C++ Signal Handling

Signals are interrupts of software which is delivered to a process by the operating system. Signals can also be issued by the OS on the basis of system or error condition. Programmers can generate interrupts by pressing Ctrl+C on a UNIX, LINUX, Mac OS X or Windows system. There is some default behavior for some but in this chapter, you will learn about how to handle the signals and manage them properly.

Some signals are there which cannot be caught by the program but there is the list of signals which programmers can in the program and take adequate actions based on the signal. The signals are defined <csignal> header file.

Here are lists of signals along with their description and working capability:

  • SIGABRT: It is used for 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: It is used for detecting an illegal instruction
  • SIGINT: It is used to receipt an interactive program interrupt signal.
  • SIGSEGV: an invalid access to storage.
  • SIGTERM: A termination request sent to the program.
  • SIGHUP: Hang-up (POSIX), its report that user's terminal is disconnected. Signal used to report the termination of the controlling process.
  • SIGQUIT: Used to terminate the process and generate a core dump.
  • SIGTRAP: Trace trap
  • SIGBUS: BUS error, which indicates an access to an invalid address.
  • SIGUSR1: User-defined signal 1
  • SIGUSR2: User-defined signal 2
  • SIGALRM: Alarm clock, which indicates expiration of a timer. Used by the alarm() function.
  • SIGTERM: Used for termination. This signal can be blocked, handled, and ignored. Generated by kill¬†command.
  • SIGCONT: This signal sent to process to make it continue.
  • SIGSTOP: Stop, unblockable. This signal is used to stop a process. This signal cannot be handled, ignored, or blocked.

The signal() function

The signal-handling library of C++ provides function signal to trap unexpected event. The syntax of the signal function is:

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

This function is set to handle the signal. It specifies a way to handle the signals with the signal number specified by sig. Parameter func specifies one of the three ways in which a signal can be handled by a program:

  • Default handling (SIG_DFL): The signal is handled by the default action for that particular signal.
  • Ignore signal (SIG_IGN): The signal is ignored and the code execution will continue even if not meaningful.
  • Function handler: A specific function is defined to handle the signal.

Here is a simple C++ program where we will catch SIGINT signal using signal() function.

Example:
#include <iostream>
#include <csignal>
  
using namespace std;

sig_atomic_t signaled = 0;
void my_handler (int param)
{
  signaled = 1;
}
int main ()
{
  void (*prev_handler)(int);
  // register signal SIGINT with handler 
  prev_handler = signal (SIGINT, my_handler); 
  raise(SIGINT);
  cout<< "signaled is: " << signaled;
}

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