Counter in C Programming - c

Question

I am trying to set up a counter for my programme to count how many times a signal was sent. I am trying to achieve the program to exit after ctrl +c was pressed twice. I have most of the code but just don't know how to link the counter to the if section. Here is my code.
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
// user-defined signal handler for alarm.
int cnt=0;
void my_handler(int signo)
{
while ( cnt < 2){
if (signo == SIGINT)
{
printf("Press ctrl c to stop\n");
exit(0);
}
}
}
int main(void)
{
signal(SIGINT,my_handler);
while(1){
printf(" I am running into infinite loop.., stop me if you dear..\n");
sleep(1); /* wait until alarm goes off */
}
} /* main */
I tried out the above code and it seems that the counter will stay at 0 forever and the programme exit immediately as soon as ctrl+c was pressed.

Answer 1

You can't do much safely in a signal handler, and definitely no I/O. For maximum portability, there are really only a handful of things a signal handler can do to affect global state:
Assign to a volatile sig_atomic_t variable
Make calls to signal
abort, exit, etc. on error
You can't do I/O safely. The safe thing to do is set a flag that the main thread can check and do your printing for you. If printing isn't necessary, the first call to your handler could just unregister itself, restoring the default behavior with SIG_DFL (or registering a new "second Ctrl-C" handler if you need to do something special) so the second Ctrl-C kills as normal.

Answer 2

Replace the while statement with an if one. If cnt is lower than 2, then print your message and count by using cnt++; else do something else

Answer 3

Signals are a little more involved to get right.
You should either use sigaction or a custom sigaction wrapper as signal doesn't have clearly defined semantics. Registering the handler may fail.
If you want to set a flag, it should be volatile sigatomic_t, you shouldn't do buffered IO in the handler.
With the wrapper and flag approach, you could do something like:
typedef void (Sigfunc)(int);
Sigfunc* reliableSignal(int signo, Sigfunc *func);
// user-defined signal handler for alarm.
volatile sig_atomic_t cnt=0;
void my_handler(int signo){
if(cnt++ == 1)
exit(0);
}
int main(void) {
if(reliableSignal(SIGINT,my_handler)<0){ perror("Signal"); exit(1); }
while(1){
printf(" I am running into infinite loop.., stop me if you dear..\n");
sleep(1); /* wait until alarm goes off */
}
} /* main */
Sigfunc* reliableSignal(int signo, Sigfunc *func)
{
struct sigaction act, oact;
act.sa_handler = func;
sigemptyset(&act.sa_mask);
act.sa_flags = 0;
if (signo == SIGALRM) {
#ifdef SA_INTERRUPT
act.sa_flags |= SA_INTERRUPT;
#endif
} else {
act.sa_flags |= SA_RESTART;
}
if (sigaction(signo, &act, &oact) < 0)
return(SIG_ERR);
return(oact.sa_handler);
}

Answer 4

In this example you must press twice Ctrl-C within 300ms.
So if you keep Ctrl-C pressed the program will stop otherwise not.
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
int ctrlcs=0;
void cchandler(int signum){
printf("Press again to quit.\n");
ctrlcs+=1;
if (ctrlcs==2) {
signal(SIGINT, SIG_DFL);
}
usleep(300000); // delay for the second Ctrl-C
}
int main(){
signal(SIGINT, cchandler);
while(1) {
printf(" I am running into infinite loop.., stop me if you dare..\n");
sleep (5);
ctrlcs=0;
}
}