Using timers in cpp

Please, have a look at the chrono header:
https://www.cplusplus.com/reference/chrono/

@Øystein Siggerud

Shall we know your query ?
FYI:
>>  1st Question
>>  Øystein Siggerud asked on 2021-01-18
>>  Please-give-me-a-programming-example-of-setitimer-unix-function-call-in-CPP  => This was resolved.

@Øystein Siggerud

From your given statement:

setitimer.cpp: In member function `void MyClass::registertimer()':
setitimer.cpp:28: error: ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function.  Say `&MyClass::timerevent'
setitimer.cpp:28: error: cannot convert `void (MyClass::*)()' to `void (*)()' in initialization

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I tried compiling the same at cygwin at windows

Reproduced your error at cygwin:
your code (29205556_base0.0.cpp)
I am using using iostream and namespace std; => for cygwin.
[code]
#include <iostream>
using namespace std;
#define INTERVAL  2
#include <signal.h>
#include <sys/time.h>

       struct itimerval tout_val;
class MyClass
{
    public :
       void settimer();
       void registertimer();
       void timerevent(void);
    private :
};

void MyClass::settimer()
{
    tout_val.it_interval.tv_sec = 0;
    tout_val.it_interval.tv_usec = 0;
    tout_val.it_value.tv_sec = INTERVAL; /* set timer for "INTERVAL (10) seconds */
    tout_val.it_value.tv_usec = 0;
    setitimer(ITIMER_REAL, &tout_val,0);
}

void MyClass::registertimer()
{
    typedef void(*fp)(void);
    fp ptr = &timerevent;
    signal(SIGALRM,(void(*)(int))ptr); /* set the Alarm signal capture */
}

void MyClass::timerevent(void)
{
     cout<<"timer event"<<endl;
     setitimer(ITIMER_REAL, &tout_val,0);
    //setitimer(ITIMER_REAL, &tout_val,0);
}
int main()
{
    MyClass myclass;
    myclass.settimer();
    myclass.registertimer();
    while(1)
    {
    }
}

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ERROR:

/usr/bin/g++ -DCYGWIN_NT -g -O -Wall 29205556_base0.0.cpp -o ./a.out
29205556_base0.0.cpp: In member function ‘void MyClass::registertimer()’:
29205556_base0.0.cpp:29:15: error: cannot convert ‘void (MyClass::*)()’ to ‘fp’ {aka ‘void (*)()’} in initialization
   29 |     fp ptr = &timerevent;
      |               ^~~~~~~~~~
make: *** [Makefile:2: all] Error 1

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I have modified your code:
$ diff 29205556_base0.0.cpp 29205556_base0.1.cpp
28c28
<     typedef void(*fp)(void);
---
>     typedef void(MyClass::*fp)(void);

/usr/bin/g++ -DCYGWIN_NT -g -O -Wall 29205556_base0.1.cpp -o ./a.out
29205556_base0.1.cpp: In member function ‘void MyClass::registertimer()’:
29205556_base0.1.cpp:30:34: warning: converting from ‘fp’ {aka ‘void (MyClass::*)()’} to ‘void (*)(int)’ [-Wpmf-conversions]
   30 |     signal(SIGALRM,(void(*)(int))ptr); /* set the Alarm signal capture */
      |                                  ^~~

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First Terminal:
$ ./a.out

Second Terminal:
$ kill -s 14 $(ps -eaf | grep a.out | awk '{ print $2}')
First Terminal:
$ ./a.out
timer event
timer event
timer event
timer event


Here "timer event" is repeated even if we do not sent kill signal.
Reason:
      #define INTERVAL  2
Hence
Replace:
      #define INTERVAL  2
With:
      #define INTERVAL  60
or seconds as per your requirement.
Recompile.

/usr/bin/g++ -DCYGWIN_NT -g -O -Wall 29205556_base0.1.cpp -o ./a.out
29205556_base0.1.cpp: In member function ‘void MyClass::registertimer()’:
29205556_base0.1.cpp:30:34: warning: converting from ‘fp’ {aka ‘void (MyClass::*)()’} to ‘void (*)(int)’ [-Wpmf-conversions]
   30 |     signal(SIGALRM,(void(*)(int))ptr); /* set the Alarm signal capture */
      |                    

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Retest
First terminal:
$ ./a.out
Second terminal:
$ kill -s 14 $(ps -eaf | grep a.out | awk '{ print $2}')
First terminal:
$ ./a.out
timer event


-Wpmf-conversions:
w Warn
p pointer to
m member
f functions
conversions
Read comment inside the code:

$ cat 29205556_base0.cpp
#ifdef CYGWIN_NT
        #include <iostream>
        using namespace std;
#else
        #include <iostream.h>
#endif
#define INTERVAL 60
#include <signal.h>
#include <sys/time.h>
struct itimerval tout_val;
class MyClass
{
        public:
                void settimer();
                void registertimer();
                void timerevent(int);
        private:
};
void MyClass::settimer()
{
        tout_val.it_interval.tv_sec = 0;
        tout_val.it_interval.tv_usec = 0;
        tout_val.it_value.tv_sec = INTERVAL; /* set timer for "INTERVAL (10) seconds */
        tout_val.it_value.tv_usec = 0;
        setitimer(ITIMER_REAL, &tout_val,0);
        return;
}
void MyClass::registertimer()
{
        // typedef void(MyClass::*fp)(int);
        auto ptr = &timerevent;
        signal(SIGALRM, (void(*)(int)) ptr); /* set the Alarm signal capture */
        return;
}
void MyClass::timerevent(int SIGNUM)
{
        if ( SIGALRM == SIGNUM )
        {
                cout << "timer event SIGALRM" << endl;
        }
        else
        {
                // This will happen when ptr is pointing to member function without an object
                // and without using static member function.
                cout << "timer event: " << (unsigned long)SIGNUM << endl;
        }
        setitimer(ITIMER_REAL, &tout_val,0);
        return;
}
int main()
{
        MyClass myclass;
        myclass.settimer();
        myclass.registertimer();
        while(1)
        {
        }
        return 0;
}

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First terminal:
$ ./a.out
Second terminal:
$ kill -s 14 $(ps -eaf | grep a.out | awk '{ print $2}')
First terminal:
$ ./a.out
timer event: 18446744073709536352

Hence handle using static member function.

Duplicate 23722832