Thread: c posix timer

> if timr period is short and before function exit, another fire occurs, what would happen?.
Signals are not queued.

Generally speaking, you should not be doing a lot of work inside signal handlers.

Code:

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

// https://en.wikipedia.org/wiki/Ackermann_function#Table_of_values
// https://www.geeksforgeeks.org/ackermann-function/
int ack(int m, int n)
{
    if (m == 0){
        return n+1;
    }
    else if((m > 0) && (n == 0)){
        return ack(m-1, 1);
    }
    else if((m > 0) && (n > 0)){
        return ack(m-1, ack(m, n-1));
    }
}

// Only call things on this list from a signal handler
// https://man7.org/linux/man-pages/man7/signal-safety.7.html
void fire(int sig) {
    static char in[] = "@-in\n";
    static char out[] = "@-out\n";
    write(1,in,strlen(in));
    if ( ++in[0] > '~' ) in[0] = '@';
    // Uncomment the next line to cause many events to be missed.
    //ack(3,11);  // burn way too much time in signal handler
    write(1,out,strlen(out));
    if ( ++out[0] > '~' ) out[0] = '@';
}

int main ( ) {
    signal(SIGALRM,fire);
    struct itimerval t;
    t.it_interval.tv_sec=0;
    t.it_interval.tv_usec=500000;
    t.it_value = t.it_interval;
    raise(SIGALRM);
    setitimer(ITIMER_REAL, &t, NULL);
    printf("Ack(3,11)=%d\n",ack(3,11));
    printf("Ack(4,1)=%d\n",ack(4,1));
    unsigned int u = sleep(5);
    printf("Sleep returned %u\n", u);
    return 0;
}

Uncomment line 30 for a totally different experience.


> how to stop a timer immediately?. How to restart the timer after stopping?.
> how to delete a timer?. How to restart the timer after deleting?.
The manual page has all these answers.
getitimer(3p) - Linux manual page

this is what i'm trying to do.
1. as soon as a flag is raised , need as instantaneously as possible to poll a status byte from serial port, then set timer off till next cycle. I found the fast timers suitable as it seems to be deterministic. But in my case when i make it too fast, the serial IO at low level crashes. I read handling the timers through threads is better than the signal handlers in these cases. I am not familiar with this yet.

2. replaced the timer with with a thread and works but pthread_create() may not be as instantaneous and deterministic as timers. I was thinking of calling pthread_create once and use some flag for stop/restart cycle. This has not worked satisfactorily yet.
any ...

So why don't you make your serial line unbuffered and call read() with a single char buffer from a thread?

The thread will block until there is a char available, and because it's unbuffered, it will return immediately with the char.