SetFilePointer sets pointer incorrectly (2 bytes more than requested)

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Hello,

I have some weird problems with SetFilePointer.

Let's there is file with this data at the beginning of the file
{ 0x1A, 0x2A, 0x3A, 0x4A, 0x5A, 0x6A, 0x7A};

hSomeObject = CreateFileA(chSomeFile, GENERIC_READ, FILE_SHARE_READ, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
SetFilePointer(hSomeObject, 2, 0, FILE_BEGIN);
ReadFile(hSomeObject, chSomeBuffer, 0xA0, &dwBytesRead, 0);

So the values in chSomeBuffer should be like {0x3A, 0x4A, 0x5A ...}
But the pointer is set to 4 instead of 2. So it looks like {0x5A, 0x6A, 0x7A ...}

I checked the call in the debugger. And yes it's called with 2 as lDistanceToMove.

So my questions is: What the heck?

Not forget to mention the SetFilePointer(hSomeObject, 0, 0, FILE_BEGIN); gives me requested output (But that's also strange as there are still 2 bytes skipped at the beginning).

Posted 6-Feb-11 9:30am

Im2N00By

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4 solutions

Solution 3

My guess is that you are comiling for UNICODE - this is easily checked, in the solution explorer right click on the project, select Properties and check General->Character Set.

This would explain why things seems to behave unexpectedly, well, it's a guess ...

Regards
Espen Harlinn

Posted 6-Feb-11 10:16am

Espen Harlinn

Comments

Sergey Alexandrovich Kryukov 6-Feb-11 16:29pm

Espen, to me, nothing indicates the use of stings or characters in the data in Question.
How can it be relevant?
--SA

Espen Harlinn 6-Feb-11 16:38pm

The number of programs, including my own, that would fail if SetFilePointer didn't work as expected, is tremendous. So according to Sherlock Holmes: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth". So basically I suspect the parts of his code we don’t see. Inadvertently compiling for unicode, while expecting ansi behavior is one possible explanation. He is using CreateFileA and not CreateFile, and that may be another hint. As I mentioned - it's a guess :)

Sergey Alexandrovich Kryukov 6-Feb-11 17:11pm

I agree on everything, except I don't know where is Unicode is in your example. The API documentation explicitly say the shift is passed in bytes.

However, I can see your point. The way OP presents information about the file suggests it's some integers, but, strictly speaking it is not said explicitly and could be a set of characters, then the size would depend on Unicode compilation option, as you suggested.

--SA

Espen Harlinn 6-Feb-11 17:19pm

As I wrote, it's a guess, mostly based on CreateFileA, and that VC++ 2010 now seems to create new projects with UNICODE on by default, that may take some people by surprise - TCHAR just became a wchar_t instead of char. As we don't see the rest of his code - it's obviously speculation on my part :)

Sergey Alexandrovich Kryukov 6-Feb-11 17:18pm

Espen, I just calculated the items in my assumption and found the pointer that the data item is 4 bytes size, so if the shift is done by 2, it was actually done by 1 element instead of 2 elements and it give exactly the same pointer as OP describes. Multiplying offset in elements by sizeof(ElementType) will give exactly expected answer. Please check up -- my answer is correct.
--SA

Espen Harlinn 6-Feb-11 17:34pm

I'm speculating that he is mixing TCHAR and printf/cout to print out his proof, and I've never had any problems using SetFilePointer the way he does. Both SetFilePointer and ReadFile operates on the byte level, SetFilePointer(hSomeObject, 2, 0, FILE_BEGIN) sets the position to the third byte in the file - that is ReadFile will start by reading the third byte - I'm a bit puzzled about why you are thinking about the "pointer that the data item is 4 bytes size" - well maybe he does something like that after he has read in the contents of the file - but that's speculation too :)

Sergey Alexandrovich Kryukov 6-Feb-11 19:21pm

Yes, yes, I got your point. My speculations are based on observing two patterns: expected and observed. I agree, it's still not 100% certain.

Please see the comment by OP: "It's binary data so theories about ASCII and UNICODE are off limit this time." Sorry, but it looks my concerns about your hypothesis turned out to be correct, won't you agree?

--SA

Im2N00By 6-Feb-11 18:18pm

#FIXED -> Problem solved by Answer 1
It's binary data so theories about ASCII and UNICODE are off limit this time. And CreateFile has two "versions" because of filepath... not because of data in the file.

But thanks guys for trying to figure it out ;)

Espen Harlinn 7-Feb-11 11:13am

Good, as the code you showed was clearly working. Seems we went a little far in our speculations about what you didn't show :)

Solution 2

It depends on the size of your data element. SetFilePointer does not know anything about if, so it point to 2 bytes, regardless. Probably, your element size is 4 bytes (8, whatever, you figure).

You should multiply offset by the result of sizeof.

—SA

Posted 6-Feb-11 10:04am

Sergey Alexandrovich Kryukov

Comments

Sergey Alexandrovich Kryukov 6-Feb-11 17:20pm

I checked up the calculation and obtained exactly the result you see and the result you expect through multiplication of the desired shift by sizeof of element type. Please check up -- will work as you expect.
--SA

Im2N00By 6-Feb-11 18:14pm

#FIXED -> Problem solved by Answer 1
Why I should multiple anything? I just needed the pointer to skip first 2 bytes. So should I do it like sizeof(unsigned char)*2 -> equal to 2*1 = 2?

Sergey Alexandrovich Kryukov 6-Feb-11 19:29pm

Thank you very much about reporting back the resolution of a pointer.
As to multiplication, I thought you wanted to shift by two elements, not by two bytes. You did not clearly indicate the size of the element, right? Besides, this is better practice. If you change you data element declaration, the code SetFilePointer with multiplication will remain valid, the code without it will be off. Don't you agree?
--SA

Im2N00By 7-Feb-11 10:30am

I should describe my problems more carefully. But it's not easy when I can't "reveal" what I'm working on (Else I could copy&paste whole function).

Sergey Alexandrovich Kryukov 7-Feb-11 21:35pm

My advice is: you develop some code specifically to post a question. It might even help you to isolate problem. The code should be as small as possible yet manifesting the problem. This is for everyone's benefit.
--SA

Solution 1

Have you checked the return value of SetFilePointer to see if it is INVALID_SET_FILE_POINTER and if so called GetLastError.

In addition the third parameter should be NULL, not 0.

Posted 6-Feb-11 10:03am

Henry Minute

Comments

Im2N00By 6-Feb-11 18:11pm

1] I removed error checks from the sample to shorten the code.
2] Ever wonder what NULL stands for? Check WinDef.h -> #define NULL 0

Henry Minute 6-Feb-11 18:38pm

1] OK
2] Never looked.

Im2N00By 7-Feb-11 10:37am

One more thing, my previous comment was little bit "offensive". I didn't mean it. I just got little bit upset about answers to my second question on other programming portal. In one part of long description I said: "I made kernel driver for that" and one of the answer was: "You can't do the g] in user mode. You will have to code a kernel driver". There was more answers like that...

So I would like to apologize.

Henry Minute 7-Feb-11 16:39pm

No need to apologize.

I was not offended, I'm too thick for that. :)

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Steve Wellens · Accepted Answer · 2011-02-06T10:35:00

Solution 4

It works for me. Are you sure about your file?

HANDLE hSomeObject;
BYTE chSomeBuffer[500];
DWORD dwBytesRead;
hSomeObject = CreateFileA("C:\\Test.bin", GENERIC_READ, FILE_SHARE_READ, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
SetFilePointer(hSomeObject, 2, 0, FILE_BEGIN);
ReadFile(hSomeObject, chSomeBuffer, 0xA0, &dwBytesRead, 0);

This site has a great free hex editor (XVI32):
http://www.chmaas.handshake.de/[^]

Posted 6-Feb-11 10:35am

Steve Wellens

Comments

Espen Harlinn 6-Feb-11 16:43pm

Good effort, my 5 - and another reason to suspect what we are not allowed to see :)

Im2N00By 6-Feb-11 18:07pm

Yea I passed wrong file name by accident. They have almost identical data -> so that's why I was confused about the 2 bytes shift.
I made this error while I copied&pasted some parts of code and didn't noticed the variable name.