Heisenbug issue with using a dll. What do I do next?

I am working on a system that uses a Voltage Controlled Oscillator chip (VCO) to help process a signal. The makers of the chip (Analog Devices) provide a program to load setup files onto the VCO but I want to be able to setup the chip from within the overarching signal processing control system. Fortunately Analog Devices also provides a DLL to interface with their chip and load setup files myself. I am programming in Visual C++ 6.0 (old I know) and my program is a dialog application.

I got the system to 开发者_高级运维work perfectly writing setup files to the card and reading its status. I then decided that I needed to handle the case where there are multiple cards attached and one must be selected. The DLL provides GetDeviceCount() which returns an integer. For some reason every time the executable runs it returns 15663105 (garbage I assume). Whenever I debug my code however the function returns the correct number of cards. Here is my call to GetDeviceCount().

typedef int (__stdcall *GetDeviceCount)();

int AD9516_Setup()
{
    int NumDevices;
    GetDeviceCount _GetDeviceCount;
    HINSTANCE hInstLibrary = LoadLibrary("AD9516Interface.dll");
    _GetDeviceCount = (GetDeviceCount)GetProcAddress(hInstLibrary,"GetDeviceCount");
    NumDevices = _GetDeviceCount();
    return NumDevices;
}

Just to be clear: every other function from the DLL I have used is called exactly like this and works perfectly in the executable and debugger. I did some research and found out that a common cause of Heisenbugs is threading. I know that there is some threading behind the scenes in the dialogs I am using so I deleted all my calls to the function except one. I also discovered that the debugger code executes slower than executable code and I thought the chip may not have enough time to finish processing each command. First I tried taking up time between each chip function call by inserting an empty for loop and when that did not work I commented out all other calls to the DLL.

I do not have access to the source code used to build the DLL and I have no idea why its function would be returning garbage in the executable and not debugger. What other differences are there between running in debugger and executing that could cause an error? What are some other things I can do to search for this error?

Some compilers/IDEs add extra padding to variables in debug builds or initialize them to 0 - this might explain the differences you're encountering between debugging and "normal" execution.

Some things that might be worth checking:
- are you using the correct calling convention?
- do you get the same return value if no devices are connected?
- are you using the correct return type (uint vs int vs long vs ..)?

Try setting _GetDeviceCount to 0 before calling the function; that could be what the debugger is doing for you.