Day52 - fasync, SIGIO, and eventfd Integration¶

Goal¶

Learn how to:

implement .fasync in a char driver
send SIGIO from kernel driver
receive asynchronous notification in userspace
bridge SIGIO into epoll using eventfd
build a unified event-driven userspace loop

1. Driver-Side fasync Support¶

Device Context¶

Add async queue:

struct fasync_struct *async_queue;

File Operations¶

Add:

.fasync = mypoll_fasync,

fasync Handler¶

static int mypoll_fasync(int fd,
                         struct file *file,
                         int on)
{
    struct mypoll_dev *mydev;

    mydev = file->private_data;

    if (!mydev)
        return -ENODEV;

    return fasync_helper(fd,
                         file,
                         on,
                         &mydev->async_queue);
}

Release Cleanup¶

mypoll_fasync(-1, file, 0);

2. Send SIGIO from Driver¶

Inside event generation path:

wake_up_interruptible(&mydev->read_queue);

kill_fasync(&mydev->async_queue,
            SIGIO,
            POLL_IN);

3. Minimal SIGIO Userspace Test¶

Userspace Flow¶

open("/dev/mypoll")
    ↓
install SIGIO handler
    ↓
F_SETOWN
    ↓
O_ASYNC
    ↓
pause()

SIGIO Handler¶

static void sigio_handler(int signo)
{
    sigio_received = 1;
}

4. Nonblocking Event Read¶

Userspace enables:

O_NONBLOCK

and drains events until:

EAGAIN

5. eventfd Integration¶

Create eventfd¶

eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC)

Signal Handler¶

Signal handler only performs:

write(eventfd)

Example:

static void sigio_handler(int signo)
{
    uint64_t value = 1;

    (void)signo;

    write(g_event_fd, &value, sizeof(value));
}

6. epoll + eventfd¶

epoll monitors eventfd:

epoll_wait()
    ↓
read(eventfd)
    ↓
drain /dev/mypoll

7. Final Architecture¶

kernel event
    ↓
mypoll_generate_event()
    ↓
queue push
    ↓
wake_up_interruptible()
    ↓
kill_fasync()
    ↓
SIGIO
    ↓
signal handler
    ↓
write(eventfd)
    ↓
epoll_wait()
    ↓
read(eventfd)
    ↓
read(/dev/mypoll)

8. Build¶

gcc -Wall -Wextra -o sigio_eventfd_epoll sigio_eventfd_epoll.c

9. Validation Checklist¶

SIGIO received successfully
eventfd wakeup works
epoll_wait wakes correctly
userspace drains driver queue
timer events received
manual trigger events received
no blocking inside signal handler

10. Files¶

Suggested files:

sigio_test.c
sigio_eventfd_epoll.c