Error with ReadFile and Overlapped

[proxy_lainux]

Hi

I have a problem with ReadFile and overlapped.

first I use ReadFile with overlapped with 0

ZeroMemory(&overlapped ,sizeof(OVERLAPPED));

hDevice = CreateFileW(zwpath, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING,  FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, NULL);

if(hDevice != INVALID_HANDLE_VALUE){

ret = ReadFile(hDevice, buff, 1024, &numerobyte, &overlapped);

}

with a for(), I can see the bytes using a printf()

for (int n=0; n<sizeof(buff); n++)  
       {  
           printf("0x%02X ", buff[n]);  
       }

and now I have one variant with a large number

crbig = 322122547

    d1 = (DWORD*)crbig;
    overlapped.Offset = d1[1]; //22122547
    overlapped.OffsetHigh = d1[0];// 00000003

I need use ReafFile twice, this second time is using overlapped() with offset and highoffset values

 d1 = (DWORD*)&crbig;
overlapped.Offset = d1[1];
overlapped.OffsetHigh = d1[0];

ret = ReadFile(hDevice, buff, 1024, &numerobyte, &overlapped);

but the for() prints the same bytes that throws me the first ReadFile, I tried cleaning the buffer with memset() or use another buffer but always show me the same bytes

Supposedly Overlapped() says to ReadFile where should go, and read 1024 bytes, and save it in a buffer, but nothing happen, buffer has the same first bytes that overlapped() = 0

this is the complete code

int main(int argc, char *argv[]){

    HANDLE hDevice;
    OVERLAPPED overlapped;
    DWORD crbig;
    BYTE buff[1024] = {0};
    DWORD numerobyte = 0, nbytes = 0;
    UINT32 ret;
    DWORD *d1;
    int offset1 = 11, offset2 = 13, offset3 = 30;
    long MFTCluster = 0, bytespercluster = 0, sectperclusters = 0, offet = 0;
    unsigned long mult = 0;

    ZeroMemory(&overlapped ,sizeof(OVERLAPPED));

    hDevice = CreateFileW(zwpath, GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING,  FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, NULL);

    if(hDevice != INVALID_HANDLE_VALUE){

        ret = ReadFile(hDevice, buff, 1024, &numerobyte, &overlapped);

    }else
    {
        return NULL;
    }

    if(ret == 0){


        ret = WaitForSingleObject(hDevice,INFINITE );

        switch (ret)
        {
        case WAIT_OBJECT_0:break;
        case WAIT_TIMEOUT:break;
        default:
            break;
        }
    }
    else
    {
        return NULL;
    }


    if((int)buff[3] == 'N' && (int)buff[4] == 'T' && (int)buff[5] == 'F' && (int)buff[6] == 'S' ){
        printf("Volumen es formato NTFS\n\n\n");
    }

    bytespercluster = endianhextodec(buff, offset1);
    sectperclusters = endianhextodec(buff, offset2);
    MFTCluster = endianhextodec(buff, offset3);

    crbig = (sectperclusters * (bytespercluster * MFTCluster));

    d1 = (DWORD*)&crbig;
    overlapped.Offset = d1[1];
    overlapped.OffsetHigh = d1[0];

    ret = ReadFile(hDevice, buff, 1024, &numerobyte, &overlapped); <<--- here's the error

    if(ret == 0){


        ret = WaitForSingleObject(hDevice,INFINITE );

        switch (ret)
        {
        case WAIT_OBJECT_0:break;
        default:
            break;
        }
    }
    else
    {
        return NULL;
    }

    for (int n=0; n<sizeof(buff); n++)  
    {  
        printf("0x%02X ", buff[n]);  
    } 

    CloseHandle(hDevice);

    getchar();
}

I try to convert VB.NET code to C code but fail overlapped

Private Sub SetReadFileOffset(ByRef NO As System.Threading.NativeOverlapped, ByRef curBig As Int64)

        Dim lowoffset() As Byte = BitConverter.GetBytes(curBig)
        Dim highoffset As Int32 = BitConverter.ToInt32(lowoffset, 0)
        Dim high As Int32
        Dim lastbytes(3) As Byte
        Array.Copy(lowoffset, 4, lastbytes, 0, 4)
        high = BitConverter.ToInt32(lastbytes, 0)
        NO.OffsetLow = highoffset
        NO.OffsetHigh = high
    End Sub

 SetReadFileOffset(NO, MFTCluster * SectperCluster * BytesPerSect)
        ret = ReadFile(Hnd, Buffer, 1024, nRead, NO)
        If ret = 0 Then
            ret = WaitForSingleObject(Hnd, INFINITE)
            Select Case ret
                Case WAIT_OBJECT_0
                Case Else
            End Select
        Else
            Return Nothing
        End If

regards

[OReubens]

you aren't quite getting the concept I fear.

overlapped io means "asynchronous".


You issue a read and supply a buffer.
The system returns from the read() immediately
the contents of the buffer is "indeterminate" until the overlapped IO completes which is signalled (so you use WaitForSingleObject, or anyone of the other functions that allow you to be informed that the read has completed)

Only when the object is signalled can you rely on the buffer having the value you intended (or having a read error).


Overlapped IO only makes sense in a multithreading type environment where you launch a read request but don't immediately need the result but can go do other stuff while the disk is being read. You return to the data when the read has completed.

if you have a situation where you Always end up with.
- Read
- Wait for read to complete
- use buffer

Then using overlapped IO is entirely pointless, you could just as well have used a regular (blocking) read instead.

[Igor Vartanov]

I try to convert VB.NET code to C code but fail overlapped

Private Sub SetReadFileOffset(ByRef NO As System.Threading.NativeOverlapped, ByRef curBig As Int64)

        Dim lowoffset() As Byte = BitConverter.GetBytes(curBig)
        Dim highoffset As Int32 = BitConverter.ToInt32(lowoffset, 0)
        Dim high As Int32
        Dim lastbytes(3) As Byte
        Array.Copy(lowoffset, 4, lastbytes, 0, 4)
        high = BitConverter.ToInt32(lastbytes, 0)
        NO.OffsetLow = highoffset
        NO.OffsetHigh = high
    End Sub

 SetReadFileOffset(NO, MFTCluster * SectperCluster * BytesPerSect)
        ret = ReadFile(Hnd, Buffer, 1024, nRead, NO)
        If ret = 0 Then
            ret = WaitForSingleObject(Hnd, INFINITE)
            Select Case ret
                Case WAIT_OBJECT_0
                Case Else
            End Select
        Else
            Return Nothing
        End If

regards

Please note, the wait function waits on file handle itself, but not on the overlapped structure handle. Therefore, the whole trick seems based on the fact that the file is written to, and written of as many bytes as your offset requires. I'm not sure how OVERLAPPED is expected to behave in the situation, i.e. I cannot predict what happens when the file is modified (WaitForSingleObject gets unblocked), but file size now is lesser than the specified read offset.

There's one important aspect with overlapped reading with offset. The read device must support a concept of seeking.