Overlapped I/O

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Overlapped I/O is an asynchronous I/O extension of the Windows APIs, which was introduced in Windows NT.

Utilizing overlapped I/O requires passing an OVERLAPPED structure to API functions that normally block, including ReadFile(), WriteFile(), and Winsock's WSASend() and WSARecv(). The requested operation is initiated by a function call which returns immediately, and is completed by the OS in the background. The caller may optionally specify a Win32 event handle to be raised when the operation completes. Alternatively, a program may receive notification of an event via an I/O completion port, which is the preferred method of receiving notification when used in symmetric multiprocessing environments or when handling I/O on a large number of files or sockets. Overlapped I/O is particularly useful for sockets and pipes. It is not well supported in Windows 9x.

The Unix equivalent of overlapped I/O is the POSIX asynchronous I/O API (AIO).

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