unix signals

Can u explain SIGCONT & SIGTSTP in a single program

SIGCONT is a signal to continue a stopped process
SIGTSTP is a signal when a terminal stops...for example when you press CTRL+Z

Example Program

Code:

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


void handler(int a)
{
   printf("SIGTSTP handler : Main has been stopped and some operations done\n");
   raise(SIGCONT);
}

void chandler(int a)
{
   printf("SIGCONT handler : Some operations done before main method continues\n");
}


int main (void)
{
   int i=0;
   signal (SIGTSTP, handler);
   signal(SIGCONT,chandler);
   for(i=0;i<5;i++)
   {
      printf("Main thread is gng to be stopped\n");
      raise(SIGTSTP);
   }
}



Output

Code:

Main thread is gng to be stopped
SIGTSTP handler : Main has been stopped and some operations done
SIGCONT handler : Some operations done before main method continues
Main thread is gng to be stopped
SIGTSTP handler : Main has been stopped and some operations done
SIGCONT handler : Some operations done before main method continues
Main thread is gng to be stopped
SIGTSTP handler : Main has been stopped and some operations done
SIGCONT handler : Some operations done before main method continues
Main thread is gng to be stopped
SIGTSTP handler : Main has been stopped and some operations done
SIGCONT handler : Some operations done before main method continues
Main thread is gng to be stopped
SIGTSTP handler : Main has been stopped and some operations done
SIGCONT handler : Some operations done before main method continues


SIGTSTP signal is used to stop a process and SIGCONT is used to continue a process and to do some special operations before a process continues.

In above example, handler() is used to handle SIGTSTP and chandler() is used to handle SIGCONT signal. Still im not very sure about signals concept. ll try to give even more better examples later.

thanks Christopher..It was a very good example

Code:


#include<signal.h>
#include<stdlib.h>
main()
{
    int pid;
    if ((pid = fork()) < 0)
    {
        perror("fork");
        exit(1);
    }

  if (pid == 0)
    {
      signal(SIGSTOP,SIG_DFL);
      system("ps -l");
      signal(SIGCONT,SIG_DFL);
      system("ps -l");
      for(;;);
    }
  else
    {
      printf("\n PARENT: sending SIGSTOP\n\n");
      kill(pid,SIGSTOP);
      sleep(3);
      printf("\n PARENT: sending SIGCONT\n\n");
      kill(pid,SIGCONT);
      sleep(3);

    }
}


[335818@oracleclient ~]$ gcc signal.c
[335818@oracleclient ~]$ ./a.out

Code:


PARENT: sending SIGSTOP

F S  UID  PID  PPID  C PRI  NI ADDR SZ WCHAN  TTY          TIME CMD
4 S  652  636  633  0  75  0 - 16527 wait  pts/3    00:00:00 bash
0 S  652  703  636  0  78  0 -  912 -      pts/3    00:00:00 a.out
1 T  652  704  703  0  78  0 -  911 finish pts/3    00:00:00 a.out
0 R  652  705  704  0  79  0 - 15886 -      pts/3    00:00:00 ps

PARENT: sending SIGCONT

F S  UID  PID  PPID  C PRI  NI ADDR SZ WCHAN  TTY          TIME CMD
4 S  652  636  633  0  75  0 - 16527 wait  pts/3    00:00:00 bash
0 S  652  703  636  0  75  0 -  912 -      pts/3    00:00:00 a.out
1 S  652  704  703  0  78  0 -  911 wait  pts/3    00:00:00 a.out
0 R  652  706  704  0  79  0 - 15886 -      pts/3    00:00:00 ps


Notice the 4rth process in the output.In this program, the child process is stopped and resumed.
In the first segment, you can notice that the child process has the status T denoting that it has been stopped.In the second segment,the same process has its status as S,showing that it has been resumed!