Handling multiple signals

Question

I have a question about handling a signal. Assume that if we recieve SIGINT signal, we should print "Recieved Signal". If within ten seconds the handler recieves another signal, it should print "Shutting Down" then exit with status 1.

I made my code like this:

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

void  handler(int);
void  secondhandler(int);
void  alrmhandler(int);

void alrmhandler (int alrmsig)
{  
  alarm(0);     
}

void secondhandler(int sig)
{
 /* after recieving second signal prints shutting down and exit */
 printf("Shutting Down\n");
 exit(1);
}

void handler ( int sig )  
{  
   /* recieve first SIGINT signal */
   printf ("Recieved Signal\n");
   /* handle for the alarm function */
   signal(SIGALRM, alrmhandler);
   /* start 10s alarm */
    alarm(10);
   /* catch second SIGINT signal within 10s*/
   signal(SIGINT, secondhandler);

 }



 int main( void )
 {
  signal(SIGINT, handler);
      printf( "Hello World!\n" );
  for ( ;; )
  {
    /* infinite loop */
  }

  return 0;
 }

I tried to compile it with dev c++, but it failed. Because SIGALRM undeclared(first use in this function).

Anyway, what I want to know is if this code is right. I actually kinda not sure with the alrmhandler(). should I ignore the SIGALRM?

Change your for-loop content to `sleep(1);` instead of having it empty, so that other processes get a chance to run, too. — ott--, Dec 01 '11 at 15:23

score 3 · Answer 1 · answered Dec 01 '11 at 15:12

3

If you are on a Windows platform, the only signals you will be able to send are : SIGABRT, SIGFPE, SIGILL, SIGINT, SIGSEGV, or SIGTERM.

answered Dec 01 '11 at 15:12

Cédric Julien

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OK, thx. But if I run this in linux, it should be working, shouldn't it? – user1075627 Dec 01 '11 at 16:20
@user1075627 : yes, the compilation should be working on a linux box, because the SIGALRM is defined in `signal.h`. – Cédric Julien Dec 01 '11 at 16:23

score 1 · Answer 2 · edited Apr 13 '17 at 12:40

You write:

what I want to know is if this code is right.

Not entirely. printf() is not async-signal-safe, and so should not be called from within a signal handler unless you are very sure it is safe to do so. It is not safe to do so within the code you provide.

The alarm() technique is, generally, race-prone. Your ten second alarm might expire in the middle of your secondhandler() function. To guard against this, you might mask out signals to compensate with a more sophisticated signal manipulation function.

There are more elegant/flexible ways of implementing the timeout you desire, but that's perhaps a question better suited for codereview.stackexchange.com.

Handling multiple signals

2 Answers2