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From: davids@ucscf.UCSC.EDU (Dave Schreiber)
Newsgroups: comp.sys.amiga.programmer
Subject: Problem with SetFunction()
Keywords: AllocMem() SetPatch() 2.02
Message-ID: <13531@darkstar.ucsc.edu>
Date: 18 Mar 91 03:45:31 GMT
Sender: usenet@darkstar.ucsc.edu
Organization: University of California, Santa Cruz
Lines: 64



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:

#include <exec/types.h>
#include <exec/exec.h>
#include <clib/exec_protos.h>

struct ExecBase *ExecBase;

APTR __regargs (*oldAllocMem)();
APTR __regargs newAllocMem(ULONG size,ULONG types);

#define NEG_AM_OFFSET 0xff3a

main()
{
   APTR temp;

   ExecBase=(struct ExecBase *)OpenLibrary("exec.library",0L);

   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();

   if(temp!=0L)
      FreeMem(temp,1000);  /*Again, doesn't crash*/

   CloseLibrary(ExecBase); /*Exits without a hitch*/
}

APTR __regargs newAllocMem(ULONG size,ULONG types)
{
   return(oldAllocMem(size,types));
}

If I change newAllocMem() so that it just returns NULL, the call to
AllocMem() in main() returns NULL; i.e. I know that calls to AllocMem() are
being routed to my module.  newAllocMem() as it is above also works without
a hitch.  But when I printf() an '\n' (or when I Permit() without undoing the
patch), it crashes.  Any suggestions?  BTW, I'm using a 3000 under 2.02
(under 1.3 it crashes as well, but it gives a 8000 0003 error).

Thanks in advance.

-- 
Dave Schreiber                               E-mail:  davids@ucscf.ucsc.edu
"It was fun learning about logic, but I don't see where or when I will ever
use it again."                               Disclaimer: