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How to use SIGEV_THREAD, sigevent for linux-timer(s) expiration handling in C?

Problem: I have timer(s) running, upon expiration of timer(s) certain function needs to be invoked. Output: There is a segfault inside Hndlr() function

As per man page of sigevent, it says,

SIGEV_THREAD – Notify the process by invoking sigev_notify_function “as if” it were the start function of a new thread. (Among the implement‐ tation possibilities here are that each timer notification could result in the creation of a new thread, or that a single thread is created to receive all notifications.)

The function (sigev_notify_function) is invoked with sigev_value as its sole argument

I did refer to this: UNIX/Linux signal handling: SIGEV_THREAD and it says,

sigev_value contains supplementary data that is passed to the function

So, I have written the following,

typedef struct Info
{
enum Status
{
    Start = 1,
    Expire = 2
} TimerStatus;

int data;
timer_t timerId;
} Info_t;

void Hndlr(union sigval *sv)
   { 
       //Upon expiry I want to set this value of t1.TimerStatus Expire:
       //t1.TimerStatus = Expire;
       //So I have done this:
       sv->sival_int = Expire;
   }

   int TimerInit(Info_t *Type)
   {
       struct sigevent sev;
       sev.sigev_notify = SIGEV_THREAD;
       sev.sigev_value.sival_ptr = Type->timerId;
       sev.sigev_value.sival_int = Type->TimerStatus;
       sev.sigev_notify_function = &Hndlr;
       sev.sigev_notify_attributes = 0;

       timer_create(CLOCK_REALTIME, &sev, &(Type->timerId));
 }

 //other code

int main(int argc, char const *argv[])
{
Info_t t1;
t1.TimerStatus = Start;

TimerInit(&t1);
//start timer
//other code
while (1)
{

    if (t1.TimerStatus == Expire)
    {
        //do something, invoke a function
    }
}

return 0;

}

Warning: assignment to ‘void (*)(__sigval_t)’ {aka ‘void (*)(union sigval)’} from 
incompatible pointer type ‘void (*)(union sigval *)’ [-Wincompatible-pointer-types]
     sev.sigev_notify_function = &Hndlr; 

Since, I am using union sigval *sv in Hndlr, I am receiving this warning.

Q) How to I pass enum type to Hndlr as pass-by-ptr and change it, ie., t1.TimerStatus = Expire

PS: I haven’t included the entire code involving timer_set() etc, and it also involves multiple instances of timer. So, How can I achieve this functionality (Q) ?

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Answer

A few mistakes in the code:

  • Wrong function prototype for timer expiry function Hndlr.
  • Setting all members of union sigval, whereas only one member must be set.
  • A variable modified and read in another thread must be atomic.

A working example (compiler options -std=c11 -pthread -W{all,extra}, linker options -std=c11 -pthread -lrt):

#include <stdatomic.h>
#include <stdlib.h>
#include <signal.h>
#include <time.h>

enum Status {
    Start = 1,
    Expire = 2
};

typedef struct {
    atomic_int status;
    int data;
    timer_t timerId;
} Info_t;

static void Hndlr(union sigval sigev_value) {
    Info_t* info = sigev_value.sival_ptr;
    atomic_store(&info->status, Expire);
}

void TimerInit(Info_t* info, unsigned seconds) {
    int r;
    struct sigevent sev;
    struct itimerspec its;

    sev.sigev_notify = SIGEV_THREAD;
    sev.sigev_value.sival_ptr = info;
    sev.sigev_notify_function = &Hndlr;
    sev.sigev_notify_attributes = 0;
    r = timer_create(CLOCK_REALTIME, &sev, &info->timerId);
    if(r)
        abort();

    its.it_interval.tv_sec = 0;
    its.it_interval.tv_nsec = 0;
    its.it_value.tv_sec = seconds;
    its.it_value.tv_nsec = 0;
    r = timer_settime(info->timerId, 0, &its, NULL);
    if(r)
        abort();
}

int main() {
    Info_t t1;
    t1.status = Start;
    TimerInit(&t1, 3);
    while(atomic_load(&t1.status) != Expire)
        ;

    return 0;
}

In this particular usage, when the timer callback function just stores into a variable, there is no need to use another thread with SIGEV_THREAD, SIGEV_SIGNAL would work just as well (setup code changes are required), as long as blocking functions that can be interrupted with the signal handle EINTR.

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