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.
Hi,
Just a thought but i might be totally wrong/out of subject: i suspect it has to do with linux timer resolution which is at 10ms (or not?) so no matter how small you set your interval your code will only be ran every 10ms ...
PS.: You query remind me of a "nice surprise" when i was working with timer on linux
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Discussion started by: senvenugopal
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LEARN ABOUT NETBSD
setitimer
GETITIMER(2) BSD System Calls Manual GETITIMER(2)NAME
getitimer, setitimer -- get/set value of interval timer
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/time.h>
int
getitimer(int which, struct itimerval *value);
int
setitimer(int which, const struct itimerval * restrict value, struct itimerval * restrict ovalue);
DESCRIPTION
The system provides each process with three interval timers, defined in <sys/time.h>. The getitimer() call returns the current value for the
timer specified in which in the structure at value. The setitimer() call sets a timer to the specified value, returning the previous value
of the timer if ovalue is not NULL.
A timer value is defined by the itimerval structure:
struct itimerval {
struct timeval it_interval; /* timer interval */
struct timeval it_value; /* current value */
};
If it_value is non-zero, it indicates the time to the next timer expiration. If it_interval is non-zero, it specifies a value to be used in
reloading it_value when the timer expires. Setting it_value to 0 disables a timer. Setting it_interval to 0 causes a timer to be disabled
after its next expiration (assuming it_value is non-zero).
The which parameter specifies the type of the timer:
ITIMER_REAL timer decrements in real time. This timer is affected by adjtime(2) and settimeofday(2). A SIGALRM signal is
delivered when this timer expires.
ITIMER_VIRTUAL timer decrements in process virtual time. It runs only when the process is executing. A SIGVTALRM signal is deliv-
ered when it expires.
ITIMER_PROF timer decrements both in process virtual time and when the system is running on behalf of the process. It is
designed to be used by interpreters in statistically profiling the execution of interpreted programs. Each time the
ITIMER_PROF timer expires, the SIGPROF signal is delivered. Because this signal may interrupt in-progress system
calls, programs using this timer must be prepared to restart interrupted system calls.
ITIMER_MONOTONIC timer decrements in monotonic time. This timer is not affected by adjtime(2) and settimeofday(2). A SIGALRM signal
is delivered when this timer expires.
Note that:
o Time values smaller than the resolution of the system clock are rounded up to this resolution (typically 10 milliseconds).
o The interaction between setitimer() and alarm(3) or sleep(3) is unspecified by the specification.
RETURN VALUES
If the calls succeed, a value of 0 is returned. If an error occurs, the value -1 is returned, and a more precise error code is placed in the
global variable errno.
ERRORS
Both functions may fail if:
[EFAULT] The value parameter specified a bad address.
[EINVAL] The which parameter was not a known timer type, or the value parameter specified a time that was too large to be handled.
SEE ALSO gettimeofday(2), select(2), sigaction(2), itimerval(3), timeradd(3)STANDARDS
The functions conform to IEEE Std 1003.1-2001 (``POSIX.1''). The later IEEE Std 1003.1-2008 (``POSIX.1'') revision however marked both as
obsolescent, recommending the use of timer_gettime(2) and timer_settime(2) instead.
HISTORY
The getitimer() function call appeared in 4.2BSD. The ITIMER_MONOTONIC functionality appeared in NetBSD 6.0.
BSD October 27, 2011 BSD
05-27-2008
