Date: 18 May 87 22:54:38 GMT
From: imagen!atari!apratt@ucbvax.Berkeley.EDU  (Allan Pratt)
Subject: Testing _shell_p.  C source and some tutorial
Comment: Extracted from digest info-atari16 87-225
To: info-atari16@score.stanford.edu

> [Simon Poole:]
>
> OK, I admit I was probaly wrong, I did some more research into how GEMBOOT
> uses _shell_p: it seems that the only reason Konrad Hahn uses it, is so
> that SETPATH can be used to change the path, the AES actually uses
> $XXXX to save the pointer to the enviroment string (have a look
> at the code pointed to by exec_os ($4FE). One doesn't actually need _shell_p
> to set the path at boot-time, so I suppose Konrad could make a version without
> using this semi-documented location (maybe one day Atari will decide on
> for what it actually is :-)).
>
>                             Simon Poole
>                             K538915@CZHRZU1A.BITNET

Thank you, Simon.  Yes, you were wrong.  Whoever posted the note to the
effect that the ROMs don't contain the word $04f6 was right.  Whoever
posted the note to the effect that you can change the string pointed to
by $04f6 (thanks to GEMBOOT) was right: the pointer at $04f6 is surely
the SAME as some other pointer (e.g.  the one used by the Desktop for
the environment of its children).  Finally, addresses like the one
I X'ed out above should not be published, because they change from
ROM to ROM (even between countries for a given ROM release).

Atari (i.e.  Landon) decided long ago what _shell_p was for (see below),
just as I said in this group.  But even if we hadn't, it is not
appropriate for you (or anybody else) to usurp system variables for your
own use.  If you don't grok the description of a variable, consider it
"Reserved for future use BY ATARI" and then yell at us to fix the
documentation.

Please remove references to _shell_p as a path pointer from your code.
It is intended as a pointer to a shell routine for executing command
lines.  At the end of this article is an example of how we hoped this
would work, and how it does work in some shells (notably Gert
Poltiek's).

*********************

Now for the big news:  if you put all your resource files in the root
of your boot device (C: for hard disk users) the AES will find them
no matter where the .PRG is started from!  This is because the environment
the AES gets initially includes "PATH=;C:\;" and it looks in this path
when you do the shell_find call (which rsrc_load uses).  Watch this space
for a program which lets you put your resources in any directory!

*********************

/----------------------------------------------\
| Opinions expressed above do not necessarily  |  -- Allan Pratt, Atari Corp.
| reflect those of Atari Corp. or anyone else. |     ...lll-lcc!atari!apratt
\----------------------------------------------/


*************** CODE EXAMPLE FOR USE OF _shell_p VARIABLE *****************

/*
 * Example of use of _shell_p variable.
 *
 * The value at _shell_p is the address of a procedure which takes a
 * string pointer as an argument, executes the string as if it were
 * typed at the shell prompt, and returns the exit status of the command.
 *
 * This code checks to see of the value at _shell_p is null (no shell
 * around) or points to the word "PATH" (adulterated by GEMBOOT).
 */

#include <osbind.h>

main()
{
    long oldstack;
    long *shell_p = (long *)0x4f6;
    long (*shell)();
    long status;
    char buf[256];

    oldstack = Super(0L);    /* get into Super mode */
    shell = *shell_p;        /* get value of _shell_p */
    Super(oldstack);        /* get back to user mode */

    /* validate _shell_p */
    if (shell == 0L) {
    printf("No shell available to execute a command\n");
    Pterm(-1);
    }
    if (strncmp(shell,"PATH",4) == 0) {
    printf("_shell_p has been adulterated: it points to \"PATH\"\n");
    Pterm(-1);
    }

    /* get a line from the keyboard using Cconrs() */
    printf("Enter a command line for the shell to execute: ");
    buf[0] = 253;
    Cconrs(buf);
    buf[2+buf[1]] = '\0';        /* null-terminate the line */

    /* execute the command */
    status = (*shell)(&buf[2]);
    printf("Return status is %ld\n",status);
    Pterm0();