[ACCEPTED]-UNIX/Linux signal handling: SIGEV_THREAD-signals

Accepted answer
Score: 17

struct sigevent is not about specifying how the process 19 will handle a signal - struct sigaction and sigaction() are how you 18 do that. Instead, struct sigevent is used to specify how 17 your process will be informed of some asychronous 16 event - like the completion of asychronous 15 IO, or a timer expiring.

The sigev_notify field specifies 14 how the event should be notified:

  • SIGEV_NONE - no notification at all. The remainder of the fields are ignored.
  • SIGEV_SIGNAL - a signal is sent to the process. The sigev_signo field specifies the signal, the sigev_value field contains supplementary data that is passed to the signal handling function, and the remainder of the fields are ignored.
  • SIGEV_THREAD - a function is called in a new thread. The sigev_notify_function field specifies the function that is called, sigev_value contains supplementary data that is passed to the function, and sigev_notify_attributes specifies thread attributes to use for the thread creation. The remainder of the fields are ignored.

Note in 13 particular that if you set SIGEV_THREAD, the sigev_signo field 12 is ignored - the struct sigevent is about specifying either a 11 thread or a signal as a notification method, not 10 about specifying a thread as the way that 9 a signal should be handled.

The struct sigevent must also 8 be passed to a function - like timer_create() - that sets 7 up the asychronous event that will be notified. Simply 6 creating a struct sigevent object does not do anything 5 special.

If you wish to use a dedicated thread 4 to handle a signal, create the thread up 3 front and have it loop around, blocking 2 on sigwaitinfo(). Use sigprocmask() to block the signal in every 1 other thread.

Score: 2

I think you are mixing up your signal handling 10 idioms here, you create a sigevent structure and 9 then do nothing with it and then use signal() within 8 the signal handler. The following code shows 7 a very simple signal handling routine based 6 on your code; note that I have changed the 5 definition of my_handler. If you need more sophisticated 4 handling then sigaction() is probably the system call 3 you need to look into.

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>
#include <time.h>

void my_handler(int sig)
 printf("my_handler caught\n");

int main()
 kill(0,SIGRTMIN); // This should invoke the signal and call the function
 while(1) ;  // Infinite loop in case the program ends before the signal gets caught!

This works under 2 cygwin on my windows box (no access to a linux 1 box at the minute).

Score: 0

I hope this works.

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>
#include <time.h>

my_handler (int sig)
    printf ("my_handler caught\n");
    signal (sig, my_handler);

main ()
    int signo;
    struct sigevent sevp;
    sigset_t set;

    if (sigemptyset (&set) == -1)
        perror ("sigemptyset");
    if (sigaddset (&set, SIGRTMIN) == -1)
        perror ("sigaddset");
    if (sigprocmask (SIG_BLOCK, &set, NULL) == -1)
        perror ("sigprocmask");

     sevp.sigev_notify = SIGEV_THREAD;
     sevp.sigev_signo = SIGRTMIN;
     sevp.sigev_value.sival_ptr = NULL;
     kill (0, SIGRTMIN);
     if (sigwait (&set, &signo) == 0)
         my_handler (signo);
         perror ("sigwait");


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