C - Linux - Timers

drigz · 08-29-2004, 07:16 AM

I want to run a set of code at set intervals, but allow my program to continue running while this is happening. I thought of two ways to do this: with a timer, or by starting a for loop (with sleep) running in the background. The timer way seemed much simpler, and I can use timers in Windows, so I thought it would be easy in Linux...

Does anybody know how to set up a timer in Linux? I have spent some time trying to figure it out from google and the contents of linux/timer.h, which I thought would have what I needed. However, whenever I try to compile a program which includes linux/timer.h, I get the error:
/usr/include/linux/timer.h:17: error: field `list' has incomplete type

Thank you.

Hko · 08-29-2004, 08:41 AM

Check the man page of "setitimer".

vijeesh_ep · 08-29-2004, 01:32 PM

Related to timer i usually heard about "jiffis" .what is it? why it is used?

shishir · 08-30-2004, 02:28 AM

jiffies to the best of my knowledge is accessible only from kernel space..
it actually is not a timer but the timers are based on the concept of jiffies..the varialble "jiffies" is updated every 1/HZ (HZ is the frequency at which the timer interrupts are generated)..on the 2.4 series kernels, this value of 100, i.e. jiffies was updated ever 10 milliseconds, in the 2.5 series onwards, HZ was changed to 1000 and thus jiffies gets updated every 1 millisecond...

this forms the basis for the timers on linux....read understandin the linux kernel to understand how the timers work...
also there is a timestamp counter that helps get resolution of more than a millisecond...
these things are explained in the timers chapter...

drigz · 08-30-2004, 09:58 AM

I read through the man page, but still can't see how I should use the getitimer and setitimer functions to get my desired behaviour. I got the impression that I should:
Declare an itimerval, like:

Code:

struct timeval my_value={1,500};
struct timeval my_interval={1,500};
struct itimerval my_timer={my_interval,my_value};

Then run getitimer on it like:

Code:

getitimer(ITIMER_REAL, *my_timer);

And then my program would recieve SIGALRM every 1.5 seconds. How can I set my program up to do something when it gets SIGALRM?

drigz · 08-30-2004, 10:26 AM

OK so far I have this:

Code:

#include <stdio.h>
#include <signal.h>
#include <sys/time.h>

typedef void (*sighandler_t)(int);

foo(int theint)
{
	printf("I just got the SIGALRM signal\n");
}

main()
{
	struct timeval my_value={1,500};
	struct timeval my_interval={1,500};
	struct itimerval my_timer={my_interval,my_value};
	getitimer(ITIMER_REAL, &my_timer);
	
	int i;
	signal(SIGALRM, (sighandler_t) foo);
	for(;;)
		i=5;
}

This compiles and runs, and if I use kill to send it the SIGALRM signal, it does what it should. However, it doesnt do anything of it's own accord. Is there something I need to do to activate the timer? Something with setitimer maybe?

drigz · 08-30-2004, 02:36 PM

Got it!

Code:

#include <stdio.h>
#include <signal.h>
#include <sys/time.h>

typedef void (*sighandler_t)(int);

foo(int theint)
{
	printf("I just got the SIGALRM signal\n");
}

main()
{
	struct timeval my_value={1,0};
	struct timeval my_interval={1,0};
	struct itimerval my_timer={my_interval,my_value};
	setitimer(ITIMER_REAL, &my_timer, 0);
	
	int i;
	signal(SIGALRM, (sighandler_t) foo);
	for( ; ; )
		i=5;
}