Re: Data alignment problems with sizeof and new
- From: astdsoftware <astdsoftware@xxxxxxxxxxxxxxxx>
- Date: Wed, 2 Nov 2005 08:42:04 -0800
Doug,
Thanks for the help. It turned out to be a #pragma pack(1) which didn't
restore the packing to normal. I thought I had looked for that before, but I
must have missed it. I didn't know about the show option. That brought it to
light very quickly. Thanks!
"Doug Harrison [MVP]" wrote:
> On Tue, 1 Nov 2005 13:12:03 -0800, astdsoftware
> <astdsoftware@xxxxxxxxxxxxxxxx> wrote:
>
> >I have a strange problem with a project involving classes with data alignment
> >padding. It appears that, in this particular project, the sizeof and new
> >operators are not taking the alignment padding into account when calculating
> >the size of an object. The struct member alignment is set to 8 (/Zp8), and
> >there are no #pragma pack directives. The test class below shows how, given a
> >class with the first member being a dword size followed by doubles, the first
> >double member is padded out to an offset of 8, increasing the memory
> >footprint by 4 bytes. Using the sizeof() operator outside the scope of the
> >class, the size returned is the sum of all of the members, but if I use the
> >sizeof operator inside the constructor, it calculates the right size.
> >
> >The real problem is that the new operator only allocates enough memory for
> >the object without any padding, and when the constructor starts initializing
> >its data members, the last one writes beyond the memory block allocated for
> >the class. If I move this code to a different project, it works just fine,
> >but I can't find any project options or anything which might explain this
> >behavior.
> >
> >Of course, the problem goes away if I change the alignment to /Zp4, simply
> >because the padding goes away, but that only works if I don't use anything
> >smaller than 4 bytes in size.
> >
> >Any clues would be greatly appreciated!
> >
> >-----------------------------------------------------------
> >// TestClass.h
> >#pragma once
> >class TestClass
> >{
> >public:
> >TestClass(void);
> >~TestClass(void);
> >unsigned int m_dwData; // Offset 0
> >double m_Data1; // Offset 8
> >double m_Data2; // Offset 16 (0x10)
> >double m_Data3; // Offset 24 (0x18)
> >double m_Data4; // Offset 32 (0x24)
> >};
> >-------------------------------------------------------------
> >// TestClass.cpp
> >
> >#include "StdAfx.h"
> >#include ".\testclass.h"
> >TestClass::TestClass(void)
> >: m_dwData(0)
> >, m_Data1(0)
> >, m_Data2(0)
> >, m_Data3(0)
> >, m_Data4(0)
> >{
> >int mysize = sizeof(*this); // Evaluates to 28h (Correct Value)
> >}
> >TestClass::~TestClass(void)
> >{
> >}
> >--------------------------------------------------------------
> >// Main.cpp
> >
> >#include ".\testclass.h"
> >TestClass* pTest;
> >pTest = new TestClass;
> >int tsize = sizeof(*pTest); // Evaluates to 24h (Incorrect Value)
> >---------------------------------------------------------------
>
> Here's a repost of an message I wrote back in May 2005 that may explain
> your problem:
>
> *****
>
> http://groups.google.com/group/microsoft.public.vc.stl/msg/37bb8ca26dbd5c01
>
> There's a bug in <winsock2.h> such that it changes the packing from the
> default 8 to 4 when WIN32 isn't #defined. You're #including c.h in two
> different ways; in a.cpp, it follows <winsock2.h>, while b.cpp doesn't
> #include <winsock2.h>. I expect this is causing various classes to have
> different layouts in these files. You can see the underlying problem if you
> add the following line to the top of c.h and following all its #include
> directives:
>
> #pragma pack(show)
>
> Then when I compile using your command line, I get:
>
> C>cl -I. -nologo -MDd -EHsc -W3 -DWINVER=0x400 a.cpp b.cpp
> a.cpp
> c:\temp\c.h(4) : warning C4810: value of pragma pack(show) == 4
> c:\temp\c.h(7) : warning C4810: value of pragma pack(show) == 4
> c:\temp\c.h(9) : warning C4810: value of pragma pack(show) == 4
> c:\temp\c.h(11) : warning C4810: value of pragma pack(show) == 4
> b.cpp
> c:\temp\c.h(4) : warning C4810: value of pragma pack(show) == 8
> c:\temp\c.h(7) : warning C4810: value of pragma pack(show) == 8
> c:\temp\c.h(9) : warning C4810: value of pragma pack(show) == 8
> c:\temp\c.h(11) : warning C4810: value of pragma pack(show) == 8
>
> I think that in general, if you're going to use the Windows SDK, you should
> #include <windows.h> before any other Windows header, which would avoid
> this problem without you having to remember to specify /DWIN32 for these
> command lines you create yourself.
>
> *****
>
> And BTW, you should avoid the /Zp option like the plague, for the reasons
> given here:
>
> http://groups.google.com/group/microsoft.public.vc.mfc/msg/8a94f9c97b8dbf09
>
> Use #pragma pack instead, either directly or through the <pshpackN.h> and
> <poppack.h> headers, to control the packing of individual structs.
>
> --
> Doug Harrison
> Visual C++ MVP
>
.
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- Data alignment problems with sizeof and new
- From: astdsoftware
- Re: Data alignment problems with sizeof and new
- From: Doug Harrison [MVP]
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