Catch SIGINT in Xcode 12.5.1

I’m not entirely sure what you goal is here. You mentioned Xcode, so it seems like your using LLDB via Xcode’s GUI. Right? And you wrote:

I want to do something when ctrl + c is pressed

Do what? Stop the debugged process in the debugger? If so, ^C isn’t how that works in Xcode. Rather, you press the Pause button (or choose Debug > Pause, or press command-control-Y).

If that’s not what you’re looking for, please clarify.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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Example:

#include <stdio.h>
#include <signal.h>
  
void sigintHandler(int sig_num)
{
    signal(SIGINT, sigintHandler);
    printf("I'm doing something \n");
    fflush(stdout);
}
  
int main ()
{
    signal(SIGINT, sigintHandler);
  
    while(1)
    {        
    }
    return 0;
}

Yes, I am using LLDB.

So, to be clear, you’re running a program in Xcode and thus using Xcode’s built-in LLDB to debug it? Right? As opposed to running the program using the lldb command line tool?

Assuming that, you’re correct that Xcode’s console panel does not map ^C to SIGINT.

I have a bunch of hints and tips on this front.

The traditional BSD approach for this is to use SIGINFO rather than SIGINT. You can generate that in Terminal using ^T.

Of course, Xcode’s console panel doesn’t support that either (-: To get around this, run your program with its standard I/O channels connect to a Terminal window. To do this, go to Product > Scheme > Edit Scheme > Run > Options and change Console from Use Xcode to Use Terminal.

You’ll also need to disable SIGINFO handling in LLDB:

(lldb) process handle -p true -s false -n false SIGINFO

When SIGINT is received by the program, I want to print out a simple string.

Example:

Your example is not safe because it calls routines that are not async signal safe. See the sigaction man page for a list of those routines (it’s very short!).

So, tying this all together:

1. Create a new command-line tool project whose main.c looks like this:

   #include <sys/time.h>
   #include <unistd.h>
   #include <stdlib.h>
   #include <stdio.h>
   
   static void handleSIGINFO(int _) {
       #pragma unused(_)
       write(STDERR_FILENO, "Hello Cruel World!\n", 19);
   }
   
   int main(int argc, char **argv) {
       #pragma unused(argc)
       #pragma unused(argv)
       signal(SIGINFO, handleSIGINFO);
       while (1) {
           struct timeval tv;
           gettimeofday(&tv, NULL);
           fprintf(stderr, "%ld.%ld\n", (long) tv.tv_sec, (long) tv.tv_usec);
           sleep(1);
       }
       return EXIT_SUCCESS;
   }
   

2. Set it to use Terminal as the console, per the instructions above.

3. Run it.

4. Choose Debug > Pause.

5. In the Xcode console, enter this:

   (lldb) process handle -p true -s false -n false SIGINFO
   NAME         PASS   STOP   NOTIFY
   ===========  =====  =====  ======
   SIGINFO      true   false  false
   

   Note You can automate this step using LLDB scripting.

6. Resume the program.

7. Switch to Terminal and press ^T.

8. You’ll see output like this:

   % … lots of Xcode goo …
   … even more Xcode goo …
   1638792673.625175
   …
   1638792683.125808
   ^T
   load: 1.37  cmd: xxot 38172 running 0.24u 0.05s
   Hello Cruel World!
   1638792683.703585
   …
   

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"