Read chardevice with libevent - c

Question

I wrote a chardevice that passes some messages received from the network to an user space application. The user space application has to both read the chardevice and send/receive messages via TCP sockets to other user-space applications. Both read and receiving should be blocking.
Since Libevent is able to handle multiple events at the same time, I thought registering an event for the file created by the chardevice and an event for a socket would just work, but I was wrong.
But a chardevice creates a "character special file", and libevent seems to not be able to block. If I implement a blocking mechanism inside the chardevice, i.e. mutex or semaphore, then the socket event blocks too, and the application cannot receive messages.
The user space application has to accept outside connections at any time.
Do you know how to make it work? Maybe also using another library, I just want a blocking behaviour for both socket and file reader.
Thank you in advance.
Update: Thanks to #Ahmed Masud for the help. This is what I've done
Kernel module chardevice:
Implement a poll function that waits until new data is available
struct file_operations fops = {
...
.read = kdev_read,
.poll = kdev_poll,
};
I have a global variable to handle if the user space has to stop, and a wait queue:
static working = 1;
static wait_queue_head_t access_wait;
This is the read function, I return -1 if there is an error in copy_to_user, > 0 if everything went well, and 0 if the module has to stop. used_buff is atomic since it handles the size of a buffer shared read by user application and written by kernel module.
ssize_t
kdev_read(struct file* filep, char* buffer, size_t len, loff_t* offset)
{
int error_count;
if (signal_pending(current) || !working) { // user called sigint
return 0;
}
atomic_dec(&used_buf);
size_t llen = sizeof(struct user_msg) + msg_buf[first_buf]->size;
error_count = copy_to_user(buffer, (char*)msg_buf[first_buf], llen);
if (error_count != 0) {
atomic_inc(&used_buf);
paxerr("send fewer characters to the user");
return error_count;
} else
first_buf = (first_buf + 1) % BUFFER_SIZE;
return llen;
}
When there is data to read, I simply increment used_buf and call wake_up_interruptible(&access_wait).
This is the poll function, I just wait until the used_buff is > 0
unsigned int
kdev_poll(struct file* file, poll_table* wait)
{
poll_wait(file, &access_wait, wait);
if (atomic_read(&used_buf) > 0)
return POLLIN | POLLRDNORM;
return 0;
}
Now, the problem here is that if I unload the module while the user space application is waiting, the latter will go into a blocked state and it won't be possible to stop it. That's why I wake up the application when the module is unloaded
void
kdevchar_exit(void)
{
working = 0;
atomic_inc(&used_buf); // increase buffer size to application is unlocked
wake_up_interruptible(&access_wait); // wake up application, but this time read will return 0 since working = 0;
... // unregister everything
}
User space application
Libevent by default uses polling, so simply create an event_base and a reader event.
base = event_base_new();
filep = open(fname, O_RDWR | O_NONBLOCK, 0);
evread = event_new(base, filep, EV_READ | EV_PERSIST,
on_read_file, base);
where on_read_file simply reads the file, no poll call is made (libevent handles that):
static void
on_read_file(evutil_socket_t fd, short event, void* arg)
{
struct event_base* base = arg;
int len = read(...);
if (len < 0)
return;
if (len == 0) {
printf("Stopped by kernel module\n");
event_base_loopbreak(base);
return;
}
... // handle message
}