Signal Handling in C++

Signals are the interrupts that force an OS to stop its ongoing task and attend the task for which the interrupt has been sent. These interrupts can pause service in any program of an OS. Similarly, C++ also offers various signals which it can catch and process in a program. Here is a list of various signals and their operations that C++ provides the user to work with.

Signals	Operations
SIGINT	produces a receipt for an active signal
SIGTERM	Sends a termination request to the program
SIGBUS	Bus error which indicates access to an invalid address
SIGILL	Detects an illegal command
SIGALRM	This is used by the alarm() function and indicates the expiration of the timer.
SIGABRT	Termination of a program, abnormally
SIGSTOP	The signal cannot be blocked, handled, and ignored and can stop a process
SIGSEGV	Invalid access to storage
SIGFPE	Overflow operations or mathematically incorrect operations like divide by zero
SIGUSR1 SIGSUR2	User-Defined Signals

This signal() function is provided by the signal library and is used to trap unexpected interrupts or events. 

Syntax:

signal(registered signal, signal handler)

The first argument is an integer, representing the signal number and second is the pointer to a signal handling function. We must keep in mind that the signal that we would like to catch must be registered using a signal function and it must be associated with a signal handling function. The signal handling function should be of the void type. 

Example: 

CPP

// CPP Program to demonstrate the signal() function
#include <csignal>
#include <iostream>
using namespace std;

void signal_handler(int signal_num)
{
    cout << "The interrupt signal is (" << signal_num
         << "). \n";

    // It terminates the  program
    exit(signal_num);
}

int main()
{
    // register signal SIGABRT and signal handler
    signal(SIGABRT, signal_handler);

    while (true)
        cout << "Hello GeeksforGeeks..." << endl;
    return 0;
}

Output: Being in an infinite loop this code will show the following output until an interrupt is faced:  

Hello GeeksforGeeks...
Hello GeeksforGeeks...
Hello GeeksforGeeks...
Hello GeeksforGeeks...

Now if we press Ctrl+C to send an interrupt, the program will exit by printing:  

Hello GeeksforGeeks...
Hello GeeksforGeeks...
Hello GeeksforGeeks...
Hello GeeksforGeeks...
The interrupt signal is (22).

raise() function

The raise() function is used to generate signals. 

Syntax:

raise( signal )

It takes an argument as any of the functions mentioned in the list. 

Example:

CPP

// CPP Program to demonstrate the raise() function
#include <csignal>
#include <iostream>

using namespace std;

void signal_handler(int signal_num)
{
    cout << "Interrupt signal is (" << signal_num << ").\n";

    // It terminates program
    exit(signal_num);
}

int main()
{
    int count = 0;
    signal(SIGSEGV, signal_handler);
    // register signal SIGSEGV and signal handler

    while (++count) {
        cout << "Hello GeeksforGeeks..." << endl;
        if (count == 5)
            raise(SIGSEGV);
    }
    return 0;
}

Output

Hello GeeksforGeeks...
Hello GeeksforGeeks...
Hello GeeksforGeeks...
Hello GeeksforGeeks...
Hello GeeksforGeeks...
Interrupt signal is (11).