Question

I have a Visual Studio 2008 C++ project for Windows Mobile 6 ARMV4I using Microsoft SQLCE 3.5. When I initialize the VARIANT component of a DBPROP structure (as below), I get a compiler warning message: C4366: The result of the unary ‘&’ operator may be unaligned.

#include <sqlce_oledb.h>

DBPROP prop = { 0 };
::VariantInit( &prop.vValue ); // warning here

I can add __unaligned casts to the line, but because VariantInit doesn’t take an __unaligned, I get another C4090 warning.

I notice that the DBPROP definition in *sqlce_oledb.h* includes packing directives for MIPS architecture:

#if defined(MIPSII_FP) || defined(MIPSIV) || defined(MIPSIV_FP)
#pragma pack(push,8)
#endif
typedef struct  tagDBPROP
    {
    DBPROPID dwPropertyID;
    DBPROPOPTIONS dwOptions;
    DBPROPSTATUS dwStatus;
    DBID colid;
    VARIANT vValue;
    }   DBPROP;
#if defined(MIPSII_FP) || defined(MIPSIV) || defined(MIPSIV_FP)
#pragma pack(pop)
#endif

So, I can make the warning go away by doing something like this:

#define MIPSIV
#include <sqlce_oledb.h>
#undef MIPSIV

But, that feels dirty. My question is: Did the designers just overlook ARM in their packing directives (meaning I should do the dirty and claim to be a MIPS processor)? Or, should I just silence the warning and ignore it? Or, is there something else I should do?

Thanks,
PaulH

Answer 1

If you plan to pass the DBPROP structure to other APIs, do NOT change its alignment, since that can change the packing and it will stop working. I notice the following comment in the header:

#if 0 
//DBPROPINFO is an unaligned structure. MIDL workaround. 42212352
typedef DBPROPINFO  *PDBPROPINFO;
#else
typedef DBPROPINFO UNALIGNED  * PDBPROPINFO;      //????????????
#endif

So it seems someone was aware of a similar problem but did not change the packing, probably to avoid breaking existing code. I don’t see the rest of your code from here, but you could try one of the following:

1. VARIANT tmp; ::VariantInit(&tmp); prop.vValue = tmp;

2. prop.vValue.vt = VT_EMPTY;