Asynchronous Input and Output

Asynchronous input and output (asynchronous I/O) allows some I/O functions to return immediately, even though an I/O request is still pending. Asynchronous I/O enables an application to continue with other processing and wait for the I/O to be completed at a later time. Asynchronous I/O is also called overlapped I/O.

The ReadFile and WriteFile functions enable an application to specify an OVERLAPPED structure that indicates where to position the file pointer before the read or write operation. The handle of the file being read from or written to must have been opened with the FILE_FLAG_OVERLAPPED flag. You can also create an event and put the handle in the OVERLAPPED structure; the wait functions can then be used to wait for the I/O operation to complete by waiting on the event handle.

An application can also wait on the file handle to synchronize the completion of an I/O operation, but doing so requires extreme caution. Each time an I/O operation is started, the operating system sets the file handle to the nonsignaled state. Each time an I/O operation is completed, the operating system sets the file handle to the signaled state. Therefore, if an application starts two I/O operations and waits on the file handle, there is no way to determine which operation is finished when the handle is set to the signaled state. If an application must perform multiple asynchronous I/O operations on a single file, it should wait on the event handle in the OVERLAPPED structure for each I/O operation, rather than on the file handle.

To cancel all pending asynchronous I/O operations, use the CancelIo function. This function only cancels operations issued by the calling thread for the specified file handle.

The ReadFileEx and WriteFileEx functions enable an application to specify a routine to execute (see FileIOCompletionRoutine) when the asynchronous I/O request is completed.

For more information, see Synchronization and Overlapped Input and Output.