Linux Device Drivers

Asynchronous Notification

Though the combination of blocking and nonblocking operations and the select method are sufficient for querying the device most of the time, some situations aren’t efficiently managed by the techniques we’ve seen so far. Let’s imagine, for example, a process that executes a long computational loop at low priority, but needs to process incoming data as soon as possible. If the input channel is the keyboard, you are allowed to send a signal to the application (using the ``INTR'' character, usually Ctrl-C), but this signalling ability is part of the tty layer, which isn’t attached to general char devices. What we need for asynchronous notification is something different. Furthermore, any input data should generate an interrupt, not just Ctrl-C.

User programs have to execute two steps to enable asynchronous notification from an input file. First, they specify a process as the ``owner'' of the file. The user ID of a file’s owner is stored in filp->f_owner by the fcntl system call when an application invokes the F_SETOWN command. Additionally, the user programs must set the FASYNC flag in the device by means of another fcntl in order to actually enable asynchronous notification.

After these two calls have been executed, the input file generates a SIGIO signal whenever new data arrives. The signal is sent to the process (or process group, if the value is negative) stored in filp->f_owner.

For example, the following lines enable asynchronous notification to ...