C++ #import and x64 builds

I already wrote earlier on 32/64-bit issues with Visual Studio. The problems are not frequent but when they happen they are pretty confusing. Here is another one today.

C++ code is simple:

    #import "libid:59941706-0000-1111-2222-7EE5C88402D2" raw_interfaces_only no_namespace

    CComPtr<IObject> pObject;
    BYTE* pnData;
    ATLENSURE_SUCCEEDED(pObject->Method((ULONG_PTR) (BYTE*) pnData));

A COM method returns a pointer to data – pretty straightforward, what could have gone wrong?

COM server vendor designed the library for easy .NET integration and defined the pointer argument as an integer value. They suppose the values to be used further with System.Runtime.InteropServices.Marshal class.

32-bit builds worked well and 64-bit builds experienced memory access violations. An attempt to consume the COM server from C# project showed the same problem: unexpected exception in the call.

The problem is that cross-compiler importing COM type library using LIBID takes 32-bit library even when it builds 64-bit code. This is the problem for both C++ #import "libid:..." and .NET COM reference using the identifier.

The type library imports as the following IDL in 32-bits:

                [in] unsigned long bufPtr);

It is supposed that 64-bit builds get the following import:

                [in] uint64 bufPtr);

Effectively though, 64-bit builds get the 32-bit import and the argument which is supposed to carry casted pointer value is truncated to 32-bits, ULONG type. Cast to ULONG_PTR in 64-bit C++ code is, of course, not helpful since it’s trimmed anyway further fitting the IDL argument type.

The same happens with C# build.

It was developer’s choice to publish ordinal type argument, they wanted this to be “better” and ended up in bitness mess. If the argument remained a pointer type in the IDL then even incorrect bitness would not necessarily result in value truncation.

All together it is unsafe to import [an untrusted] type library using LIBID when it comes to 64-bit builds. It’s 32-bit library to be taken and it can result in incorrect import. Instead, such build should explicitly point to 64-bit type library, for example:

#if defined(_WIN64)
    #import "Win64\ThirdParty.DLL" raw_interfaces_only no_namespace
    //#import "libid:59941706-0000-1111-2222-7EE5C88402D2" raw_interfaces_only no_namespace
    #import "Win32\ThirdParty.DLL" raw_interfaces_only no_namespace

Too bad! libid looked so nice and promising.

Leave a Reply