triggering user space with kernel

Question

I need to send a string from kernel to a user space function without asking for it in particular from the user space, sort of triggering a function or application in the use

Answer 1

Here's how my process works, I would be interested in any suggestions for improvements as well:

1. Start the kernel module

2. Start user space application, which sends a custom command to the kernel module to register the user space PID for kernel module signals. In my case this was via a write to /dev/mymodule. The kernel module registers the PID:

   ...
   printk("registering a new process id to receive signals: %d\n", current->pid);
   signal_pid = current->pid;
   ...
   

   The user space application also registers a handler with the kernel for certain types of signals.

   void local_sig_handler(int signum) {
           printf("received a signal from my module\n");
           fflush(stdout); }
   ...
   signal(SIGIO, local_sig_handler);
   

3. Kernel module generates a signal

   ...
   struct siginfo info;
   struct task_struct *t;
   info.si_signo=SIGIO;
   info.si_int=1;
   info.si_code = SI_QUEUE;        
   
   printk("<1>IRQ received: %d\n", irq);
   printk("<1>searching for task id: %d\n", signal_pid);
   
   t= pid_task(find_vpid(signal_pid),PIDTYPE_PID);//user_pid has been fetched successfully
   
   if(t == NULL){
           printk("<1>no such pid, cannot send signal\n");
   } else {
           printk("<1>found the task, sending signal\n");
           send_sig_info(SIGIO, &info, t);
   }
   

4. Kernel relays the signal to the application's handler