Date: Thu, 30 Apr 87 14:57:09 EDT
From: Michael Fischer <fischer-michael@YALE.ARPA>
Subject: SET_ENV.C  (SET_ENV, _SHELL_P, MSH, and GEMBOOT)
To: info-atari16@score.stanford.edu
Comment: Extracted from Digest 87-198

Here is the little program I promised that sets *_SHELL_P and gets
around the problem of MSH not being able to find its profile when
using GEMBOOT.

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/* SET_ENV.C

   Purpose:  Rewrites the system path string.

   Usage:  set_env [username]

   Description:

   Writes the string "USER=<username>\0PATH=\0\0" into the string
   buffer pointed to by _shell_p, where <username> is the optional
   command line argument to set_env.  If <username> is omitted,
   then the string "PATH=\0\0" is written instead.

   The Mark Williams shell msh.prg (V2.0) uses this string as its
   initial environment, so environment variables USER and PATH will
   be defined when msh starts up (or just PATH if run with no
   argument).  The profile can then modify its action based on the
   contents of USER.  For example, if USER=MIKE, the profile line
        C:\${USER}.MSH
   will cause msh to run the shell script in C:\MIKE.MSH.

   Set_env is particularly useful in conjunction with gemboot.prg,
   Konrad Hahn's program that solves the 40 folder problem.  By
   eliminating the semicolons that gemboot leaves in the system
   path string, it fixes the problem with msh not being able to
   find its profile file.  It should be placed in the .BAT startup
   file invoked by gemboot.  For example, putting
        SET_ENV MIKE
   in the gemboot startup file MIKE.BAT allows msh to subsequently
   customize itself according to MIKE's preferences.

   Caution:  set_env does not modify the pointer _shell_p but instead
   writes into memory pointed to by _shell_p.  If insufficient memory
   has been allocated to contain the new string, set_env will simply
   overwrite whatever lies beyond, possibly crashing the system
   or causing other problems.  Gemboot seems to allocate a buffer
   of 80 or more bytes, and I have not noticed any problems using
   set_env with gemboot, but other programs that set _shell_p might
   not be so generous.

   Author:   Michael J. Fischer
   Address:  Arpanet:  fischer@yale.arpa
             Bitnet:   fischer@yale-cs

   Distribution:
   THIS PROGRAM IS PLACED IN THE PUBLIC DOMAIN WITHOUT WARRANTY
   OF ANY KIND.  IT MAY BE FREELY COPIED, MODIFIED, OR REDISTRIBUTED
   AT WILL.
*/

#include <stdio.h>

#define _shell_p 0x4f6L        /* system path variable */
long peekl();

main(argc, argv)
int argc;
char *argv[];
{
   char *s = (char *) peekl(_shell_p);

   if (argc >= 2) {
       strcpy(s, "USER=");
       strcat(s, argv[1]);
       s += strlen(argv[1]) + 6;
   }
   strcpy(s, "PATH=");
   s += 6;
   *s = '\0';
}
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--Mike Fischer
      Arpanet:  fischer@yale.arpa
      Bitnet:   fischer@yale-cs