C++ Signal Handling

Saturday, May 2, 2015

C++ Signal Handling
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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 signalfunction 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.

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