Virtual-Function-Table pointer is misplaced in object memory
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Thread: Virtual-Function-Table pointer is misplaced in object memory

  1. #1
    Join Date
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    Virtual-Function-Table pointer is misplaced in object memory

    I have found that when I dump a C++ object from memory to a file - it seems that there is a misplacement of the last Virtual-Function-Table pointer - in that appears at the beginning. The result is that the gdump information based on this object dump (using green hills) is incorrect. I copied the contents of the gdump information below. The executable is compiled in linux.

    Basically MEIO::CameraStatus contains an item that relates to its parent class (line 188). Then it has 18 items that are all Diagnostics::EventsCounter items. Now for each Diagnostics::EventsCounter item there is a Virtual-Function-Table Info Pointer as its last item. All is fine and good except that the last item of MEIO::CameraStatus which is _selfReset is missing its last item of 4 bytes (which is the Virtual-Function-Table Info Pointer ). On the other hand - right before the first Diagnostics::EventsCounter item ("_vidErrors") - there is an extra 4 bytes which happens to be the Virtual-Function-Table Info Pointer. As I said the gdump information file does not see this.
    Can anyone explain why the object memory "moves" the last Virtual-Function-Table Info Pointer to the beginning (right before _vidErrors) and is there a way to "fix" this?

    ///////////////////////////////////////////////////////////////////////////

    "MEIO::CameraStatus" val:0x000002f0 ind208,-1) Struct-Begin Info
    188: "" offset 0, Parent-Class Private Info C++ Struct ref = 114
    189: "_vidErrors" offset 160, Member Info C++ Struct ref = 128
    190: "_vdiErrors" offset 480, Member Info C++ Struct ref = 128
    191: "_vidLogicalErrors" offset 800, Member Info C++ Struct ref = 128
    192: "_vidDrops" offset 1120, Member Info C++ Struct ref = 128
    193: "_resyncAttempts" offset 1440, Member Info C++ Struct ref = 128
    194: "_imageRegMismatchErrors" offset 1760, Member Info C++ Struct ref = 128
    195: "_camInvalidHeaderErrors" offset 2080, Member Info C++ Struct ref = 128
    196: "_regWriteHILErrors" offset 2400, Member Info C++ Struct ref = 128
    197: "_externalErrorFlags" offset 2720, Member Info C++ Struct ref = 128
    198: "_i2cErrors" offset 3040, Member Info C++ Struct ref = 128
    199: "_i2cNotOnTime" offset 3360, Member Info C++ Struct ref = 128
    200: "_temperature" offset 3680, Member Info C++ Struct ref = 108
    201: "_readTemperatureErrors" offset 4096, Member Info C++ Struct ref = 128
    202: "_readFrameCountErrors" offset 4416, Member Info C++ Struct ref = 128
    203: "_histogramsMismatchErrors" offset 4736, Member Info C++ Struct ref = 128
    204: "_ccftDecisionsMismatchErrors" offset 5056, Member Info C++ Struct ref = 128
    205: "_ccftCRCErrors" offset 5376, Member Info C++ Struct ref = 128
    206: "_selfReset" offset 5696, Member Info C++ Struct ref = 128
    207: "" val:0xffffffff ind187,-1) Struct-End Info



    128: "Diagnostics::EventsCounter" val:0x00000028 ind138,-1) Struct-Begin Info
    129: "_handler" offset 0, Member Info Reference to C++ Struct ref = 138
    130: "_name" offset 32, Member Info Pointer to C++ Char
    131: "_total" offset 64, Member Info C++ UInt
    132: "_inCurrentFrame" offset 96, Member Info C++ UInt
    133: "_consecutiveFrames" offset 128, Member Info C++ UInt
    134: "_severity" offset 160, Member Info C++ Struct ref = 123
    135: "_eventAppData" offset 256, Member Info Pointer to C++ Struct ref = 121
    136: "__vptr" offset 288, Virtual-Function-Table Info Pointer to C++ Struct ref = 490
    137: "" val:0xffffffff ind128,-1) Struct-End Info
    Last edited by VictorN; July 30th, 2014 at 04:15 PM. Reason: too long "slash" string

  2. #2
    Join Date
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    Re: Virtual-Function-Table pointer is misplaced in object memory

    Quote Originally Posted by akiva pat View Post
    The result is that the gdump information based on this object dump (using green hills) is incorrect.
    In that case why don't you ask the providers of gdump?

  3. #3
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    Re: Virtual-Function-Table pointer is misplaced in object memory

    "memory dumping" like you're doing here is not part of the c++ standard,
    so the results will be dependant on the compiler used (even version could matter) and which settings you used.
    a vtable isn't even the only way for a compiler to achieve a solution for virtual functions.

    looking at the above.
    CameraStatus only has a reference to new virtuals
    and it contains a reference to the parent object, for which you can find it's vtable there.

  4. #4
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    Re: Virtual-Function-Table pointer is misplaced in object memory

    Well, from what you're saying - there is nothing that I can do about this issue as it is non standard and somewhat arbitrary. Am I correct?

    What is your point from stating:

    "looking at the above.
    CameraStatus only has a reference to new virtuals
    and it contains a reference to the parent object, for which you can find it's vtable there. "

  5. #5
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    Re: Virtual-Function-Table pointer is misplaced in object memory

    I meant to say that how the compiler solves the 'virtual' issue is entirely proprietary to the compiler. Many compilers solve it with a vtable, but that isn't the only approach.


    Part2:
    it's how I made sense of it...
    somewhat hard to make much sense without also seeing the class definition.

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