I'm reading a volume (logical drive) with ReadFile. I'm using DeviceIoControl with FSCTL_ALLOW_EXTENDED_DASD_IO code, because I want to have access to all (including the last) bytes and had an issue trying to read last 512 bytes (ReadFile successed, but reported 0 bytes read) and saw advice to use it. Unfortunately, ReadFile fails being called after that DeviceIoControl called.
In code it looks like this (all success checks are omitted for the brevity):
HANDLE fd;
DWORD junk;
int lenToBeRead = 0x1000;
DWORD nread;
char* alignedBuf = new char[lenToBeRead];
fd = CreateFile("path to volume", FILE_READ_DATA,
FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL, OPEN_EXISTING, 0, NULL)) //success
DeviceIoControl(fd, FSCTL_ALLOW_EXTENDED_DASD_IO,
NULL, 0, NULL, 0, &junk, (LPOVERLAPPED) NULL) //success
ReadFile(fd, alignedBuf, (DWORD) lenToBeRead, &nread, NULL)
// fails with 0x57 code, ERROR_INVALID_PARAMETER
All work with fd handle is synchronous.
EDIT. I solved the problem. I was trying to read last bytes. So my volume was of length L = 0x...200 and I had my handle on position pos = L - 0x200. What I had done before I did the FSCTL_ALLOW_EXTENDED_DASD_IO thing - I cut lenToBeRead to fit in remaining space (so, if it was 0x1000, it would change to 0x200), because I had found that ReadFile did not guarantee read all the bytes to the EOF in case of lenToBeRead is greater than amount of bytes remained from current handle position. This did not help, ReadFilewas still returning with success and 0 bytes read. I deleted that fix and then used FSCTL_ALLOW_EXTENDED_DASD_IO, which deliver me then ReadFile failing with ERROR_INVALID_PARAMETER on lenToBeRead = 0x1000. I totally forgot about the first fix and remembered now and now it works.
I found the solution and add it to the question body.
What one has to keep in mind when working with ReadFile is to control arguments (length) to not cross the boundary of the file.
I had tried it as a fix before doing the FSCTL_ALLOW_EXTENDED_DASD_IO thing and it did not help. But combination of the FSCTL_ALLOW_EXTENDED_DASD_IO thing and the boundary check gave me wanted result - I could read that last bytes.
Related
I am using the win32 function GetRegionData(...) to extract the exact rectangles which make up the invalidated paint region in response to a WM_PAINT message.
The following code works correctly and the second call to GetRegionData succeeds.
DWORD uRegionSize = GetRegionData(hRgn, sizeof(RGNDATA), NULL); // Send NULL request to get the storage size
RGNDATA* pData = (RGNDATA*)(new char[uRegionSize]); // Allocate space for the region data
pData->rdh.dwSize = uRegionSize;
DWORD uSizeCheck = GetRegionData(hRgn, uRegionSize, pData);
if (uSizeCheck != uRegionSize) {
// FAIL!
delete[] pData;
return;
}
...
do stuff with rectangles
...
But when I tried to move the data buffer to a member variable allocated on the stack, GetRegionData fails every time returning 0.
In my header:
char UpdateRegionData[LOTS_MORE_BYTES_THAN_NEEDED];
In my cpp:
DWORD uRegionSize = GetRegionData(hRgn, sizeof(RGNDATA), NULL); // Send NULL request to get the storage size
RGNDATA* pData2 = (RGNDATA*)UpdateRegionData;
pData2->rdh.dwSize = uRegionSize;
DWORD uSizeCheck = GetRegionData(hRgn, uRegionSize, pData2);
if (uSizeCheck != uRegionSize) {
// FAIL!
return;
}
The only thing different between the 2 versions is the memory allocation, but the second one fails. GetLastError() returns code 183 which is ERROR_ALREADY_EXISTS which doesn't seem to make much sense.
Thanks to Raymond for pointing out the size error - that was indeed an error, but it was not cause of the issue. The actual cause was byte alignment. The project I am working on has its byte alignment set to a default of 1. When I specified 4 byte alignment for the buffer using __declspec(align(4)) then the problem was solved.
How can I read a file from 100th byte, using ReadFile function and C++
I used this code but it reads the first 100 bytes of the file
I want to read the second 100 bytes
hndl = CreateFileW(L"1.txt", GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
SetFilePointer(hndl, 100, NULL, FILE_BEGIN);
ReadFile(hndl, pbytReadBuffer, 100, NULL, &ol);
The ReadFile API offers two distinct ways to set the starting offset when doing synchronous I/O:
By using the implicitly stored file pointer (which can be manipulated through the SetFilePointer API call).
Or by passing an explicit offset through the OVERLAPPED structure.
Your code fails because you are setting the implicitly stored file pointer, but then (presumably) pass a zero-initialized OVERLAPPED structure, which ignores the file pointer (see Synchronization and File Position for more information).
Either of the following solutions will work. First, by using the implicitly stored file pointer. This is useful, when you want to read chunks of a file in consecutive calls:
hndl = CreateFileW(L"1.txt", GENERIC_READ, 0, nullptr,
OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
// Move the file pointer to offset 100
SetFilePointer(hndl, 100, NULL, FILE_BEGIN);
// Read contents from the current offset
DWORD dwBytesRead{0};
ReadFile(hndl, pbytReadBuffer, 100, &dwBytesRead, nullptr);
Alternatively, you can pass an OVERLAPPED structure to pass the offset. This ignores the implicitly stored file pointer. It is slightly more efficient, because it doesn't need two calls to the file I/O API.
hndl = CreateFileW(L"1.txt", GENERIC_READ, 0, nullptr,
OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
OVERLAPPED ol{};
// Set the offset from the start of the file
ol.Offset = 100;
ReadFile(hndl, pbytReadBuffer, 100, nullptr, &ol);
Note that error handling is elided in these samples for brevity. In real code, you always must check for errors.
you need code like this
HANDLE hFile = CreateFile(L"c:\\windows\\notepad.exe", FILE_GENERIC_READ, FILE_SHARE_READ, 0, OPEN_EXISTING, 0, 0);
if (hFile != INVALID_HANDLE_VALUE)
{
OVERLAPPED ov = {};
ov.Offset = 100;
UCHAR buf[100];
ULONG cb;
if (ReadFile(hFile, buf, sizeof(buf), &cb, &ov))
{
}
else
{
GetLastError();
}
CloseHandle(hFile);
}
you not need call SetFilePointer - much better set offset direct in OVERLAPPED structure
EDIT
for every read/write operation we need to specify the starting byte offset. we can do this directed by setting values in Offset and OffsetHigh from OVERLAPPED.
or indirect only if file opened if synchronous mode - I/O Manager can use the current file position in FILE_OBJECT - so we can not direct set offset - it will be get from FILE_OBJECT.CurrentByteOffset. FILE_OBJECT.CurrentByteOffset we can set with SetFilePointer also every read/write operation update this offset - move forward to count of bytes which readed/ written. of course this is correct only if file used in synchronous mode when all operations with file is sequential
if we use direct offset in OVERLAPPED he and used - so FILE_OBJECT.CurrentByteOffset is ignored - this mean that previous call to SetFilePointer - also lost all effect - will be used offset from OVERLAPPED and after read operation FILE_OBJECT.CurrentByteOffset will be updated based on offset + bytes readed
BOOL SetDeviceID(HANDLE device,char *id){//
char data[2];
data[0]=0x02;
data[1]=0x27;
DWORD dwPtr=SetFilePointer(device,0x33,//distance
NULL,//
FILE_BEGIN);
if(dwPtr==INVALID_SET_FILE_POINTER) cout<<GetLastError()<<endl;
BOOL result=WriteFile(device,data,2,NULL,NULL);
//cout<<GetLastError()<<endl;
if(result==false)cout<<"Fail WRITE "<<endl;
return TRUE;
}
HANDLE GetDeviceHandle(char *path){
HANDLE handle= CreateFile(LPCSTR(path),
GENERIC_ALL,//
0,
NULL,
OPEN_EXISTING,
NULL,
NULL);
if(handle==INVALID_HANDLE_VALUE){
cout<<"fail to createfile()"<<endl;
exit(1);
}
else return handle;
}
this is some code of my works.
I am going to read/write directly device(usb)
on ReadFile() case, It was successful.
But, I have tried to call SetFilePointer
But GetLastError returned 87. it means invalid input
What is the problem? on my code
shortly, CreateFile,ReadFile is ok but SetFilePointer and WriteFile failed
When you are directly accessing a disk device you cannot seek to positions in the middle of a sector. The position must always be a multiple of the sector length. And 0x33 is not a mutiple of your sector length.
What you will need to do is read an entire sector. Modify the bytes that need to be modified. And finally write back the entire sector.
I am getting odd results when using RegQueryValueEx and I cannot figure out why.
This is what I had set up before making the RegQueryValueEx
DWORD dataSize;
TCHAR data[256];
The first time I call
LONG ret = RegQueryValueEx( hKey, dataKey, NULL, NULL, (LPBYTE)data, &dataSize);
ret is equal to 234 (ERROR_MORE_DATA)
But when I call the same thing on the next line
LONG ret2 = RegQueryValueEx( hKey, dataKey, NULL, NULL, (LPBYTE)data, &dataSize);
ret2 is equal to 0 (ERROR_SUCCESS)
Why would this function return ERROR_MORE_DATA the first time I call it, then return ERROR_SUCESS on the same call on the very next line?
I attempted to change TCHAR data[1024] but I got the exact same results. Any ideas?
Complete code:
for( int i=0; i<NUM_HISTORY; i++){
CString dataKey = getDataKey(i);
DWORD dataSize = 1024;
TCHAR data[1024];
LONG ret = RegQueryValueEx( hKey, dataKey, NULL, NULL, (LPBYTE)data, &dataSize);
LONG ret2 = RegQueryValueEx( hKey, dataKey, NULL, NULL, (LPBYTE)data, &dataSize);
// Breakpoint to see what ret and ret2 are equal to
int j = 0;
}
This is by design. The first call failed because you specified as size that was too small. But what you didn't count on is that it also updated your dataSize variable. To tell you how much memory to allocate so the call can succeed.
So the second call succeeded since you now specify a size that's exactly correct. But without doing the other thing you needed to do, actually make the buffer bigger. Nothing much good can happen when the call then causes a buffer overflow and corrupt your stack frame, be sure to use the /RTC compile option so you'll get a runtime error from that.
You avoided this problem by increasing the buffer size from 256 to 1024. But your code is still incorrect, your program will fail miserably if the registry value ever gets larger than 1024 bytes. Don't use a local array, use the new operator or malloc() to allocate the buffer so it can never fail like this. Or simply fail the call and declare "bad data".
Also note another bug, the dataSize is in bytes but the buffer is TCHAR, not a byte. Which is probably why you didn't corrupt the stack frame, the buffer was big enough by accident. You don't want to rely on accidents like this. Consider a helper class like CRegKey to avoid these kind of mistakes.
I am trying to use combination of functions CreateFileMapping , MapViewOfFile, FlushViewOfFile.
the total buffer size is more than the mapped view.
example buffer is 50KB. and mapped view is 2KB. in such scenario,
i want to write the total buffer to a physical file, using the above function.
First part i am able to write to file. but the remaining part how to write to file. I mean, how to move to next page and write the next part of data.
#define MEM_UNIT_SIZE 100
-first module...Memory map creator
GetTempPath (256, szTmpFile);
GetTempFileName (szTmpFile, pName, 0, szMMFile);
hFile = CreateFile (szMMFile, GENERIC_WRITE | GENERIC_READ, FILE_SHARE_WRITE,
NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_TEMPORARY, NULL);
HANDLE hFileMMF = CreateFileMapping( hFile ,NULL,PAGE_READWRITE,0,
(MEM_UNIT_SIZE),pName)
-second module... Memory writer
long lBinarySize = 1000;
long lPageSize = MEM_UNIT_SIZE;
HANDLE hFileMMF = OpenFileMapping(FILE_MAP_WRITE,FALSE,pMemName);
LPVOID pViewMMFFile = MapViewOfFile(hFileMMF,FILE_MAP_WRITE,0,0, lPageSize );
CMutex mutex (FALSE, _T("Writer"));
mutex.Lock();
try
{
ASSERT(FALSE);
CopyMemory(pViewMMFFile,pBinary,lPageSize); // write
FlushViewOfFile(pViewMMFFile,lPageSize);
// first 100 bytes flushed to file.
//how to move to next location and write next 900 bytes..<---??
}
catch(CException e)
{
...
}
please share if you have any suggestion.
thanks in advance,
haranadh
Repeat your call to MapViewOfFile with a different range.
as described in the following link,
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366761(v=VS.85).aspx
can you please check "allocation granularity", I think you should use this parameter to set the values for "dwFileOffsetLow" or "dwFileOffsetHigh".