Asynchronous I/O
form of input/output processing
Asynchronous I/O is a form of input/output processing that allows a program to initiate an I/O operation and continue processing other tasks before the I/O operation completes. Unlike non‑blocking I/O, which returns control immediately but may require repeated polling, asynchronous I/O enables the system or API to notify the program when the operation finishes. The term “overlapped I/O” is used for asynchronous I/O in the Windows API.
Input and output operations on a computer can be extremely slow compared to the processing of data. An I/O device can incorporate mechanical devices that must physically move, such as a hard drive seeking a track to read or write; this is often orders of magnitude slower than the switching of electric current. For example, during a disk operation that takes ten milliseconds to perform, a processor that is clocked at one gigahertz could have performed ten million instruction-processing cycles.
A simple approach to I/O would be to start the access and then wait for it to complete. But such an approach, called synchronous I/O or blocking I/O, would block the progress of a program while the communication is in progress, leaving system resources idle. When a program makes many I/O operations (such as a program mainly or largely dependent on user input), this means that the processor can spend almost all of its time idle waiting for I/O operations to complete.
Alternatively, it is possible to start the communication and then perform processing that does not require that the I/O be completed. This approach is called asynchronous input/output.
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