Reading sectors

[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

[Victor Nijegorodov]

When using CFile::Open you should pass the CFileException* parameter to get the failure cause if CFile::Open fails. See the example in [CFile::Open](https://docs.microsoft.com/en-us/cpp/mfc/reference/cfile-class?view=vs-2019#open)