What is a Signal?

Signals are interrupts delivered to a process by the operating system, which can potentially terminate our program prematurely. For example, pressing "Ctrl+C" on various operating systems such as Linux, UNIX, Mac OS X, or Windows generates an interrupt. Signals come in various types; some can be caught by the program, while others cannot.

Below is a list of catchable signals in a program and the corresponding actions that can be taken based on each signal. To handle signals in C++, we need to include the <csignal>header file.

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Types of Signals:

TYPESIGNALDESCRIPTION
Int (signals)SIGABRTSignal Abort - Abnormal termination, such as initiated by the abort function.
Int (signals)SIGFPESignal Floating-point Exception - An erroneous arithmetic operation, such as an overflow or divide by zero.
Int (signals)SIGILLSignal Illegal Instruction - Invalid function image, such as an illegal instruction, often due to code corruption or an attempt to execute data.
Int (signals)SIGINTSignal Interrupt - Interactive attention signal, usually generated by the application user.
Int (signals)SIGSEGVSignal Segmentation Violation - Invalid access to storage, like reading or writing outside the allocated memory.
Int (signals)SIGTERMSignal Terminate - A request sent to the program for termination.
Function (handlers)SIG_DFLDefault Handling - The signal is handled by the default action for that particular signal.
Function (handlers)SIG_IGNIgnore Signal - The signal is ignored.
Function (handlers)SIG_ERRSignal Error - A special return value indicating failure.

Signal() Function:

The signal()function is provided by the C++ signal-handling library to trap unexpected and unwanted events. Below is the general form of the signal()function:

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

This function requires two arguments:

  • An integer specifying the signal number.
  • A pointer specifying the signal-handling function.

Here's a simple example where we catch an SIGINT(Interrupt Signal) using the signal()function. To catch a specific signal in our program, we must register it using the signal function and associate it with a signal handler.

SIGNAL() - C++ Copy to Clipboard  
#include <iostream>
#include <csignal> // Required for signal handling

using namespace std;

void signalHandler(int signum) {
   cout << "Interrupt signal (" << signum << ") received.\n";
   // Perform necessary cleanup here
   // Terminate the program
   exit(signum);
}

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

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

   return 0;
}
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When we compile and execute our program, it works normally. If we press "Ctrl+C" to interrupt the execution, our program catches the signal and exits the loop.

Raise() Function:

We can generate signals using the raise()function. This function requires an argument, which is an integer signal number.

int raise(int sig);

Here, sigis the signal number to send, such as SIGINT, SIGABRT, SIGFPE, SIGILL, SIGSEGV, SIGTERM, or SIGHUP. Below is an example where we raise a signal internally using the raise()function:.

RAISE() - C++ CODE Copy to Clipboard  
#include <iostream>
#include <csignal> // Required for signal handling

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 << "Sleeping...." << endl;

      if (i == 3) {
         raise(SIGINT);
      }

      sleep(1);
   }

   return 0;
}

In this example, the program raises the SIGINTsignal internally when iequals 3. The signal handler catches the signal and terminates the program accordingly.

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AUTHOR

Sardar Omar

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