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Asked: 2026-05-10T22:45:30+00:00

I have some code in MS VC++ 6.0 that I am debugging. For some

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I have some code in MS VC++ 6.0 that I am debugging. For some reason, at this certain point where I am trying to delete some dynamically allocated memory, it breaks and I get a pop up message box saying ‘User Breakpoint called from code at blah blah’.. then the Disassembly window pops up and I see

*memory address* int      3

The odd thing is, there is NOWHERE in the code that I am calling an assembly instruction like this (I think asm int 3 is a hardware break command for x86?)..

what could be causing this?

EDIT: ANSWER: My code was ‘walking off the end’ of an array, but only in the locations marked by Visual Studio debug with 0xFDFDFDFD, which is called a NoMan’sLand fence.. I think its also called an Off-by-one error.. This array was unrelated to the point where i was freeing the memory when the error was occuring. Which made it harder to spot.. 🙁

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1 Answer

  1. You’re probably hitting code in the debug heap routines that have found heap corruption.

    What does the call stack look like when you’ve hit the Int 3?

    Edit: Based on the stack trace in your comments, the routine _CrtIsValidHeapPointer() is saying that the pointer being freed is bad. Here’s the snippet of code from MSVC’s DBGHEAP.C source:

        /*      * If this ASSERT fails, a bad pointer has been passed in. It may be      * totally bogus, or it may have been allocated from another heap.      * The pointer MUST come from the 'local' heap.      */     _ASSERTE(_CrtIsValidHeapPointer(pUserData));

    pUserData would be the value of the pointer you’re deleteing.

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