Program terminates if I use scanf - c++

Question

I am trying to implement the timer in code, All the example that I found are using while(1) or for(;;) but when I tried with using scanf my program terminates. Is it getting any value on stdin because if I use scanf two times then timer is called two time before exiting from program.
Here is my sample code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <netinet/in.h>
#include <signal.h>
#include <time.h>
#include <linux/socket.h>
#include <time.h>
#define SIGTIMER (SIGRTMAX)
#define SIG SIGUSR1
static timer_t tid;
static timer_t tid2;
void SignalHandler(int, siginfo_t*, void* );
timer_t SetTimer(int, int);
int main(int argc, char *argv[]) {
int t;
printf("SIGRTMAX %d\n", SIGRTMAX);
printf("SIGRTMIN %d\n", SIGRTMIN);
struct sigaction sigact;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = SA_SIGINFO;
sigact.sa_sigaction = SignalHandler;
// set up sigaction to catch signal
if (sigaction(SIGTIMER, &sigact, NULL) == -1) {
perror("sigaction failed");
exit( EXIT_FAILURE );
}
// Establish a handler to catch CTRL+c and use it for exiting.
sigaction(SIGINT, &sigact, NULL);
tid=SetTimer(SIGTIMER, 1000);
struct sigaction sa;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_SIGINFO;
sa.sa_sigaction = SignalHandler;
// set up sigaction to catch signal
if (sigaction(SIG, &sa, NULL) == -1) {
perror("sa failed");
exit( EXIT_FAILURE );
}
// Establish a handler to catch CTRL+c and use it for exiting.
sigaction(SIGINT, &sa, NULL);
tid2=SetTimer(SIG, 1000);
// for(;;); or while(1) Working properly
scanf("%d", &t); /// Program terminates
return 0;
}
void SignalHandler(int signo, siginfo_t* info, void* context)
{
if (signo == SIGTIMER) {
printf("Command Caller has ticked\n");
}else if (signo == SIG) {
printf("Data Caller has ticked\n");
} else if (signo == SIGINT) {
timer_delete(tid);
perror("Crtl+c cached!");
exit(1); // exit if CRTL/C is issued
}
}
timer_t SetTimer(int signo, int sec)
{
static struct sigevent sigev;
static timer_t tid;
static struct itimerspec itval;
static struct itimerspec oitval;
// Create the POSIX timer to generate signo
sigev.sigev_notify = SIGEV_SIGNAL;
sigev.sigev_signo = signo;
sigev.sigev_value.sival_ptr = &tid;
if (timer_create(CLOCK_REALTIME, &sigev, &tid) == 0)
{
itval.it_value.tv_sec = sec / 1000;
itval.it_value.tv_nsec = (long)(sec % 1000) * (1000000L);
itval.it_interval.tv_sec = itval.it_value.tv_sec;
itval.it_interval.tv_nsec = itval.it_value.tv_nsec;
if (timer_settime(tid, 0, &itval, &oitval) != 0)
{
perror("time_settime error!");
}
}
else
{
perror("timer_create error!");
return NULL;
}
return tid;
}
So, How can I resolve this problem ?
Any help would be Appreciated.
Thanks, Yuvi

Answer 1

Signal interrupts scanf. If you add perror('scanf') after scanf output will be:
SIGRTMAX 64
SIGRTMIN 34
Command Caller has ticked
Data Caller has ticked
scanf: Interrupted system call
If you replace scanf with:
do {
errno = 0;
scanf("%d", &t);
} while(errno == EINTR);
scanf will be retried when it fail with Interrupted system call.

Answer 2

You absolutely cannot use printf and perror in a signal handler. They are not re-entrant.
Also, it's possible that scanf will return with an error if your program receives a signal while in it. If scanf returns EOF, check errno, it will probably be EINTR.