VC++ HeapAlloc inside function gives null pointer - c++

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)