All,
A message from Ben Darnell a couple of weeks ago
prompted me look up callbacks in Rodes' & McKeehan's
book. It says:
"The GCC compiler's calling conventions differ
from those in the Palm OS. In particular, the
GCC compiler expects at startup that it can set
up the A4 register (which it uses to access
global variables) and that it will remain set
throughout the life of the application.
Unfortunately, this is not true when a GCC
application calls a Palm OS routine that either
directly or indirectly calls back to a GCC
function (a callback).
"The most common example of this occurrence is
when we've installed an event handler for a form
with FrmSetEventHandler. Once we've done that,
a call to FrmDispatchEvent (a Palm OS routine)
can call our form's event handler (a GCC function,
if we've compiled our application with GCC).
At this point, if our event handler tries to
access global variables, it'll cause a spectacular
application crash."
and then goes on to talk about the CALLBACK_PROLOGUE and
CALLBACK_EPILOGUE macros.
Before discovering this, however, I wrote three non-
trivial applications using GCC but without these macros.
They each access global and static variables, either
directly in the event-handlers or in other functions
these call, but they seem to survive testing (both real
and POSE/gremlin) and field use under OS3.0 and 3.3
without crashing, spectacularly or otherwise.
Have now inserted the macros (which at least don't break
anything) but would be interested if anyone can confirm
that they really are necessary and, if so, in what
circumstances.
Regards, Nick
-----Original Message-----
From: Ben Darnell [mailto:[EMAIL PROTECTED]]
Sent: Saturday, 18 December 1999 4:08
To: [EMAIL PROTECTED]
Subject: Re: GCC/Callbacks
On Thu, Dec 16, 1999 at 04:35:07PM -0500, Thomas Ward wrote:
> Hi All,
>
> I have a routine that in some places in my code I call directly, and in
> other places I call via a callback. Is there any reason why that shouldn't
> work? Since I am using GCC, do I need to put CALLBACK_PROLOGUE and
> CALLBACK_EPILOGUE in such a routine? Can it cause a problem to have
> CALLBACK_PROLOGUE and CALLBACK_EPILOGUE where they are NOT needed?
Functions called by your code do not need CALLBACK_{PRO,EPI}LOGUE,
although there is no problem with having them unnecessarily (as long as
they are properly balanced - e.g. you must have a CALLBACK_EPILOGUE
before every return). Functions called by PalmOS (e.g. form event
handlers, list draw functions) must have CALLBACK_{PRO,EPI}LOGUE if they
access global variables.
-Ben
--
Ben Darnell [EMAIL PROTECTED]
http://thoughtstream.org
