IOCOMPLETIONROUTINE

IOCOMPLETIONROUTINE is a callback function used in Windows operating system programming to handle asynchronous I/O operations. When an asynchronous I/O request is issued to the operating system, a pointer to an IOCOMPLETIONROUTINE function can be provided. This routine is then invoked by the system once the I/O operation has completed, whether successfully or with an error.

The primary purpose of an IOCOMPLETIONROUTINE is to allow an application to perform actions after an I/O operation finishes without blocking the main thread of execution. This is crucial for maintaining responsiveness in applications, especially those with graphical user interfaces or high-throughput requirements. The routine receives information about the completed I/O operation, such as the number of bytes transferred and the status of the operation.

When an asynchronous I/O operation is initiated using functions like ReadFileEx or WriteFileEx, the application can specify the address of the IOCOMPLETIONROUTINE. The operating system's I/O manager calls this routine when the I/O operation completes. This allows the application to process the data, update its state, or initiate further operations based on the outcome of the I/O. The routine itself can also signal the completion event or perform other necessary cleanup. This mechanism is a fundamental part of efficient asynchronous I/O handling in Windows.