OVERLAPPED structure

Contains information used in asynchronous (overlapped) input/output (I/O).

Syntax

typedef struct _OVERLAPPED {
  ULONG_PTR Internal;
  ULONG_PTR InternalHigh;
  union {
    struct {
      DWORD Offset;
      DWORD OffsetHigh;
    };
    PVOID  Pointer;
  };
  handle    hEvent;
} OVERLAPPED, *LPOVERLAPPED;

Members

Internal

Type: ULONG_PTR

Reserved for operating system use. This member, which specifies a system-dependent status, is valid when the IStreamAsync::OverlappedResult function returns without setting the extended error information to ERROR_IO_PENDING.

InternalHigh

Type: ULONG_PTR

Reserved for operating system use. This member, which specifies the length of the data transferred, is valid when the IStreamAsync::OverlappedResult function returns TRUE.

Offset

Type: DWORD

File position at which to start the transfer. The file position is a byte offset from the start of the file. The calling process must set this member before it calls the IStreamAsync::ReadAsync or IStreamAsync::WriteAsync function.

OffsetHigh

Type: DWORD

High-order word of the file position at which to start the transfer.

Pointer

Type: PVOID

Reserved.

hEvent

Type: handle

Handle to an event that is set to the signaled state when the operation has been completed. The calling process must set this member either to zero or a valid event handle before it calls any overlapped functions. To create an event object, use the CreateEvent function. This function returns a handle that can be used to synchronize simultaneous I/O requests for a device.

Functions such as IStreamAsync::ReadAsync and IStreamAsync::WriteAsync set this handle to the nonsignaled state before they begin an I/O operation. When the operation has completed, the handle is set to the signaled state.

Functions such as IStreamAsync::OverlappedResult and the wait functions reset auto-reset events to the nonsignaled state. Therefore, if an auto-reset event is used, the application can stop responding if it waits for the operation to complete and then calls IStreamAsync::OverlappedResult.

Remarks

This structure should always be initialized to zero before it is used in a function call. If it is not, the function can fail and return ERROR_INVALID_PARAMETER.

Use the IStreamAsync::CancelIo function to cancel an asynchronous I/O operation.

A common mistake is to reuse an OVERLAPPED structure before the previous asynchronous operation has been completed. Use a separate structure for each request. Create an event object for each thread that processes data.

Requirements