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 BSD
signal
SIGNAL(3F)SIGNAL(3F)NAME
signal - change the action for a signal
SYNOPSIS
integer function signal(signum, proc, flag)
integer signum, flag
external proc
DESCRIPTION
When a process incurs a signal (see signal(3C)) the default action is usually to clean up and abort. The user may choose to write an
alternative signal handling routine. A call to signal is the way this alternate action is specified to the system.
Signum is the signal number (see signal(3C)). If flag is negative, then proc must be the name of the user signal handling routine. If
flag is zero or positive, then proc is ignored and the value of flag is passed to the system as the signal action definition. In particu-
lar, this is how previously saved signal actions can be restored. Two possible values for flag have specific meanings: 0 means "use the
default action" (See NOTES below), 1 means "ignore this signal".
A positive returned value is the previous action definition. A value greater than 1 is the address of a routine that was to have been
called on occurrence of the given signal. The returned value can be used in subsequent calls to signal in order to restore a previous
action definition. A negative returned value is the negation of a system error code. (See perror(3F))
FILES
/usr/lib/libU77.a
SEE ALSO signal(3C), kill(3F), kill(1)NOTES
f77 arranges to trap certain signals when a process is started. The only way to restore the default f77 action is to save the returned
value from the first call to signal.
If the user signal handler is called, it will be passed the signal number as an integer argument.
4.2 Berkeley Distribution May 15, 1985 SIGNAL(3F)
05-27-2008
