title: Problem with signals, timer (SIGEV_SIGNAL)
body: <p>I have a small piece of code, where i have used a POSIX timer: timer_create( ). The timer is created using the signal approach (SIGEV_SIGNAL) - as SIGEV_THREAD not supported on our platform. When the timer expires, it generates a signal, SIGUSR1, to notify of it's expiration, and there is a corresponding handler, to catch this signal, inside this handler (in actual program, not shown on code) i have a flag which sets, once the signal given by timer is caught.</p>
  
  <p>Uptill this, everything is fine: <strong>Problem is, suppose if the test program also generates same signal as the timer (SIGUSR1 in this case), then the same flag is set, not by timer. So there is no way to discriminate, wether the signal received at signal handler is that of a timer, or any other test program.</strong></p>
  
  <p>Can you help me, to resolve this issue?</p>
  
  <p>Thanks in advance.</p>
  
  <pre><code>enter code here
  #include &lt;stdlib.h&gt;
  #include &lt;time.h&gt;
  #include &lt;stdio.h&gt;
  #include &lt;signal.h&gt;
  #include &lt;errno.h&gt;
  #include &lt;string.h&gt;
  
  
  void sig_handlerTimer1(int);
  
  time_t timerid;
  int main()
  {
       int i;
       static struct sigaction sa;
  
  
       static struct sigevent sevp;  // argument to timer_create
       static struct itimerspec its; // argument to timer_gettime
  
       memset (&amp;sevp, 0, sizeof (struct sigevent));
       sevp.sigev_value.sival_ptr = &amp;timerid;
       sevp.sigev_notify = SIGEV_SIGNAL;
       sevp.sigev_notify_attributes = NULL;
       sevp.sigev_signo = SIGUSR1;
       sevp.sigev_notify_function=sig_handlerTimer1;
  
       /* Setting timer interval */
       its.it_interval.tv_sec = 0;
       its.it_interval.tv_nsec = 0;
  
       /* Setting timer expiration */
       its.it_value.tv_sec = 2;  // First expiry after 1 sec
       its.it_value.tv_nsec = 0;
  
       /* Setting the signal handlers before invoking timer*/
       sa.sa_handler = sig_handlerTimer1;
       sa.sa_flags = 0;
       sigaction(SIGUSR1, &amp;sa, NULL);
  
       if (timer_create(CLOCK_REALTIME, &amp;sevp, &amp;timerid) == -1)
       {
               fprintf(stderr, "LeakTracer (timer_trackStartTime): timer_create failed   to create timer. " \
                      "Leak measurement will be for entire duration of the execution   period:%s \n", strerror(errno));
              return;
  
        }
  
       if (timer_settime(timerid, 0, &amp;its, NULL) == -1)
       {
               fprintf(stderr, "LeakTracer (timer_trackStartTime): timer_settime failed  to set the timer. " \
                      "Leak measurement will be for entire duration of execution period:%s \n", strerror(errno));
               return;
  
        }
  
        for(i=0; i&lt;10; i++)
        {
               printf("%d\n",i);
               if(i==3) {
                   kill(getpid(), SIGUSR1); // SIGUSR1 also generated by test program  which reaches same handler and sets flag (THIS IS UN-DESIRABLE)
                }
               sleep(1);
         }
  
    }
  
    void sig_handlerTimer1(int signum)
    {
  
       int flag = 1;
       printf("Caught signal: %d\n",signum); // How to understand this signal caught, is that of test program of timer expiry?
       if (timer_delete(timerid) &lt; 0)
       {
               fprintf(stderr, "timer deletion failed. " \
                       "This may result in some memory leaks (sig_handlerTimer1):%s \n", strerror(errno));
        }
     }
  </code></pre>
  
  <p>Added: <strong>Is there, any way to know inside the handler wether the timer has really expired or not, once signal is caught?</strong></p>