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VB Forum / Win API / April 2008CopyMemory within a callback
Hi guys, I need to call the CopyMemory function in VB6 from within a callback, all works well within the VB Dev Environment, but causes a crash once compiled. I have spent a few days researching this problem on the net and it looks to be related to the fact that calling a function from a declare causes the vbaSetSystemError to be set, which then subsequently causes the callback to fail. I have seen suggestions that using a Type Library could resolve the problem, however when using the TLB file to define CopyMemory, I receive a cannot 'Read' memory error, and switching back to using the Declare within a module I receive a cannot 'Write' memory error. (Please note that these errors only occur in the compiled exe, and within the Vb Environment everything works great. I'd be really grateful for any your suggestions you may have. Regards, James > Hi guys, > [quoted text clipped - 12 lines] > occur in the compiled exe, and within the Vb Environment everything works > great. Yes, you do need to use a typelib if the callback is in another thread (what can do safely is really limited) I expect you are getting errors down to difference in ByVal and ByRef and param types. The params in my typelib are: pDst [in] void* pSrc [in] void* ByteLen [in] long  SignatureDean Earley (dean.earley@icode.co.uk) i-Catcher Development Team iCode Systems Hi Dean, Thanks for your suggested typlib changes, I tried them however my code still unfortunately crashed when compiled with a memory read error. I suspect you are right about problems with byref/byval etc, I've included below a snippet of the code, hopefully you will be able to spot something? ' The following are present in a module. Public Type EVENT_GENERIC_CONTAINER ContainerLength As Long ContainerType As Integer Code As Integer TransactionID As Long Parameter As Long End Type Private pevent As EVENT_GENERIC_CONTAINER Public Sub EventCallback(CameraHandle As Long, ByVal Context As Long, ByVal EventVoid As Long) ' any attempt to run RtlMoveMemory fails when compiled! RtlMoveMemory pevent, ByVal EventVoid, 16 ' rest of code here End Sub Regards, James > > Hi guys, > > [quoted text clipped - 22 lines] > pSrc [in] void* > ByteLen [in] long > Hi Dean, > [quoted text clipped - 22 lines] > ' rest of code here > End Sub It more depends what the original CopyMemory declare looking like I used: Public Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (ByRef pDst As Any, ByRef pSrc As Any, ByVal ByteLen As Long) SignatureDean Earley (dean.earley@icode.co.uk) i-Catcher Development Team iCode Systems Hi Dean, How do you mean that it depends on the original copymemory declare? When I used your suggested Typelib parameters, I can successfully use CopyMemory in the Vb Environment, however I'm seeing a cannot 'read' memory error once compiled. As everything is working fine in the VB Dev Environment doesn't that mean that I have already got the correct by ref or by val settings? Regards, James > > Hi Dean, > > [quoted text clipped - 27 lines] > Public Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" > (ByRef pDst As Any, ByRef pSrc As Any, ByVal ByteLen As Long) > Hi Dean, > How do you mean that it depends on the original copymemory declare? When I [quoted text clipped - 3 lines] > As everything is working fine in the VB Dev Environment doesn't that mean > that I have already got the correct by ref or by val settings? Are you sure it is the copymemory itself that is causing the crash? What you can call from another thread is VERY limited (no declares, no com objects, no other classes, etc..) When running in the IDE, VB marshals it all to the same thread so all the problems "disappear" SignatureDean Earley (dean.earley@icode.co.uk) i-Catcher Development Team iCode Systems Hi Dean, Many thanks, yes it looks like I've been so caught up in trying to get the CopyMemory working that I never noticed that other, more innocent operations (like form1.print "Callback Triggerd") where actually causing the crash. This leads me to one last question, if i may, how would one perform an action such as form1.print "Event Triggered" from within the callback? Would utilizing CreateThread from within the callback be the solution? Regards, James > > Hi Dean, > > How do you mean that it depends on the original copymemory declare? When I [quoted text clipped - 11 lines] > When running in the IDE, VB marshals it all to the same thread so all > the problems "disappear" > Hi Dean, > [quoted text clipped - 5 lines] > action such as form1.print "Event Triggered" from within the callback? Would > utilizing CreateThread from within the callback be the solution? No, as it is already in another thread, effectively isolating it from the rest of the app. Perhaps the simplest method is to use Send/PostMessage (using a variable containing the forms hWnd) and subclass that to receive the message. SignatureDean Earley (dean.earley@icode.co.uk) i-Catcher Development Team iCode Systems > Hi guys, > [quoted text clipped - 17 lines] > Regards, > James You'll have to take a closer look to your CopyMemory call. Is it really a CopyMemory call or a MoveMemory call? Also check if the parameters are passed correctly. For me it seems like the IDE is more relax about the way the parameters are passed in, once compiled it must be correctly, otherwise the app crashes. I've made the same mistake before, but depending on the situation you have to make adjustments to the call. Sinna |
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