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From: bairds@eecs.cs.pdx.edu (Shawn L. Baird)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Problem with SetFunction()
Keywords: AllocMem() SetPatch() 2.02
Message-ID: <1995@pdxgate.UUCP>
Date: 18 Mar 91 07:49:31 GMT
Article-I.D.: pdxgate.1995
References: <13531@darkstar.ucsc.edu>
Sender: news@pdxgate.UUCP
Lines: 60

davids@ucscf.UCSC.EDU (Dave Schreiber) writes:



>I've been trying to patch AllocMem() as an experiment, and haven't been
>able to get it to work.  I manage to patch it correctly with SetFunction(),
>but when I try to print a carriage return using printf, the machine crashes
>with a 8000 0006 error (CHK instruction).  Here's the code:

[ stuff deleted ]

>   Forbid();      /*Stop multitasking*/

>             /*Patch AllocMem()*/
>   oldAllocMem=SetFunction(ExecBase,NEG_AM_OFFSET,newAllocMem);

>   temp=(APTR)AllocMem(1000,0L); /*Goes to my routine & allocates*/
>                                 /*the memory without a problem*/

>   printf("%x ",temp);  /*Crashes here if there's a \n in that line*/
>		  /*Otherwise, it just continues on fine*/

>	       /*Undo patch*/
>   printf("%x\n",SetFunction(ExecBase,NEG_AM_OFFSET,oldAllocMem));

>   Permit();

[ more stuff deleted ]

Here's my $0.02. Since you've stopped multi-tasking the console device,
which is another task, cannot run. Thus the crash. Since stdout doesn't
flush until it gets a newline... Try this instead:

   Forbid();      /*Stop multitasking*/

             /*Patch AllocMem()*/
   oldAllocMem=SetFunction(ExecBase,NEG_AM_OFFSET,newAllocMem);

   Permit();

   temp=(APTR)AllocMem(1000,0L); /*Goes to my routine & allocates*/
                                 /*the memory without a problem*/

   printf("%x\n",temp);  /*Crashes here if there's a \n in that line*/
		  /*Otherwise, it just continues on fine*/

   Forbid();

	       /*Undo patch*/
   tmpfunc=SetFunction(ExecBase,NEG_AM_OFFSET,oldAllocMem));

   Permit();

   printf("%x\n",tmpfunc); /* print out the address of the old routine */
   /* if tmpfunc != newAllocMem then somebody else probably also did a
      SetFunction() while we were running I guess */

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