Asynchronous I/O

• Asynchronous I/O means allowing a process to start an I/O operation and proceeding with other work while the I/O occurs

• UNIX I/O occurs asynchronously if you use select

• A process asks the select system call to tell which of a collection of file descriptors is ready to finish I/O.

• After calling select the process can call read or write to perform I/O which is at that time no more than a copying of data to/from kernel space with real I/O either already done or scheduled for later.

• A server process can use select to determine which of a collection of sockets it can read without blocking.