In C++ when interrupted with ctrl-c call a functio

Question

I want to write a few extra lines to a file when interrupted with ctrl-c before the program dies. However the location of the file is not hard coded so I need something more than normal interrupt handling as explained here. What is the best way to do this?

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Motivation:
I'm doing time dependent finite element simulations. Sometimes I forget or mis-estimate a reasonable tmax and the simulation would take a very long time to complete, so I interrupt it with ctrl-c. However I still want to use the data generated up to that point. In particular there's an xml file which needs a few lines appended after the simulation is finished to close the tags. Of course it's possible to do this by hand but it's a pain, so I'm try to automate it.

Answer 1

Based on the answer to "How can I catch a ctrl-c event? (C++)" we can install a signal handler that simply raises an exception. Then we can catch the exception and run whatever standard c++ code we like.

Here's a working example:

#include <iostream>
#include <exception>
#include <signal.h>
#include <stdlib.h>


class InterruptException : public std::exception
{
public:
  InterruptException(int s) : S(s) {}
  int S;
};


void sig_to_exception(int s)
{
  throw InterruptException(s);
}


int main()
  {
    // Taken from answer to "How can I catch a ctrl-c event? (C++)"
    struct sigaction sigIntHandler;
    sigIntHandler.sa_handler = sig_to_exception;
    sigemptyset(&sigIntHandler.sa_mask);
    sigIntHandler.sa_flags = 0;
    sigaction(SIGINT, &sigIntHandler, NULL);


    try
      {
        std::cout << "Inside try" << std::endl;

        // Do something with output so the loop doesn't get optimised out
        double i = 0;
        while(i < 1e30) {i++;} // loop until interrupted
        std::cout << i << std::endl;
      }
    catch(InterruptException& e)
      {
        std::cout << "Write something to file" << std::endl;
        std::cout << "Caught signal " << e.S << std::endl;
        return 1;
      }

    return 0;
  }

This approach does have at least one downside: we have to wrap everything after the handler is installed in a try, otherwise interrupt signals would cause an abort().

Answer 2

One common scheme for loop-based scientific code is to have a global volatile sig_atomic_t boolean indicating whether a signal was caught and add it to the loop condition.

e.g.

volatile sig_atomic_t interrupted=false; 
...
void signal_handler(int s)
{
   // ...
   interrupted=true;
}
...
while (!converged && !interrupted)
{
     // Perform computations
}
// Save file properly