VC++ HeapAlloc inside function gives null pointer

Question

I am trying to use HeapAlloc() to allocate a buffer used by SetupDiGetDeviceRegistryProperty().

Inside GetDeviceInformation() I have:

HANDLE hHeap = GetProcessHeap();

while (SetupDiEnumDeviceInfo(DeviceInfoSet, MemberIndex++, DeviceInfoData)) 
{
    DWORD DataT;
    LPTSTR buffer = NULL;
    DWORD buffersize = 0;
    // port of device
    DWORD portnum = 0;

    GetRegistryProperty(DeviceInfoSet, DeviceInfoData, SPDRP_FRIENDLYNAME, 
         &DataT, buffer, &buffersize, &buffersize);

    if (!buffer) 
    {
        cerr << "Null Ptr!" << endl;
        exit(1);
    }
    // Do stuff, uninstall device


    if (buffer) HeapFree(hHeap, NULL, buffer); 

    }
}

And inside GetRegistryProperty() I have:

void GetRegistryProperty(HDEVINFO DeviceInfoSet, PSP_DEVINFO_DATA DeviceInfoData, 
    DWORD Property, PDWORD DataT, LPTSTR buffer, PDWORD buffersize, PDWORD size)
{


    HANDLE hHeap = GetProcessHeap();

    while (!SetupDiGetDeviceRegistryProperty(
        DeviceInfoSet,
        DeviceInfoData,
        Property, //SPDRP_FRIENDLYNAME or SPDRP_CLASS
        DataT, //&DataT
        (PBYTE)buffer,
        *buffersize,
        size)) //&buffersize
    {
        if (GetLastError() == ERROR_INSUFFICIENT_BUFFER)
        {
            // Change the buffer size.
            if (buffer) HeapFree(hHeap, NULL, buffer); 

            // Double the size to avoid problems on 
            // W2k MBCS systems per KB 888609. 
            buffer = (LPTSTR)HeapAlloc(hHeap, HEAP_ZERO_MEMORY |
                HEAP_GENERATE_EXCEPTIONS, *buffersize * 2);
        }
        else
        {
            // error handling
            break;
        }
    }
}

HeapAlloc() works as expected (the buffer is filled with the property) until GetRegistryProperty() returns. At this point, the buffer is always NULL. Is this also expected? How can I return a char * pointing to an array that lives past the life of the function that created it? I assume that I don't understand how HeapAlloc() works.

The reason I have it in a separate function is that I would like to call GetRegistryProperty() multiple times with different DWORD Propertys. Before I moved the code to a separate function it worked perfectly.

Answer 1

Pass the buffer by reference (note the LPTSTR&) :

void GetRegistryProperty(HDEVINFO DeviceInfoSet, PSP_DEVINFO_DATA DeviceInfoData, 
    DWORD Property, PDWORD DataT, LPTSTR& buffer, PDWORD buffersize, PDWORD size)

Answer 2

You are passing buffer by value, so in GetRegistryProperty, when you reassign it, you simply overwrite the copy of the pointer in GetRegistryProperty.

Change the signature of GetRegistryProperty to:

void GetRegistryProperty(HDEVINFO DeviceInfoSet, PSP_DEVINFO_DATA DeviceInfoData, DWORD Property, PDWORD DataT, LPTSTR& buffer, PDWORD buffersize, PDWORD size)