Application I/O Interface

4. Blocking vs. Nonblocking devices.
   • This is about whether a program should wait for I/O to be ready, or let the program continue executing other instructions, without waiting for the I/O data transfer to finish.
   • With blocking I/O, a program is moved to the "wait" queue when an I/O request is made, and moved back to the ready queue (to continue running) when the request completes, allowing other processes to run in the meantime while the current program waits.
   • With nonblocking I/O, the I/O request returns immediately, whether the requested I/O operation has (completely) occurred or not. This allows the program to check for available data without getting hung completely if it is not there.
   • A nonblocking I/O call returns quickly, with a return value that indicates how many bytes were transferred.