Type Cast conversion for x64 build : C, C++, C4312

May 9, 2008 06:44pm pdt

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03.27.2008 at 01:59PM PDT, ID: 23275637

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8.2

Type Cast conversion for x64 build

Zones: C++ Programming Language, C Programming Language, Windows 64-bit

Tags: C, C++, C4312

I'm working on converting a printer driver to run on x64, and have a couple errors I'm not sure what to do with. I have a lot of type cast warnings, but only a few errors. As this is a driver build and uses the DDK compiler, I can't really disable the portability checking.

Any help would be appreciated.

         
             //Generates error C4312: 'type cast' : conversion from 'DWORD' to 'LPTSTR' of greater size
pOptParam->pData  = (LPTSTR) *pTemplateInfo++; // pointer to the data
 
//Generates error C4312: 'type cast' : conversion from 'DWORD' to 'BYTE *' of greater size
pb = (BYTE*)(((DWORD)pb + 3) & ~3); // DWORD align
 
//Generates error C4312: 'type cast' : conversion from 'int' to 'HBRUSH' of greater size
FillRect( lpDIS->hDC, & lpDIS->rcItem, (HBRUSH)(nIndex +1) );

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Question Stats

Zone: Programming

Question Asked By: hess_joel

Solution Provided By: evilrix

Participating Experts: 2

Solution Grade: B

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03.27.2008 at 08:07PM PDT, ID: 21227716

Rank: Master

josgood:

This link may help
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/win64/win64/rules_for_using_pointers.asp

I'm assuming that the code contains pointer casts. The article also mentions that HANDLE is defined as void*, and I believe HBRUSH is a HANDLE.

03.28.2008 at 12:57AM PDT, ID: 21228581

Rank: Wizard

evilrix:

You can stop this warning by disabling the /Wp64 option in your project properties.
[Configuration Properties | C/C++ | General ===> Detect 64-bit Portability Issues]

You can also disable the warning on a per instance basis using #pragma warning, example below.

Before you disbale this warning I suggest you read the caveats documented on the MSDN page.
http://msdn2.microsoft.com/en-us/library/h97f4b9y.aspx

         
           
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             #include "windows.h"
 
int main()
{
	DWORD dw = 0;
 
// Disable warning C4312
#ifdef _WIN64
#pragma warning( push )
#pragma warning( disable : 4312 )
#endif
 
	// This line normally generates warning C4312
	LPTSTR s = (LPTSTR) dw;
 
// Reenable warning C4312
#ifdef WIN64
#pragma warning( pop )
#endif
 
	return 0;
}

           
         

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03.29.2008 at 04:41AM PDT, ID: 21236196

Rank: Wizard

evilrix:

I forgot to mention that you can also disable just that specific compiler warning by adding /wd4312 to the compiler command line settings in the project properties page.

/wd n
Disables the specified compiler warning where n is the compiler warning number.

http://msdn2.microsoft.com/en-us/library/thxezb7y.aspx

03.29.2008 at 07:07AM PDT, ID: 21236583

hess_joel:

As I said, I can't disable the error because the is a device driver using the DDK compiler.

I'm looking for a solutiont that will not just ignore the error.

03.29.2008 at 07:23AM PDT, ID: 21236637

Rank: Master

josgood:

I can't test to see whether the generated code actually does what you intend, but I can say that the lines marked /*fix*/ remove the compilation error when building with VS2005.

#include <windows.h>

struct OptParam { LPTSTR pData; };
struct DIS { HDC hDC; RECT rcItem; };

int main() {
//Generates error C4312: 'type cast' : conversion from 'DWORD' to 'LPTSTR' of greater size
OptParam* pOptParam = new OptParam;
int* pTemplateInfo = 0;
//pOptParam->pData = (LPTSTR) *pTemplateInfo++; // pointer to the data
/*fix*/ pOptParam->pData = (LPTSTR) (INT_PTR) *pTemplateInfo++; // pointer to the data

//Generates error C4312: 'type cast' : conversion from 'DWORD' to 'BYTE *' of greater size
BYTE* pb = 0;
//pb = (BYTE*)(((DWORD)pb + 3) & ~3); // DWORD align
/*fix*/ pb = (BYTE*)(((INT_PTR)pb + 3) & ~3); // DWORD align

//Generates error C4312: 'type cast' : conversion from 'int' to 'HBRUSH' of greater size
int nIndex = 0;
DIS* lpDIS = new DIS;
//FillRect( lpDIS->hDC, & lpDIS->rcItem, (HBRUSH)(nIndex +1) );
/*fix*/ FillRect( lpDIS->hDC, & lpDIS->rcItem, (HBRUSH)(INT_PTR)(nIndex +1) );
}

03.29.2008 at 07:56AM PDT, ID: 21236776

Rank: Wizard

evilrix:

>> As I said, I can't disable the error because the is a device driver using the DDK compiler.
It's not an error it's a warning and if you want to do this properly and remove the warning without disabling it you'll need to rework your code to use compatible types -- after all this is what the compiler is bitching about!

You may be able to introduce different levels of casting to prevent the compiler from giving the warning but that doesn't change the fundamental fact that you are casting types that are potentially incompatible. That said, if you know the values being cast are safe to cast (because the values won't cause overflow) then disabling the warning is the simplest way to do this otherwise you potentially make your code hard to follow/read just to hide a warning! If you want to ensure the cast is safe at runtime you can always use the Boost numeric cast operator, which performs runtime validation; an exception if thrown of the cast is unsafe and you'll have to handle that situation. You can then disable the warning and know your code will do the right thing at runtime time.
http://www.boost.org/libs/numeric/conversion/doc/numeric_cast.html

03.29.2008 at 08:09AM PDT, ID: 21236817

Rank: Master

josgood:

evilrx, I'm thinking that you're on the right track here, but (using the example I posted) when I

pOptParam->pData = boost::numeric_cast<LPTSTR>(pTemplateInfo); // pointer to the data

then I get
error C2440: 'static_cast' : cannot convert from 'int *const ' to 'char *'

what's with the const?

03.29.2008 at 09:03AM PDT, ID: 21237012

Rank: Wizard

evilrix:

The boost::numeric_cast can't cast between numeric and pointer types as it's an unsafe cast so what you'll need to do is cast the numeric value to a suitable numeric type that will cast to your pointer type without the warning and then use reinterpret_cast to cast to the pointer type. Since the size_t type is sized specific to the platform (it is guaranteed to be an unsigned integer type that is suitable for addressing ranges of memory) you may find this is the most suitable type; however, you'll need to play a little. I don't have a 64bit compiler at home so I am not in a position to test this unfortunately. Below is an example.

This might look a bit of a kludge and, guess what, it is! The only safe way to fix this is to rewrite the code to use 64bit safe types. Also, you should note that when casting incompatible pointer types (pointer to pointer or numeric to pointer for example) the only safe thing you can do with them is to cast back to the original type and any other usage is, at best, undefined and non-portable.

         
             #include "windows.h"
#include <boost/cast.hpp>
 
int main()
{
	DWORD dw = 0;
 	LPTSTR s = reinterpret_cast<LPTSTR>(boost::numeric_cast<size_t>(dw));
 	return 0;
}

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Accepted Solution

03.29.2008 at 09:12AM PDT, ID: 21237081

Rank: Master

josgood:

Thank you, evilrx. Now I see my mistake.

>>The only safe way to fix this is to rewrite the code to use 64bit safe types
I completely agree. *Especially* for a driver!

hess_joel, that's the real answer. Coercing the compiler into silence is risky enough with an application. A driver, as you know, has access to all of memory, including ring 0 memory. 'nuff said.

Assisted Solution

03.29.2008 at 09:55AM PDT, ID: 21237240

Rank: Wizard

evilrix:

>> Thank you, evilrix. Now I see my mistake.
No worries, I probably should have explained it a little better to start with :)

>> Coercing the compiler into silence is risky...
Agreed! A cast is always a potential problem, even when you are sure it's safe. Casts can hide problems, you are basically telling the compiler to shut up, you know best. In this case the compiler is fighting back, for good reason -- the code isn't 64bit safe. Consider; a cast that is safe today may not be tomorrow (code can change) and a cast will hide any potential warnings that may come out of future modifications. The boost cast does offer some protection in the form of runtime checks, but you'll then have the overhead of having to write additional exception handlers to deal with when it does go wrong.

Anyway, I don't think there's much more that can be said on this. I hope the info josgood and I have given is helpful. Sorry but there is no simple answer to this... it's either rewrite the code to make it safe, disable the warning or cast away the warning and live with the potential that in the future something may cause it to mis-behave.

-Rx.

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