina State University
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balmer@urz.unibas.ch writes:
>Someone proposed to write a program called "J" which would be executed in
>this case. The program J will actually execute, the problem is  that the
>shell tries to execute the next bytes of the binary file. You then get an
>error message like "Error 216: can't execute verflow".

Good point.  The shell will execute "J", and then try to use the next bytes
(after the max readln point, or after a $0D seen as readln delimiter) in the
originally named binary file (which shell thought was a script file)
as a new script line.

This brings up a neat trick with shell scripts tho.  The thing to remember
is that whenever a script line is executed, the standard input of the called
program (assuming no "<" redirection is done) is the file itself... which
in this case is the binary module!

Thus the program called "J" has a standard input conduit to the binary file.
Now all it has to do is to Seek its standard input path (0) to the end of
the binary, and voila... when J ends, its parent shell thinks the "script"
is at the end of file and exits.  In BASIC, such a program would be...

PROCEDURE J
DIM modsize:INTEGER
SEEK #0,4 \ (* seek to module size location in header
GET  #0,modsize \ (* read in module size
SEEK #0,modsize-1 \ (* seek to end of module/"file"

PRINT "You are trying to execute a module"
PRINT "from within your current data dir "
END

The J program could even look up the name of the target binary, and
perhaps load and/or execute it.

This standard input trick also allows you to write commands which loop
and do other "goto"-ish moves within a script file... by doing a Seek
to wherever you'd like your calling shell to continue from.  
The simplest example is a program which repeats the script file:

PROCEDURE rewind
SEEK #0,0
END

Put that at the end of your script, and it'll execute over and over.
  best - kevin <kdarling@catt.ncsu.edu>