Writing Device Drivers

Differences Between Synchronous and Asynchronous I/O

Data transfers can be synchronous or asynchronous. The determining factor is whether the entry point that schedules the transfer returns immediately or waits until the I/O has been completed.

The read(9E) and write(9E) entry points are synchronous entry points. The transfer must not return until the I/O is complete. Upon return from the routines, the process knows whether the transfer has succeeded.

The aread(9E) and awrite(9E) entry points are asynchronous entry points. Asynchronous entry points schedule the I/O and return immediately. Upon return, the process that issues the request knows that the I/O is scheduled and that the status of the I/O must be determined later. In the meantime, the process can perform other operations.

With an asynchronous I/O request to the kernel, the process is not required to wait while the I/O is in process. A process can perform multiple I/O requests and allow the kernel to handle the data transfer details. Asynchronous I/O requests enable applications such as transaction processing to use concurrent programming methods to increase performance or response time. Any performance boost for applications that use asynchronous I/O, however, comes at the expense of greater programming complexity.