Reading sectors

[_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)

[Lost User]

It is just the same as OpenFile behind the scenes so nothing to do with CFile. Your call probably fails because you are not including the share flags in your call to CFile::Open(), which are required when trying to access a device.

[_Flaviu]

Thanks, solved.