HeapAlloc causes excpetion - Windows 7 x64

Question

Before some time i coded my own little db editor program, i was coding it from the zero using Win API's so its not very small project. It was working fine on all OS till now, i have Win 7 x64 with all latest updates and my application is crashing with 0xC000005 exception because of some of the Heap functions(HeapAlloc or HeapFree, i use nothing else), i tried replacing HeapAlloc & HeapFree with VirtualAlloc and VirtualFree and it was all fine, but i dont want to use the virtual memory.... Something else, i tried to attach with debugger to trace the problem, but when i attach debugger its not crashing, then i tried to display MessageBox to trace where it crashes, but when i display MessageBox its not crashing too....

My application is running as 32bit process. Coded in C.

Anyone had similar problem ?

Answer 1

Firstly, both HeapAlloc and VirtualAlloc allocate virtual memory.

My guess as to what is happening is that you are writing past the boundary of the allocated memory. The reason why this does not work with HeapAlloc is that it allocates exactly the amount of memory you request. With VirtualAlloc, the size returned is the requested size rounded up to the next page boundary. In your case, this gave a bit more leeway (even though your code is still doing the wrong thing).

In terms of why it has been working so far: you just got lucky. Look carefully at the code accessing the allocated memory and if you get stuck, post the relevant part up here. If the debugger isn't helping and the bug is easily reproducible, just comment out parts of the code until you locate the line causing the crash.

Answer 2

Have you attached it to Debug version of your application? If the problem does not appear in debug version then you should check what warnings (on highest level) generate your code, maybe you will find some uninitialized variables. If nothing here, then you might use some static analysis tool to help with finding bugs - like PVS-Studio http://www.viva64.com/.

You can also compile Release version with debugging information enabled, this way when problem arrises you should be able to attach to your application with debugger and see callstack with function names. To make it easier to debug, disable code optimizations.

You can also try gflags from windows debugger tools, this program will trigger breakpoint each time you write outside of buffer boundary. This is really helpfull tool because not all buffer overruns end up with exceptions. Use it on application with debugging information enabled, and preferably with code optimizations off. To enable gflags for your app use:

gflags /p /enable myapp.exe /full (http://msdn.microsoft.com/en-us/library/windows/hardware/ff543097%28v=vs.85%29.aspx)