byte array to struct - how to access struct members

Question

I have the following byte array that I am casting to the struct below: ** I know it's not in the proper format here "0x80", etc, but in my code it is.

unsigned char ReadBuffer[512] = { 80 00 00 00 50 00 00 00 01 00 40 00 00 00 01 00 00 00 00 00 00 00 00 00 FF F2 00 00 00 00 00 00 40 00 00 00 00 00 00 00 00 00 30 0F 00 00 00 00 00 00 30 0F 00 00 00 00 00 00 30 0F 00 00 00 00 33 20 C8 00 00 00 0C 42 E0 2A 0F 9F B9 00 00 FF}

typedef struct MFT_ATTRIBUTE {
    DWORD dwType;
    DWORD dwFullLength;
    BYTE uchNonResFlag;
    BYTE uchNameLength;
    WORD wNameOffset;
    WORD wFlags;
    WORD wID;
    LONG n64StartVCN;
    LONG n64EndVCN;
    WORD wDatarunOffset;
    WORD wCompressionSize;
    BYTE uchPadding[4];
    LONGLONG n64AllocSize;
    LONGLONG n64RealSize;
    LONGLONG n64StreamSize;
} MFT_ATTRIBUTE, *P_MFT_ATTRIBUTE;

MFT_ATTRIBUTE* mft_attribute = (MFT_ATTRIBUTE*)ReadBuffer[0];

When I try to print the members, for some reason I get the some kind of incremental values:

printf("%x ",&mft_attribute->dwType);
printf("%x ",&mft_attribute->dwFullLength);
printf("%x ",&mft_attribute->uchNonResFlag);
printf("%x ",&mft_attribute->uchNameLength);

Output:
0x80 0x84 0x88 0x89

Can someone help me clarify this?

Why don't you write unsigned char ReadBuffer[512] = { 0x80, 0x00, 0x00, and so on? What you are showing in your code snippet doesn't compile. — Wolf
– Wolf, Commented Oct 13, 2015 at 14:16

Wolf · Accepted Answer · 2015-10-13 14:22:56Z

3

You are printing addresses, not values. That's why the output is increasing this way:

0x80 - base address, same as the first member dwType
0x84 - second member, dwFullLength, sizeof (dwType) apart from start
0x88 - third member, uchNonResFlag, again an offset of 4, sizeof(dwFullLength)
0x89 - 4th member, the offset is 1, which is sizeof (uchNonResFlag)

Remove the & before mft_attribute in your output code:

printf("%x ", mft_attribute->dwType);
printf("%x ", mft_attribute->dwFullLength);
printf("%x ", mft_attribute->uchNonResFlag);
printf("%x ", mft_attribute->uchNameLength);

edited Oct 13, 2015 at 14:22

answered Oct 13, 2015 at 14:14

Wolf

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3 Comments

msbir Over a year ago

Thank you, this was the other half of the answer.

Wolf Over a year ago

Fine. But how does this { 0 1 2 } syntax compile on your computer?

msbir Over a year ago

it doesn't, but my data is created dynamically so I didn't want to spend the effort to add the " "

Manos Nikolaidis · Accepted Answer · 2015-10-13 14:24:31Z

1

You are casting the first element of the array to a pointer to your struct.

MFT_ATTRIBUTE* mft_attribute = (MFT_ATTRIBUTE*)ReadBuffer[0];

You want to cast a pointer to the first element :

MFT_ATTRIBUTE* mft_attribute = (MFT_ATTRIBUTE*) (ReadBuffer + 0);

Also as @Wolf pointed out this prints pointers not values pointed to:

printf("%x ",&mft_attribute->dwType);

You need instead

printf("%x ", mft_attribute->dwType);

edited Oct 13, 2015 at 14:24

answered Oct 13, 2015 at 14:14

Manos Nikolaidis

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Comments

AnatolyS · Accepted Answer · 2015-10-13 14:15:33Z

0

Change your cast to the following:

MFT_ATTRIBUTE* mft_attribute = (MFT_ATTRIBUTE*)&ReadBuffer[0];

or to

MFT_ATTRIBUTE* mft_attribute = (MFT_ATTRIBUTE*)ReadBuffer;

answered Oct 13, 2015 at 14:15

AnatolyS

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