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Top Forums Programming signal handling question Post 302199447 by fox_hound_33 on Tuesday 27th of May 2008 01:00:12 AM
Old 05-27-2008
signal handling question
Hello all,

I am starting to learn signal handling in Linux and have been trying out some simple codes to deal with SIGALRM. The code shown below sets a timer to count down. When the timer is finished a SIGALRM is produced. The handler for the signal just increments a variable called count. This is repeated until the user hits ‘q' in the keyboard. The code is shown below:
Code:
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/time.h>
#include <stdlib.h>

void my_action(int);

int count = 0;

int main(int argc, char** argv)
{
	struct sigaction sigalrm_action;
	struct itimerval timer;
	
	timer.it_interval.tv_sec = 0;	//Deal only in usec
	timer.it_interval.tv_usec = 1000;
	timer.it_value.tv_sec = 0;	//Deal only in usec
	timer.it_value.tv_usec = 1000;	

	sigalrm_action.sa_handler  = my_action;	
	sigemptyset(&sigalrm_action.sa_mask);
	sigalrm_action.sa_flags = 0;
	
	sigaction(SIGALRM, &sigalrm_action, 0);				

	printf("Hit any key to start, q to exit\n");	
	getchar();	
    	
	if(setitimer(ITIMER_REAL, &timer,NULL) != 0){
		perror("Error starting timer");
		exit(1);
	}    	
	while(getchar()!= 'q');	
	printf("Bye bye\n");
	return 0;
}

void my_action(int signum)
{	
	count++;
	printf("Count is %d\n", count);	
}

The problem I am facing is this, when I set the timer for 1000000usec it works fine (i.e 1sec). However if I keep reducing the usec time to 100000, 10000, 1000 etc the timing seems to be too slow. The count variable is not being incremented as fast as it should be. Why is this? I have a hunch I am doing some silly mistake here but I am not sure what it is.

Any help would be greatly appreciated.
Last edited by Yogesh Sawant; 05-27-2008 at 03:11 AM.. Reason: added code tags
 

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LEARN ABOUT REDHAT

setitimer

GETITIMER(2)						     Linux Programmer's Manual						      GETITIMER(2)

NAME

       getitimer, setitimer - get or set value of an interval timer

SYNOPSIS

       #include <sys/time.h>

       int getitimer(int which, struct itimerval *value);
       int setitimer(int which, const struct itimerval *value, struct itimerval *ovalue);

DESCRIPTION

       The system provides each process with three interval timers, each decrementing in a distinct time domain.  When any timer expires, a signal
       is sent to the process, and the timer (potentially) restarts.

       ITIMER_REAL    decrements in real time, and delivers SIGALRM upon expiration.

       ITIMER_VIRTUAL decrements only when the process is executing, and delivers SIGVTALRM upon expiration.

       ITIMER_PROF    decrements both when the process executes and when the  system  is  executing  on  behalf  of  the  process.   Coupled  with
		      ITIMER_VIRTUAL,  this  timer is usually used to profile the time spent by the application in user and kernel space.  SIGPROF
		      is delivered upon expiration.

       Timer values are defined by the following structures:
	    struct itimerval {
		struct timeval it_interval; /* next value */
		struct timeval it_value;    /* current value */
	    };
	    struct timeval {
		long tv_sec;		    /* seconds */
		long tv_usec;		    /* microseconds */
	    };

       The function getitimer fills the structure indicated by value  with  the  current  setting  for	the  timer  indicated  by  which  (one	of
       ITIMER_REAL,  ITIMER_VIRTUAL,  or  ITIMER_PROF).   The element it_value is set to the amount of time remaining on the timer, or zero if the
       timer is disabled.  Similarly, it_interval is set to the reset value.  The function setitimer sets the indicated  timer	to  the  value	in
       value.  If ovalue is nonzero, the old value of the timer is stored there.

       Timers decrement from it_value to zero, generate a signal, and reset to it_interval.  A timer which is set to zero (it_value is zero or the
       timer expires and it_interval is zero) stops.

       Both tv_sec and tv_usec are significant in determining the duration of a timer.

       Timers will never expire before the requested time, instead expiring some short, constant time afterwards, dependent on	the  system  timer
       resolution  (currently  10ms).  Upon expiration, a signal will be generated and the timer reset.  If the timer expires while the process is
       active (always true for ITIMER_VIRT) the signal will be delivered immediately when generated.  Otherwise the delivery will be offset  by  a
       small time dependent on the system loading.

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EFAULT value or ovalue are not valid pointers.

       EINVAL which is not one of ITIMER_REAL, ITIMER_VIRT, or ITIMER_PROF.

CONFORMING TO

       SVr4, 4.4BSD (This call first appeared in 4.2BSD).

SEE ALSO

       gettimeofday(2), sigaction(2), signal(2)

BUGS

       Under  Linux,  the  generation  and delivery of a signal are distinct, and there each signal is permitted only one outstanding event.  It's
       therefore conceivable that under pathologically heavy loading, ITIMER_REAL will expire before the signal from  a  previous  expiration  has
       been delivered.	The second signal in such an event will be lost.

Linux 0.99.11							    1993-08-05							      GETITIMER(2)
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