Signals are the interrupts delivered to a process by the operating system which can terminate a program prematurely. You can generate interrupts by pressing Ctrl+C on a UNIX, LINUX, Mac OS X or Windows system.

There are signals which can not be caught by the program but there is a following list of signals which you can catch in your program and can take appropriate actions based on the signal. These signals are defined in C++ header file <csignal>.

Signal	Description
SIGABRT	Abnormal termination of the program, such as a call to abort
SIGFPE	An erroneous arithmetic operation, such as a divide by zero or an operation resulting in overflow.
SIGILL	Detection of an illegal instruction
SIGINT	Receipt of an interactive attention signal.
SIGSEGV	An invalid access to storage.
SIGTERM	A termination request sent to the program.

The signal() function:

C++ signal-handling library provides function signal to trap unexpected events. Following is the syntax of the signal() function:

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

Keeping it simple, this function receives two arguments: first argument as an integer which represents signal number and second argument as a pointer to the signal-handling function.

Let us write a simple C++ program where we will catch SIGINT signal using signal() function. Whatever signal you want to catch in your program, you must register that signal using signal function and associate it with a signal handler. Examine the following example:

#include <iostream>
#include <csignal>

using namespace std;

void signalHandler( int signum ) {
   cout << "Interrupt signal (" << signum << ") received.\n";

   // cleanup and close up stuff here  
   // terminate program  

   exit(signum);  

}

int main () {
   // register signal SIGINT and signal handler  
   signal(SIGINT, signalHandler);  

   while(1){
      cout << "Going to sleep...." << endl;
      sleep(1);
   }

   return 0;
}

When the above code is compiled and executed, it produces the following result:

Going to sleep....
Going to sleep....
Going to sleep....

Now, press Ctrl+c to interrupt the program and you will see that your program will catch the signal and would come out by printing something as follows:

Going to sleep....
Going to sleep....
Going to sleep....
Interrupt signal (2) received.

The raise() function

You can generate signals by function raise(), which takes an integer signal number as an argument and has the following syntax.

int raise (signal sig);

Here, sig is the signal number to send any of the signals: SIGINT, SIGABRT, SIGFPE, SIGILL, SIGSEGV, SIGTERM, SIGHUP. Following is the example where we raise a signal internally using raise() function as follows:

#include <iostream>
#include <csignal>

using namespace std;

void signalHandler( int signum ) {
   cout << "Interrupt signal (" << signum << ") received.\n";

   // cleanup and close up stuff here  
   // terminate program  

   exit(signum);  

}

int main () {
   int i = 0;
   // register signal SIGINT and signal handler  
   signal(SIGINT, signalHandler);  

   while(++i){
      cout << "Going to sleep...." << endl;
  
      if( i == 3 ){
         raise( SIGINT);
      }
  
      sleep(1);
   }

   return 0;
}

When the above code is compiled and executed, it produces the following result and would come out automatically:

Going to sleep....
Going to sleep....
Going to sleep....
Interrupt signal (2) received.