Mixing managed and unmanaged code issue

Question

I have the following code

    System::Void MainForm::initLoadCell(){
        //Open the first found LabJack U3 over USB.
        lngErrorcode = OpenLabJack (LJ_dtU3, LJ_ctUSB, "1", TRUE, &lngHandle);

        //Load defualt config
        lngErrorcode = ePut (lngHandle, LJ_ioPIN_CONFIGURATION_RESET, 0, 0, 0);

        //Setup FIO0 as an analogue input port
        lngErrorcode = ePut (lngHandle, LJ_ioPUT_ANALOG_ENABLE_BIT,0,1,0);

        //Obtain error string
        char* errorcode = new char;
        ErrorToString(lngErrorcode, errorcode);

        // Convert the c string to a managed String.
        String ^ errorString = Marshal::PtrToStringAnsi((IntPtr) (char *) errorcode);

        MainForm::textBox_LoadCellError->Text = errorString;

        Marshal::FreeHGlobal((IntPtr)errorcode);
}

This works when I run the program directly from Visual Studio but when I build .exe file and run as standalone I get the following error

Problem signature:
Problem Event Name: APPCRASH
Application Name:   BenchTester.exe
Application Version:    0.0.0.0
Application Timestamp:  52f4c0dd
Fault Module Name:  ntdll.dll
Fault Module Version:   6.1.7601.18247
Fault Module Timestamp: 521ea8e7
Exception Code: c0000005
Exception Offset:   0002e3be
OS Version: 6.1.7601.2.1.0.256.1
Locale ID:  3081
Additional Information 1:   0a9e
Additional Information 2:   0a9e372d3b4ad19135b953a78882e789
Additional Information 3:   0a9e
Additional Information 4:   0a9e372d3b4ad19135b953a78882e789

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I know that it is caused by the following line

ErrorToString(lngErrorcode, errorcode);

This is a call to 3rd party code, I assume the error has to do with the code not dealing with the unmanaged code correctly but I am unsure where. Could someone please point me in the right direction.

Answer 1

I don't know what ErrorToString expects as arguments, but I'd say it is a char* representing a pointer to a buffer where it can store the result string.

In this case your code:

//Obtain error string
char* errorcode = new char;
ErrorToString(lngErrorcode, errorcode);

looks wrong (it is allocating a single char).

Try changing it to:

//Obtain error string
char* errorcode = new char[1024];
ErrorToString(lngErrorcode, errorcode);

and see if this works (in this case don't forget you need to release the memory later).

Hope this helps.

Answer 2

You're making all of this far too complicated. Instead of

    //Obtain error string
    char* errorcode = new char;
    ErrorToString(lngErrorcode, errorcode);

    // Convert the c string to a managed String.
    String ^ errorString = Marshal::PtrToStringAnsi((IntPtr) (char *) errorcode);

    MainForm::textBox_LoadCellError->Text = errorString;

    Marshal::FreeHGlobal((IntPtr)errorcode);

All you need is:

    //Obtain error string
    char errorString[1024];
    ErrorToString(lngErrorcode, errorString);
    MainForm::textBox_LoadCellError->Text = gcnew System::String(errorString);

You originally had two problems: not having a big enough buffer, and mismatching allocation and deallocation functions. After using new you must use delete, not Marshal::FreeHGlobal. But there was no reason to do any dynamic allocation at all.