Re: C Question ...

From:

"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>

Newsgroups:

microsoft.public.vc.language

Date:

Wed, 12 Aug 2009 19:26:53 -0400

Message-ID:

<#J7LjR6GKHA.3736@TK2MSFTNGP04.phx.gbl>

It is quite common for the compiler to insert padding (unused bytes) into
struct declarations. If you need to declare a struct with no padding use
#pragma pack, just before and just after the declaration:

#pragma pack(push)
#pragma pack(1) // one byte packing
typedef ....
{ };
#pragma pack(pop) // restore normal packing

"John" <johnthompson1@hotmail.com> wrote in message
news:D1586ED2-26E6-44EF-A99E-5FBA8B02B091@microsoft.com...

I have the following struct:

typedef struct _MINE
{
UCHAR a[6];
UCHAR B[6];
USHORT C;
USHORT D;
USHORT E;
UCHAR F;
UCHAR G;
USHORT H;
UCHAR I[6];
UINT J;
UCHAR K[6];
UINT L;
} MINE, *PMINE;

When I try to use this in code, I have this ...
PMINE a;
a = (PMINE) someBuffer;

Now, someBuffer has all of the correct stuff filled into it. When I set
"a" to point to it, everything in "a" is fine except the last UINT L. For
some reason, I get this:

a->A == OK;
a->B == OK;
a->C == OK;
a->D == OK;
a->E == OK;
a->F == OK;
a->G == OK;
a->H == OK;
a->I == OK;
a->J == OK;
a->K == OK;
a->L == NOT OK;

a->L = Almost OK. It appears as though a->L is really only 2 bytes
instead of 4 in the debugger. Also, it also appears as though a->L starts
pointing 2 bytes after a->K. So, in memory, it looks like this:

a->K 00, 01, 02, 03, 04, 05; FINE
06 07; Two bytes that I need but are skipped
a->L = 08, 09;

Then a bunch of zeros appear after 09. If I set my struct to this:

typedef struct _MINE
{
UCHAR a[6];
UCHAR B[6];
UCHAR C[2];
UCHAR D[2];
UCHAR E[2];
UCHAR F;
UCHAR G;
UCHAR H[2];
UCHAR I[6];
UCHAR J[4];
UCHAR K[6];
UCHAR L[4];
} MINE, *PMINE;

Everything appears to point to the right stuff. However, if I do this
then I have to do some type of memcpy from L into a UINT. I don't want to
do that because it is extrememly ghetto, and this should be working. Can
anybody tell what's wrong? BTW, this is a kernel driver.

Thanks.

--
Scott McPhillips [VC++ MVP]