Q50514: Flip/Swap Setting Crucial to Debugging PM Apps with Children
Article: Q50514
Product(s): See article
Version(s): 2.30
Operating System(s): OS/2
Keyword(s): ENDUSER | | mspl13_basic
Last Modified: 10-NOV-1989
Using protected mode CodeView (CVP) to debug Presentation Manager (PM)
programs that spawn one or more child processes can be rather
difficult. CVP works correctly for debugging just the parent PM
process, but once you begin CodeView with the /O option to specify
that you also want to debug child processes, you may find that you
hang the computer quite regularly during your debugging session.
CodeView itself does not have a problem with PM programs even if they
do invoke child processes. It is only when you use the /O switch to
debug the children at the same time that you run into difficulty. This
is because of problems with CodeView conflicting with the OS/2 kernel
as you bring a process to the foreground for debugging, while a
system-dependent process like the PM parent is relegated to the
background where it can become blocked.
For example, if the parent gets to a point where it is waiting on a
message, but it is not in the foreground, there is no way to get the
focus back to this waiting process and you are essentially hung. The
parent cannot process the message because it must be in the foreground
to do so, and CodeView cannot continue until the message is processed,
so it just waits.
The key to debugging the parent and child processes of a PM program at
the same time is the setting of Flip/Swap on the Options menu. Under
CodeView, Flip/Swap ON forces messaging, Flip/Swap OFF does not.
Therefore, you must turn Flip/Swap on and off as you go, depending on
which particular part of the program you are currently debugging. The
Flip/Swap setting is crucial in determining whether you hang the
system or not.
The ability to debug child processes from the parent's CodeView
session became an available option beginning with CVP Version 2.30. By
specifying the /O switch on the command line, CVP allows you to trace
into child processes. (See the CVP 2.30 Note below if you are using
that particular version of CodeView.)
As far as actually debugging child processes in PM programs, you must
proceed in an exacting manner. Unfortunately, the specifics are
different for every program, so an all-encompassing set of debugging
procedures cannot be devised. Nevertheless, the following general
guidelines can be used for debugging most PM applications with their
accompanying child processes:
1. It is only with the /O option that any of this becomes critical.
2. You need to debug in a full screen and not a PM window.
3. Remember, Flip/Swap ON forces messaging, Flip/Swap OFF does not.
Therefore, while you begin debugging in the parent program, you
should have Flip/Swap ON, since you need messaging whenever you
are doing any of the window initialization routines.
4. Set a breakpoint in the parent somewhere after the window
initialization code, but before the call to start the child
process. It varies, but it usually works to put the breakpoint at
the "while get message - dispatch message" loop or in the
ClientWndProc procedure at the main switch statement.
5. You must make sure that the breakpoint is placed in a position
where you will stop at it before the prompt appears to ask you if
you want to debug the child, but after ALL the window
initialization has been completed. You will also probably need to
set another breakpoint at the point where the parent program is
going to begin executing again after you have finished working
with the child.
6. Do a GO, and when the breakpoint is reached, turn Flip/Swap OFF.
Make sure that the prompt to debug the child is not already
visible or you will lock up.
7. Do a GO (or some traces) and you should get the prompt to debug
the child. Answer "Y" and you should be able to debug the child
process at this point.
8. Use the Process command (the "|") to gain access to the child.
Never use CTRL+ESC or ALT+ESC to switch to the other processes
because these will almost assuredly cause you to hang.
9. Go ahead and set breakpoints, watches, etc. in the child and do
your debugging.
10. After the child process is completed, use the Quit command to exit
the child process's CodeView screen or use the Process command to
reselect the parent process.
11. Once you have returned to the parent process, and before you do a
Restart (or load), or before you do ANY other window
initialization, you MUST turn Flip/Swap ON again.
12. Remember to always be aware of where you are in the program and
what the current state of Flip/Swap is, since turning Flip/Swap on
or off at the wrong time will almost always cause you to hang.
Again, this is only the general outline of steps to follow and each
application requires different specific steps. Don't be too surprised
if you still run into occasional lock-ups.
CVP 2.30 Note: When using the /O switch with CVP Version 2.30, there
is a bug that may also cause you to hang. The problem involves using
/O on the command line and then saying "No" when asked if you want to
debug the child. So, with this version of CodeView, you should always
answer "Yes" when asked if you wish to debug the child.
For more information on this problem, use the following query:
CodeView 2.30 hangs debugging child processes
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