覆盖Ctrl-C

Question

I am supposed override the Ctrl C signal and use it to print a message. It is not supposed to end the program.

What happens so far is that when Ctrl C is pressed it prints the message, but ends the program.

When I asked my professor he told me to do this: You need to make your signal handler keep from continuing to process the signal. Right now the signal is being handled by your code and then going on to the parent handler.

Is there a method I am supposed to add or do i need to move the signal installers someplace?

This is my code so far:

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>  
#include <sys/types.h>
#include <signal.h> 
#include "Input.h"
#include "CircleBuff.h"

//void handler_function(int signal_id);

void catch_int(int sig_num){

    //reset the signal handler again to catch_int, for next time
    signal(SIGINT, catch_int);
    //print message
    printf("Print History");
    fflush(stdout);
}

void printHistory(CircleBuff hist){
    cout << "Complete History:\n" << endl;
    hist.print();
    cout << endl;
}

int main(int argc, char** argv){

  struct sigaction signal_action;                /* define table */
  signal_action.sa_handler = catch_int;   /* insert handler function */
  signal_action.sa_flags = 0;                    /* init the flags field */
  sigemptyset( &signal_action.sa_mask );     /* are no masked interrupts */
  sigaction( SIGINT, &signal_action, NULL ); /* install the signal_action */

  do{


  //My code: where the value report will be assigned within.

  } while(report != 1)

}

Answer 1

Whoa, way too much code to sift through. However, if you use the C standard library, you should get the desired behaviour. Here's a C++ version:

#include <iostream>
#include <csignal>

sig_atomic_t sigflag = 0;

void sighandler(int s)
{
  // std::cerr << "Caught signal " << s << ".\n"; // this is undefined behaviour
  sigflag = 1; // something like that
}

int main()
{
  std::signal(SIGINT, sighandler);

  // ... your program here ...

  // example: baby's first loop (Ctrl-D to end)
  char c;
  while (std::cin >> c)
  {
    if (sigflag != 0) { std::cerr << "Signal!\n"; sigflag = 0; }
  }
}

This will catch Ctrl-C (which raises SIGINT ), and the signal handler is not replaced, so it'll fire every time, and nobody is terminating the program.

Note that signal handlers are inherited by fork() ed children.

The Posix function sigaction() allows you to register "one-shot" handlers which are replaced by the standard handler after they're invoked once. That's more advanced and Posix-specific, though.

Edit: As @Dietrich points out, you should never do any real work inside a signal handler. Rather, you should set a flag (I provided an example), and check for that flag inside your loop (and print the message there). I'll amend the example for that, too.

Answer 2

Have you considered that inside your while loop there may be an interruptable function failing with 'EINTR'. You could might be able to fix it by using :

sa.sa_flags = SA_RESTART;

Or, just checking errno for it and looping. Are you sure the program isn't reaching the end of termination. Try putting a print statement at the end of main.