Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!asuvax!mcdphx!udc!willcox
From: willcox@urbana.mcd.mot.com (David A Willcox)
Newsgroups: comp.sys.mac.hypercard
Subject: Re: XCMD's cannot set global variables.
Message-ID: <1269@urbana.mcd.mot.com>
Date: 31 May 90 13:53:12 GMT
References: <3707@darkstar.ucsc.edu> <1266@urbana.mcd.mot.com> <3821@darkstar.ucsc.edu>
Organization: Motorola Microcomputer Division, Urbana, IL
Lines: 78

>>In article <3707@darkstar.ucsc.edu> I write:
>>>I believe that the book "XCMD's for HyperCard" by Gary Bond mentions that
>>>XCMDs cannot create global variables.
>>
>>Am I missing something?  I don't have any documentation available here
>>at work, but I could swear that there are a pair of callbacks called
>>something like GetGlobal() and SetGlobal() that let you get and set
>>globals from an XCMD.  I've used them, and they work.

>Yes, GetGlobal and SetGlobal exist and work fine, but in my experience,
>strange things happen if the global variables do not exist before the
>XCMD tries to Get or Set them.  (Strange things == the next XCMD that
>gets the variable finds it empty.)

>The best solution is to declare "Global foo" in some script that is always
>executed before the XCMD is invoked.

I couldn't believe this since I've used SetGlobal() and GetGlobal() in
XCMDs without "global foo" in any script.  I went home and tried it, and
found that you are absolutely wrong - the "global foo" is not required. 
Source for my very simple minded example in Think C 3.?  is below. 

BUT THEN...  As a last step in checking this out, I tried my example
on the old klunker machine at work, and found that it didn't work -
the "global foo" was necessary in the script.  The difference appears
to be that the MAC at work still has HC version 1.0.1, while I have
version 1.2.something.

My conclusion - If you have a "modern" version of HyperCard, then you
can create globals from within an XCMD/XFCN; the "global" in your
script is not necessary.  If you have an old version of HyperCard, the
"global" is necessary.  Sounds to me like a bug that has been fixed.

David A. Willcox		"Just say 'NO' to universal drug testing"
Motorola MCD - Urbana		UUCP: ...!uiucuxc!udc!willcox
1101 E. University Ave.		INET: willcox@urbana.mcd.mot.com
Urbana, IL 61801		FONE: 217-384-8534

Sample XCMD:

/*
 * Invoke this as "GetSetGlobal arg".  It remembers arg, and
 * returns the previous value of arg.
 */

#include <MacTypes.h>
#include <HyperXCmd.h>
#include <SetUpA4.h>

#define nil 0L

#define GLOB_NAME "\pDummyGlobal"

pascal void main (XCmdBlock *);

Handle CopyHand(h)
Handle *h;
{
    long len;
    Handle ret;
    
    len = strlen (*h) + 1;
    ret = NewHandle (len);
    strcpy (*ret, *h);	/* yeah, I should lock them ... */
    return (ret);
}

pascal void main (paramPtr)
XCmdBlock *paramPtr;
{
    RememberA0();
    SetUpA4();
    
    paramPtr->returnValue = GetGlobal (paramPtr, (StringPtr)GLOB_NAME);
    SetGlobal (paramPtr, (StringPtr)GLOB_NAME, CopyHand (paramPtr->params[0]));

    RestoreA4();
}