1. The ‘fasync()’ driver-method Two viewpoints on implementing asynchronous notification: the application’s and the driver’s

2. Chapter 6 of LDD3 Our textbook’s chapter on Enhanced Char Drivers provides details on how programs can utilize asynchronous notification when it’s supported by the underlying drivers We will survey the essential elements and we will present a pair of demonstrations: –An application: ‘pipedemo.cpp’ –A device-driver: ‘nicasync.c’

3. The ‘fcntl()’ library-function #include // the header-file int fcntl( int fd, int cmd ); // gets info int fcntl( int fd, int cmd, int arg ); // sets info F_GETOWN and F_SETOWN// owner F_GETFL and F_SETFL// flags F_GETSIG and F_SETSIG// signal

4. The ‘sigaction()’ library-function #include // the header-file struct sigaction sa = {0}, oa;// data-object void action( int signo, siginfo_t *si, void *data ); sigemptyset( &sa.sa_mask ); sa.sa_sigaction = action; sa.sa_flags = SA_SIGINFO; sigaction( SIGnumber, &sa, &oa );

5. Enabling async notification int fd = open( devicename, O_RDWR ); fcntl( fd, F_SETOWN, getpid() ); int flags = fcntl( fd, F_GETFL ); fcntl( fd, F_SETFL, flags | FASYNC ); fcntl( fd, F_SETSIG, SIGnumber ); pause();// go to sleep until signaled

6. The device-driver duties #include // for kill_fasync() int my_signal = SIGIO;// default signal-ID struct fasync_struct *my_asqueue; int my_fasync( int fd, struct file *fp, int on ) { return fasync_helper( fd, fp, on, &my_asqueue ); } // plus a few modifications in other driver-methods

7. Interrupt-handler: ‘my_isr()’ Use this function to issue the notification when a new network-packet is received: kill_fasync( &asqueue, SIGno, POLL_IN ); Use this function to issue the notification when a packet-transmission completes: kill_fasync( &asqueue, SIGno, POLL_OUT );

8. ‘release()’ Use: fasync_helper( -1, file, 0, &asqueue ); // to disable asynchronous notifications for // this device-file’s descriptor

9. Non-blocking i/o, signal wakeups Our driver-modifications in ‘nicasync.c’ included support for non-blocking reads and non-blocking writes (one line each) Our driver-modifications in ‘nicasync.c’ now support the handling of signals that awaken tasks from read or write sleeps (i.e., if ( wait_event_interruptible() < 0 ) return -ERESTARTSYS

10. In-class exercise Compile and install our ‘nicasync.c’ Linux module (a RealTek driver with ‘fasync()’) Then write a revised version of our earlier ‘trychat.cpp’ demo-program that uses the asynchronous notification capability for handling the multiplexed input (from the network device and the keyboard device)

11. The ‘trychat’ packet-format dest’n hardware address (6 bytes) source hardware address (6 bytes) type (2 bytes) 0x0635 ethernet frame-header (14 bytes) count (2 bytes) nicchat protocol frame-data (50 bytes) network-order: big-endian local-order: little-endian