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What does ReadFile return? TRUE or FALSE?
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The ReadFile function returned FALSE, which tell me that reading has failed.
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In this case you have to call GetLastError to obtain the exact reason of the failure!
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I didn't feed correctly the buffer. After I setup correctly that parameter, ReadFile return TRUE.
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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.
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_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]; unsigned char buffer[dwLen]; ReadFile(hVolume, buffer, dwLen, &dwNum, NULL);
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Is not accepted:
unsigned char buffer[dwLen];
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.
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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
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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.
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If you are writing MFC code then you should be using CFile to handle all your I/O.
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I should know that ... thank you.
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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);
file.Open(_T("F:"), CFile::modeRead | CFile::osSequentialScan);
file.Open(_T("F:\\"), CFile::modeRead | CFile::osSequentialScan);
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);
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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
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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.
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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
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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 !
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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.
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Understood now, I didn't paid attention on that function (lpDistanceToMoveHigh, is obviously now, or, at least I think is obviously).
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struct TestStruct
{
int x;
int z;
};
TestStruct S;
TestStruct Funct()
{
return S;
}
Funct().x = 100;
Will this work?
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Yes, but Func() is not really necessary, since you have already declared S. You can just say S.x = 100;
Creating a function to return a structure (or anything else) is usually necessary when you cannot declare the relevant object at compile time, but need to create it dynamically. Something like:
TestStruct* Funct()
{
TestStruct* pS = new TestStruct;
pS->x = 0;
ps->z = 0;
return pS;
}
TestStruct* pNewStruct = Funct();
pNewStruct->x = 100;
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Hello,
the code itself will work, but will not do what I suppose you expect.
Here, Funct returns a copy of the TestStruct S.
So Funct().x = 100 will affect 100 in a temporary TestStruct that you can not use after that.
I don't know why you want to use a function to access your S structure, but the behavior you look for is probably :
TestStruct& Funct()
{
return S;
}
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