lpOverlapped

lpOverlapped is a common name used in Windows API programming to refer to a pointer to an OVERLAPPED structure, i.e., LPOVERLAPPED. This pointer is used to support asynchronous (overlapped) input/output operations in many file and device I/O functions, such as ReadFile, WriteFile, ReadFileEx, and WriteFileEx. The lpOverlapped parameter allows a caller to initiate an I/O operation without blocking the calling thread and to obtain completion status later.

The OVERLAPPED structure contains fields that the system uses to track and complete the asynchronous operation. It includes Internal and InternalHigh, which the I/O subsystem uses to store status information. It also includes a 64-bit offset represented by Offset and OffsetHigh, which specifies the starting byte offset for the operation on byte-addressable devices, or a Pointer that can be used by some I/O types. The hEvent member is a handle to an event object that can be signaled when the operation completes. In addition to single-threaded event signaling, overlapped I/O can be coordinated with I/O completion ports for scalable asynchronous processing.

In typical usage, a function may return immediately with ERROR_IO_PENDING if the operation is initiated asynchronously. The application can wait on the event referenced by hEvent, or retrieve completion through an I/O completion port or by calling GetOverlappedResult, to determine the final status and the number of bytes transferred. The memory backing the OVERLAPPED structure and any associated buffers must remain valid until completion.

See also: OVERLAPPED, LPOVERLAPPED, ReadFile, WriteFile, GetOverlappedResult, Io completion ports.