5

So I'm trying to implement a signal handler for the SIGTSTP signal in a child process.

Basically what I'm trying to achieve is this:

  1. Start child Process
  2. Make the parent wait on the child process
  3. call Sleep on the Child process for x seconds.
  4. Before the sleep finishes execution, I want to send a Ctrl+Z signal. This signal should stop the child process, but resume the parent process. The parent process should then know the process id of the stopped process.

I run it using the command: ./testsig sleep 10

This is my code so far:

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



volatile sig_atomic_t last_proc_stopped;
volatile sig_atomic_t parent_proc_id;

 void handle_stp(int signum)
  {
    if(getpid()==parent_proc_id)
    {
        kill(parent_proc_id,SIGCONT);
        signal(SIGTSTP,handle_stp);
    }
    else
    {
        last_proc_stopped=getpid();
      kill(parent_proc_id,SIGCONT);
    }   
  }



  void main(int argc, char *argv[])
  {

    int childid=0,status;

    signal(SIGTSTP,SIG_IGN);

    parent_proc_id=getpid();


    childid=fork();

    if(childid>=0)
    {

      if(childid==0)//child
      {


        signal(SIGTSTP,handle_stp);
        strcpy(argv[0],argv[1]);
        strcpy(argv[1],argv[2]);
        argv[2]=NULL;
        printf("Passing %s %s %s\n",argv[0],argv[1],argv[2]);
        execvp(argv[0],argv);
      }

      else
      {
        wait(&status);

        printf("Last Proc Stopped:%d\n",last_proc_stopped);

      }
    }

    else
    {
      printf("fork failed\n");
    }
  }

Currently, it seems like ctrl+Z has some kind of effect (but definitely not the one I want!)

When I hit ctrl+Z in the middle of the child executing sleep, the cursor continues to blink for the remainder of the (in my case 10) seconds, but control does not reach the parent process.

Without hitting ctrl+Z, control returns to the parent as expected.

What am I doing wrong?

I have seen this answer as well, but I'm not really able to understand it:

After suspending child process with SIGTSTP, shell not responding

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RohanC
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1 Answers1

4

You have two processes:

  • The parent, which ignores the signal,

  • The child, which sets a handler, then execs another process - this will clear the code of the signal handler from the memory (the executed program will be loaded in place of the calling process), therefore will also clear the signal settings as well. So, your signal handler function will never be called. Is it possible to signal handler to survive after “exec”?

What you could do to achieve your goal?

  • The parent should ignore the signal,

  • The child should leave the default signal handling (which stops it),

  • The parent should use waitpid() to see if the child process exited or was stopped, and act accordingly (this involves actually killing the stopped child process).

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Laszlo Valko
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  • Oh I see. So is there a different approach to accomplish what I want to do? – RohanC Oct 14 '14 at 23:06
  • Also, if they're both not executing the signal handler- what is preventing the parent process from resuming when the child finished execution? – RohanC Oct 14 '14 at 23:10
  • I don't know, on my computer, it does resume running after `wait()` returns. There are two more mistakes: `last_proc_stopped` will not get set, so I get a 0 instead, and your magic with copying `argv` strings is gross, you should not really write into any of those strings. – Laszlo Valko Oct 14 '14 at 23:18
  • Haha I realise that. It's just a quick code to understand signal handlers. What I meant was, what prevents the parent from resuming when you hit ctrl+z while the child is executing sleep? – RohanC Oct 14 '14 at 23:26
  • Try running it with `strace -f -v`. – Laszlo Valko Oct 14 '14 at 23:42