Can a Timer event experience reentrancy? - timer

Question

I'm examining some dreadful legacy code that has a Timer event with some lengthy code that contains DoEvents calls. A simplified version looks something like this:
Private Sub tmrProcess_Timer()
'Run some slow processing code here
DoEvents
'More slow code here
DoEvents
'Lots more slow code and the occasional DoEvents here
If booComplete Then
tmrProcess.Enabled = False
End If
End Sub
The timer has it's Interval set to 250 and the slow code could take up to thirty or so seconds to complete. Note that there is a button on the form that sets booComplete = True when it is clicked.
Given that VB6 is single threaded and that timer messages are low priority is it at all possible for the Timer event to be re-entered during a DoEvents call or will the VB6 runtime block execution of a Timer event if the Timer event is currently executing?
This reference has some relevant information. In particular it states that WM_PAINT messages are combined into a single message but there is no mention of whether or not WM_TIMER messages are combined.

Answer 1

i expected it would reenter, but it seems not to
have a look at the following test project:
'1 form with:
' 1 timer control : Name=Timer1
Option Explicit
Private Sub Form_Load()
WindowState = vbMaximized
Timer1.Interval = 2000
End Sub
Private Sub Timer1_Timer()
Static intCount As Integer
Dim sngTime As Single
intCount = intCount + 1
Print CStr(Now) & " Timer event fired " & CStr(intCount)
sngTime = Timer + 3
Do While sngTime > Timer
DoEvents
Loop
Print CStr(Now) & " End of timer event " & CStr(intCount)
End Sub
you will see a "start" 2 seconds after the form loads
you will see an "end" 3 seconds after that
you will see a "start" 2 seconds after the previous "end" showed
you will see an "end" 3 seconds after the "start"
...
if the timer would be reentered i would expect 2 seconds between each "start", but there appears to be 3+2=5 seconds between each "start"
removing the DoEvents doesn't change the behaviour, it just changes the time at which the texts are printed

Answer 2

To avoid reentry, disable the timer till the time taking logic is executed and then enable again.
Private Sub tmrProcess_Timer()
tmrProcess.Enabled = False
'your time taking logic goes here ......
tmrProcess.Enabled = True
End Sub

Answer 3

First, most code containing DoEvents doesn't require it, it's a magical word that people feel compelled to incant (but without without knowing why).
DoEvents allows reentrancy of anything, not just timers.
Your's is a TimerProc. If you had chosen a message then the wm_timer message only comes if the message queue is empty and you ask "are their any messages?". If their is a paint, timer, or mouse move type message pending then if the queue is empty only then are they available.
Despite the obviousnous this is the link it came from:
> mk:#MSITStore:C:\Program%20Files\Microsoft%20Visual%20Studio\MSDN\2001OCT\1033\kbvb.chm::/Source/vbapps/q118468.htm
(of course you have to have same libraries as me installed for this to work.)
Why do you assume the source is the internet?
Definition of DoEvents in Visual Basic for Applications
Q118468
The information in this article applies to:
Microsoft Visual Basic for Applications version 1.0 Microsoft Excel
for Windows, versions 5.0, 5.0c Microsoft Excel for the Macintosh,
versions 5.0, 5.0a Microsoft Excel for Windows 95, versions 7.0, 7.0a
Microsoft Excel 97 for Windows Microsoft Excel 98 Macintosh Edition
SUMMARY The DoEvents function surrenders execution of the macro so
that the operating system can process other events. The DoEvents
function passes control from the application to the operating system.
Some instances in which DoEvents may be useful include the following:
Hardware I/O
Delay Loops
Operating System Calls
DDE Deadlocking
This article also discusses potential problems associated with the
DoEvents function.
MORE INFORMATION
Hardware I/O If your code waits for an input from any I/O device, the
DoEvents function speeds up the application by multitasking. As a
result, the computer does not seems to pause or stop responding (hang)
while the code is executing.
Example:
Open "com1" For Input As #1 Input #1, x Do Until x = Chr(13) DoEvents
'... '... Input #1, x Loop Delay Loops In a delay loop, the DoEvents
function can allow the CPU operating system to continue with any
pending jobs.
Example:
X = Timer() Do While X + 10 > Timer()
DoEventsLoop Operating System Calls When Visual Basic calls the
operating system, the operating system may return the control even
before processing the command completely. Doing so may prevent any
macro code that depends on an object generated by the call from
running. In the example below, the Shell function starts the Microsoft
Word application. If Word is not yet open, any effort to establish a
DDE link to it will halt the code. By using DoEvents, your procedure
makes sure that an operation, such as Shell, is completely executed
before the next macro statement is processed.
Example:
z% = Shell("WinWord Source.Doc",1) DoEvents ... ... DDE Deadlocking
Consider a situation in which a Visual Basic macro calls an
application that is waiting for a second application to get some data.
If the macro does not give control to the second application, the
result is a deadlock. In DDE conversations between multiple
applications, using DoEvents removes the possibility of this type of
deadlocking. Problems Associated with DoEvents Using too many nested
DoEvents statements may deplete the stack space and therefore generate
an "Out of Stack Space" error message. This error is referring to the
application stack space allocated to the Microsoft Excel application.
Make sure the procedure that has given up control with DoEvents is not
executed again from a different part of your code before the first
DoEvents call returns; this can cause unpredictable results.
Once DoEvents relinquishes control to the operating system, it is not
possible to determine when Microsoft Excel will resume the control.
After the operating system obtains control of the processor, it will
process all pending events that are currently in the message queue
(such as mouse clicks and keystrokes). This may be unsuitable for some
real- time data acquisition applications.
REFERENCES For more information about DoEvents, click the Search
button in Help and type:
doevents
Additional query words: Sendkeys keystroke Wait XL98 XL97 XL7 XL5
Keywords : Issue type : Technology : kbHWMAC kbOSMAC kbExcelSearch
kbZNotKeyword6 kbExcel95 kbExcel500 kbExcel98 kbExcel95Search
kbExcel97Search kbExcel98Search kbExcelMacsearch kbVBASearch
kbZNotKeyword3 kbExcel500Mac kbExcel500aMac kbExcel500c kbExcel95a
kbVBA100
Last Reviewed: January 17, 2001 © 2001 Microsoft Corporation. All
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