[DllImport("kernel32.dll")]
static extern bool WriteFileGather(IntPtr hFile, [Out] FILE_SEGMENT_ELEMENT []
   aSegmentArray, uint nNumberOfBytesToWrite, IntPtr lpReserved,
static extern bool WriteFile(IntPtr hFile, byte [] lpBuffer,
   uint nNumberOfBytesToWrite, out uint lpNumberOfBytesWritten,
   [In] ref System.Threading.NativeOverlapped lpOverlapped);

or:

[DllImport("kernel32.dll", SetLastError=true)]
static extern unsafe int WriteFileGather(IntPtr hFile,
  FILE_SEGMENT_ELEMENT* aSegmentArray, int nNumberOfBytesToWrite,
  IntPtr lpReserved, System.Threading.NativeOverlapped* lpOverlapped);
static extern unsafe int WriteFile(IntPtr handle, IntPtr buffer,
  int numBytesToWrite, IntPtr numBytesWritten, NativeOverlapped* lpOverlapped);

User-Defined Types:

[StructLayout(LayoutKind.Explicit, Size = 8)]
internal struct FILE_SEGMENT_ELEMENT
{
  [FieldOffset(0)]
  public IntPtr Buffer;
  [FieldOffset(0)]
  public UInt64 Alignment;
}

Notes:

The documentation for the Scatter/Gather functions states that the buffer addresses used must be page aligned (not just storage aligned). Indeed, passing a managed allocation, such as a byte array, causes error 87 (Invalid Parameter). This function is asynchronous only so an use the AsyncResult and Overlapped pattern established by the FileStream.

[DllImport("kernel32.dll", BestFitMapping=true, CharSet= CharSet.Ansi)]
    static extern bool WriteFile(IntPtr hFile, System.Text.StringBuilder lpBuffer,
     uint nNumberOfBytesToWrite, out uint lpNumberOfBytesWritten,
     [In] ref System.Threading.NativeOverlapped lpOverlapped);

Important

Remember to add the attribute CharSet = CharSet.Ansi,

If you use:

    static extern bool WriteFile(IntPtr hFile, System.Text.StringBuilder lpBuffer,
     uint nNumberOfBytesToWrite, out uint lpNumberOfBytesWritten,
     [In] ref System.Threading.NativeOverlapped lpOverlapped);

for Hooking into a process and see what an application want's to write using windows APIs, remember to clean up the System.Text.StringBuilder lpBuffer by deleting the junk data that remains concatenated.

You can do it using:

string text = lpBuffer.ToString().Substring(0, (int)nNumberOfBytesToWrite).Replace("\r\n", " ")

Tips & Tricks:

It is recommended that VirtualAlloc be used to allocate an unmanaged block of memory for use with this function. The array of file segments can be effeciently constructed without heap allocation using stackalloc. You will need to get the system's page size through a call to GetSystemInfo.

...and this will remove the junk data from lpBuffer

Sample Code:

// Prepares the array of file segments given an IntPtr[] ABuffers (only uses ACount of the buffers so that these arrays can be pooled)
// The size of each buffer (ASize) must be a multiple of the system's page size (get using GetSystemInfo)
int LPerBufferPageCount = ASize / FPageSize;
FILE_SEGMENT_ELEMENT* LElements = stackalloc FILE_SEGMENT_ELEMENT[(ACount * LPerBufferPageCount) + 1];
for (int LElementIndex = 0; LElementIndex < ACount; LElementIndex++)
{
  for (int LPageIndex = 0; LPageIndex < LPerBufferPageCount; LPageIndex++)
   LElements[(LElementIndex * LPerBufferPageCount) + LPageIndex].Buffer =
    (IntPtr)((uint)ABuffers[LElementIndex] + (LPageIndex * FPageSize));
}
LElements[(ACount * LPerBufferPageCount)].Buffer = IntPtr.Zero;

Alternative Managed API:

No managed API as of the 1.1 Framework.

VB Signature:

<DllImport("kernel32.dll", SetlastError:=True)> _
  Private Shared Function WriteFile(ByVal hFile As IntPtr, ByVal Buffer As IntPtr,
    ByVal nNumberOfBytesToWrite As Integer, ByRef lpNumberOfBytesWritten As Integer,
    ByRef lpOverlapped As OVERLAPPED) As <MarshalAs(UnmanagedType.Bool)> Boolean
  End Function

User-Defined Types:

None.

Notes:

The documentation states that if using unbuffered IO, the memory must be "storage aligned" (aligned to the sector size of the storage device). Either this is not enforced, or managed allocations (specifically byte arrays) are automatically storage aligned because there seems to be no problem using it. Note that storage aligned and page aligned are not the same, and managed allocations are not in general page aligned (required for WriteFileScatter).

In fact the buffer must be aligned to the granularity of the SCSI/IDE adapter's DMA alignment (PDEVICE_OBJECT->AlignmentRequirement). The MSDN documentation is written for the lamers and probably it seemed safe to restrict them using the sector size (which is >= 512 byte, while most modern SCSI cards use <=4 byte requirement).

For more information on unbuffered IO in .NET see:

http://arxiv.org/abs/cs.PF/0502012

Warning! GetOverlappedResult writes to the address of the buffer specified in the ORIGINAL OPERATION (ie/ ReadFile or WriteFile). .NET may move the address of the buffer before GetOverlappedResult returns, resulting in a buffer overflow. Use AllocHGlobal and FreeHGlobal or otherwise ensure the buffer is pinned between the two calls.

Tips & Tricks:

NET 2.0 use the SafeFileHandle version

If you are passing a System.Threading.NativeOverlapped struct in (for lpOverlapped) then there's no need to also call SetFilePointer or SetFilePointerEx. The offset in the OVERLAPPED structure trumps wherever the file position was previouly set to.

Sample Code:

  Public Sub Write(ByVal Packet As stArcnetPacket, ByVal DataLength As Int16)
    Dim dwErrorCode As FarcConstants
    Dim dwBytesWritten As Int32
    Dim ip As IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(Packet))
    Marshal.StructureToPtr(Packet, ip, True)
    If WriteFile(hDriver, ip, DataLength + 6, dwBytesWritten, Nothing) Then
      Marshal.FreeHGlobal(ip)
    Else
      dwErrorCode = Marshal.GetLastWin32Error
      Marshal.FreeHGlobal(ip)
      Throw New System.IO.IOException("Write fails. Errorcode: " & dwErrorCode.ToString, _
        New System.ComponentModel.Win32Exception(dwErrorCode))
    End If
  End Sub

Alternative Managed API:

System.IO.FileStream

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