CODEWITHSUNDEEP
cinterrupt-handlingsigintsignal-handling
bardao
bardao

Reputation: 954

What is si_int in siginfo and how does it affect signal propagation

As part of sending interrupts to user space I am seeing the following code snippet around but I don't understand how si_int affects the signal propagation.

static int SendToUsrSpace(void) {
  struct siginfo info;
  struct task_struct *t;
  int ret;

  memset(&info, 0, sizeof(struct siginfo));
  info.si_signo = 44;
  info.si_code = SI_QUEUE;
  info.si_int = 1234; // WHY NOT 36 or any random number?
  [...]
  ret = send_sig_info(44, &info, t);
  [...]
}

Upvotes: 2

Views: 975

Answers (1)

jhelphenstine
jhelphenstine

Reputation: 423

I think the explanation is in sigqueue(3). From the text:

       sigqueue() sends the signal specified in sig to the process whose PID
       is given in pid.  The permissions required to send a signal are the
       same as for kill(2).  As with kill(2), the null signal (0) can be
       used to check if a process with a given PID exists.

       The value argument is used to specify an accompanying item of data
       (either an integer or a pointer value) to be sent with the signal,
       and has the following type:

           union sigval {
               int   sival_int;
               void *sival_ptr;
           };

       If the receiving process has installed a handler for this signal
       using the SA_SIGINFO flag to sigaction(2), then it can obtain this
       data via the si_value field of the siginfo_t structure passed as the
       second argument to the handler.  Furthermore, the si_code field of
       that structure will be set to SI_QUEUE.

So, 1234 looks like an arbitrary value that a process can pull (if it has a handler for this signal). It looks like a way to send data with the signal.

Additional reading: I arrived at sigqueue from sigaction(2), which lays out the data type for siginfo_t.

Upvotes: 1

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