Reading sectors

_Flaviu

I have written a little piece of code to read few (first 512) bytes from USB drive. here is the code:

HANDLE hVolume = ::CreateFile(sVolume, GENERIC\_READ,
	FILE\_SHARE\_READ | FILE\_SHARE\_WRITE, 
	NULL, OPEN\_EXISTING, FILE\_FLAG\_SEQUENTIAL\_SCAN, NULL);
TRACE("Volume handle: %p\\n", hVolume);
LONG lDistLow = 0;
PLONG plDistHigh = 0;
if (INVALID\_SET\_FILE\_POINTER == SetFilePointer(hVolume, lDistLow, plDistHigh, FILE\_BEGIN) &&
	NO\_ERROR != GetLastError())
{
	TRACE("Error: %d\\n", GetLastError());
	return FALSE;
}
else
{
	TRACE("%d\\t%d\\n", lDistLow, plDistHigh);
}

DWORD dwLen = 512;
DWORD dwNum = 0;
void\* pBuffer = NULL;
ReadFile(hVolume, pBuffer, dwLen, &dwNum, NULL);
TRACE("%p\\t%d\\t%d\\n", pBuffer, dwLen, dwNum);

but the result is:

Volume handle: 00000438
0 0
00000000 512 0

this pBuffer NULL tell me that I am not able to read that sectors ... why ? The program is running as admin and sVolume is feed by _T("\\\\.\\F:"), first TRACE macro printed is a prove that is OK that part. SetFilePointer is used to position the reading part, and I intend to setup offset to the beginning, right ?

Victor Nijegorodov

What does ReadFile return? TRUE or FALSE?

Graham Breach

Your pBuffer value is NULL, so you are trying to read data into the memory address 0 - you have to allocate some space to read the data into and use that for your pBuffer value. For 512 bytes a stack-based array should do the trick.

Lost User

_Flaviu wrote:

pBuffer NULL tell me that I am not able to read that sectors

No, pBuffer = NULL; tells you that you have not allocated any space to pBuffer. And don't use void* unless you are trying to allocate nothing. You can allocate space either of the following ways.

unsigned char* pBuffer = new unsigned char[dwLen]; // allocate some space for the data to be read into
// or
unsigned char buffer[dwLen]; // allocate on the stack
ReadFile(hVolume, buffer, dwLen, &dwNum, NULL);

_Superman_

As others have pointed out, the issue is with pBuffer not associated with any allocated memory. In addition to this, I would like to point out one more flaw in your code, although it doesn't matter in this case. The third parameter to SetFilePointer[^] must be an address of a LONG variable. PLONG does not declare a LONG variable. It's only a pointer to a LONG variable. Here SetFilePointer will actually try to write to memory 0. What you need to do is declare a LONG variable and provide its address -

LONG highValue; SetFilePointer(..., ..., &highValue, ...);

You can also pass a nullptr, if you're not intersted in that value.

«_Superman_» _I love work. It gives me something to do between weekends.

_Microsoft MVP (Visual C++) (October 2009 - September 2013)

Polymorphism in C

_Flaviu

The ReadFile function returned FALSE, which tell me that reading has failed.

_Flaviu

It will interest me, not this time, but in the real program. I modified the code like this:

LONG lValueLow = 0;
LONG lValueHigh = 0;
if (INVALID\_SET\_FILE\_POINTER == SetFilePointer(hVolume, lValueLow, &lValueHigh, FILE\_BEGIN) &&
	NO\_ERROR != GetLastError())
{
	TRACE("Error: %d\\n", GetLastError());
	return FALSE;
}
else
{
	TRACE("%d\\t%d\\n", lValueLow, lValueHigh);
}

Thank you Superman !

_Flaviu

Is not accepted:

unsigned char buffer[dwLen]; // error C2057: expected constant expression

Of course, putting 512 is ok, but I need to allocate that length of buffer through a variable. I prefer this option, on stack, as far as I know, is faster than heap.

Victor Nijegorodov

In this case you have to call GetLastError to obtain the exact reason of the failure!

_Flaviu

I didn't feed correctly the buffer. After I setup correctly that parameter, ReadFile return TRUE.

Lost User

You can fix that easily by doing it this way:

#define SECTOR_SIZE 512
DWORD dwLen = SECTOR_SIZE;
DWORD dwNum = 0;
char buffer[SECTOR_SIZE];
ReadFile(hVolume, buffer, dwLen, &dwNum, NULL);

If you are using C++ then instead of #define you can use this:

const int SECTOR_SIZE = 512

_Flaviu

Good idea. I have tried in this way:

CByteArray arrByte;
arrByte.SetSize(512);
BOOL bRet = ReadFile(hVolume, arrByte.GetData(), dwLen, &dwNum, NULL);

and seem to go well.

Lost User

You do not need lValueHigh unless the distance to move is a 64 bit value: just specify NULL instead. Also I suggest you study the difference between a variable and a pointer to a variable, you seem somewhat confused about it.

_Flaviu

Understood now, I didn't paid attention on that function (lpDistanceToMoveHigh, is obviously now, or, at least I think is obviously).

Lost User

If you are writing MFC code then you should be using CFile to handle all your I/O.

_Flaviu

I'll try to use CFile Class | Microsoft Docs[^] Can I replace SetFilePointer as well with something from MFC yard ?

Victor Nijegorodov

[CFile Class | Microsoft Docs](https://docs.microsoft.com/en-us/cpp/mfc/reference/cfile-class?view=vs-2019#seek)

Lost User

CFile Class.Seek | Microsoft Docs[^]. You need to read through all the documentation for the class to see what is available.

_Flaviu

I should know that ... thank you.

_Flaviu

Good idea. I have few troubles with accessing USB drive with CFile, but once I'll solve it, the code will be simple. Here is my trial, none on them has worked:

file.Open(_T("\\\\.\\F:"), CFile::modeRead | CFile::osSequentialScan); // return FALSE

file.Open(_T("F:"), CFile::modeRead | CFile::osSequentialScan); // return FALSE

file.Open(_T("F:\\"), CFile::modeRead | CFile::osSequentialScan); // return FALSE

HANDLE hVolume = ::CreateFile(sVolume, GENERIC\_READ,
	FILE\_SHARE\_READ | FILE\_SHARE\_WRITE, 
	NULL, OPEN\_EXISTING, FILE\_FLAG\_SEQUENTIAL\_SCAN, NULL);
CFile file(hVolume);
CFileStatus status;
file.GetStatus(status); // return FALSE