PROGRAMMING INTERVIEWS

Note: SIGKILL and SIGSTOP cannot be ignored.

If a process wishes to handle a signal then in the code, the process has to register a signal handling function to the kernel. The following is the prototype of a signal handling function :

void <signal handler function> (int sig)

You can specify in this function how you want to handle it.

To invoke above function when a signal occurs, you need to register this function to kernal. To get the signal handler function registered to the kernel, the signal handler function pointer is passed as second argument to the ‘signal’ function. The prototype of the signal function is :

void (*signal(int signo, void (*func )(int)))(int);

Where func is the signal handler function, we defined.

Below is a program to handle SIGTERM signal. Run this program and send a SIGTERM signal to this to see what happens.

#include<cstdio>
#include<csignal>

void sig_handler(int signo)
{
 if (signo == SIGTERM)
   printf("received SIGTERM\n");
}

int main(void)
{
 if (signal(SIGTERM, sig_handler) == SIG_ERR)
 printf("\ncan't catch SIGTERM\n");
 // wait so that we can easily issue a signal to this process
 while(1)
   sleep(1);
 return 0;
}

As told earlier that KILL and STOP cannot be handled. Change above program to handle these two signals to see how the ‘signal’ system call responds in that case.

PS: Apart from handling the standard signals, we can also have user defined signals that can be sent and handled.