How can I catch a ctrl-c event?

Question

How do I catch a Ctrl+C event in C++?

Meistersinger

Console application, windows application, or what? – Boccie

Which OS Windows, Linux, etc.. – Crambo

Well, it's a Qt app, but I'm running it from the console during development. (This is Linux) – Meistersinger

Answer 1

signal isn't the most reliable way as it differs in implementations. I would recommend using sigaction. Tom's code would now look like this :

#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

void my_handler(int s){
           printf("Caught signal %d\n",s);
           exit(1); 

}

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

   struct sigaction sigIntHandler;

   sigIntHandler.sa_handler = my_handler;
   sigemptyset(&sigIntHandler.sa_mask);
   sigIntHandler.sa_flags = 0;

   sigaction(SIGINT, &sigIntHandler, NULL);

   pause();

   return 0;    
}

Flop

I think my_handler should take int s as it's argument. sig_t is itself a function pointer type. – Tribromoethanol

<stdlib.h>, etc - it's C, not C++. In C++ you should use <cstdlib> – Vyky

Works on OSX. In my main.mm file, I didn't even need to include signal.h, stdlib.h, or unistd.h. I just included stdio.h and Foundation/Foundation.h, and then if I needed to do std::cout stuff for outputting a shutdown message, I included iostream. – Kowalczyk

printf() isn't async-signal-safe, so can't be used inside signal handler. – Dineric

These functions are not available on Windows. – Glossary

would be nice to have some explanation about sa_mask and sa_flags. – Blakely

@Glossary No doubt you've figured it out by now, but for anyone else wondering: these are posix calls, which work on Linux and Unix systems. For windows you have to use the windows api. – Alluvium

@FelisPhasma: Being Posix doesn't really have much to do with it. Windows includes support for a decent number of Posix functions. – Glossary

Is there a reason to exit(1) instead of exit(s) in my_handler ? For me this loses information if for instance you want to manage (or just log) exit value ($?). – Lattie

Answer 2

For a Windows console app, you want to use SetConsoleCtrlHandler to handle CTRL+C and CTRL+BREAK.

See here for an example.

Miquelmiquela

Answer 3

You have to catch the SIGINT signal (we are talking POSIX right?)

See @Gab Royer´s answer for sigaction.

Example:

#include <signal.h>
#include <stdlib.h>
#include <stdio.h>

void my_handler(sig_t s){
           printf("Caught signal %d\n",s);
           exit(1); 

}

int main(int argc,char** argv)
{
   signal (SIGINT,my_handler);

   while(1);
   return 0;

}

State

Yes, it's POSIX. I forgot to add Linux to the question. – Meistersinger

signal() behaves differently, depending if it follows BSD or SysV style. sigaction() is preferable. – Timer

I know it is old, but this does not compile in g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0. I had to change void my_handler(sig_t s) to void my_handler(sig_atomic_t s). – Seineetmarne

Answer 4

Yeah, this is a platform dependent question.

If you are writing a console program on POSIX, use the signal API (#include <signal.h>).

In a WIN32 GUI application you should handle the WM_KEYDOWN message.

Stepup