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#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define STRINGIFY(x) #x
#define VERSION_STRING(x) STRINGIFY(x)

#ifndef VERSION
#define VERSION HEAD
#endif

static void sigdown(int signo) {
  psignal(signo, "Shutting down, got signal");
  exit(0);
}

static void sigreap(int signo) {
  while (waitpid(-1, NULL, WNOHANG) > 0)
    ;
}

int main(int argc, char **argv) {
  int i;
  for (i = 1; i < argc; ++i) {
    if (!strcasecmp(argv[i], "-v")) {
      printf("pause.c %s\n", VERSION_STRING(VERSION));
      return 0;
    }
  }

  if (getpid() != 1)
    
    fprintf(stderr, "Warning: pause should be the first process\n");

  if (sigaction(SIGINT, &(struct sigaction){.sa_handler = sigdown}, NULL) < 0)
    return 1;
  if (sigaction(SIGTERM, &(struct sigaction){.sa_handler = sigdown}, NULL) < 0)
    return 2;
  if (sigaction(SIGCHLD, &(struct sigaction){.sa_handler = sigreap,
                                             .sa_flags = SA_NOCLDSTOP},
                NULL) < 0)
    return 3;

  for (;;)
    pause();
  fprintf(stderr, "Error: infinite loop terminated\n");
  return 42;
}

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zerun.dong$ man waitpid
WAIT(2)                     BSD System Calls Manual                    WAIT(2)

NAME
     wait, wait3, wait4, waitpid -- wait for process termination

SYNOPSIS
     #include <sys/wait.h>

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:~# man waitpid
WAIT(2)                                                      Linux Programmer's Manual                                                      WAIT(2)

NAME
       wait, waitpid, waitid - wait for process to change state

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All of these system calls are used to wait for state changes in a child of the calling process, and obtain information about the child whose
state has changed.  A state change is considered to be: the child terminated; the child was stopped by a signal; or the child was resumed by
a  signal.   In the case of a terminated child, performing a wait allows the system to release the resources associated with the child; if a
wait is not performed, then the terminated child remains in a "zombie" state (see NOTES below).

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#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
   pid_t cpid, w;
   int wstatus;

   cpid = fork();
   if (cpid == -1) {
       perror("fork");
       exit(EXIT_FAILURE);
   }

   if (cpid == 0) {            
       printf("Child PID is %ld\n", (long) getpid());
       if (argc == 1)
           pause();                    
       _exit(atoi(argv[1]));

   } else {                    
       do {
           w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
           if (w == -1) {
               perror("waitpid");
               exit(EXIT_FAILURE);
           }

           if (WIFEXITED(wstatus)) {
               printf("exited, status=%d\n", WEXITSTATUS(wstatus));
           } else if (WIFSIGNALED(wstatus)) {
               printf("killed by signal %d\n", WTERMSIG(wstatus));
           } else if (WIFSTOPPED(wstatus)) {
               printf("stopped by signal %d\n", WSTOPSIG(wstatus));
           } else if (WIFCONTINUED(wstatus)) {
               printf("continued\n");
           }
       } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
       exit(EXIT_SUCCESS);
   }
}

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~$ ./a.out
Child PID is 70718
stopped by signal 19

continued
stopped by signal 19
continued
^C

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~# ps aux | grep a.out
zerun.d+   70717  0.0  0.0   4512   744 pts/0    S+   06:48   0:00 ./a.out
zerun.d+   70718  0.0  0.0   4512    72 pts/0    S+   06:48   0:00 ./a.out
root       71155  0.0  0.0  16152  1060 pts/1    S+   06:49   0:00 grep --color=auto a.out
~#
~
~#
~
zerun.d+   70717  0.0  0.0   4512   744 pts/0    S+   06:48   0:00 ./a.out
zerun.d+   70718  0.0  0.0   4512    72 pts/0    T+   06:48   0:00 ./a.out
root       71173  0.0  0.0  16152  1060 pts/1    S+   06:49   0:00 grep --color=auto a.out
~#
~
~#
~
zerun.d+   70717  0.0  0.0   4512   744 pts/0    S+   06:48   0:00 ./a.out
zerun.d+   70718  0.0  0.0   4512    72 pts/0    S+   06:48   0:00 ./a.out
root       71296  0.0  0.0  16152  1056 pts/1    R+   06:49   0:00 grep --color=auto a.out

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apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  shareProcessNamespace: true
  containers:
  - name: nginx
    image: nginx
  - name: shell
    image: busybox
    securityContext:
      capabilities:
        add:
        - SYS_PTRACE
    stdin: true
    tty: true

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~$ kubectl attach -it nginx -c shell
If you don't see a command prompt, try pressing enter.
/ # ps aux
PID   USER     TIME  COMMAND
    1 root      0:00 /pause
    8 root      0:00 nginx: master process nginx -g daemon off;
   41 101       0:00 nginx: worker process
   42 root      0:00 sh
   49 root      0:00 ps aux

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/ # kill -HUP 8
/ # ps aux
PID   USER     TIME  COMMAND
    1 root      0:00 /pause
    8 root      0:00 nginx: master process nginx -g daemon off;
   42 root      0:00 sh
   50 101       0:00 nginx: worker process
   51 root      0:00 ps aux

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~# ls -l /proc/140756/ns
total 0
lrwxrwxrwx 1 root root 0 May  6 09:08 cgroup -> 'cgroup:[4026531835]'
lrwxrwxrwx 1 root root 0 May  6 09:08 ipc -> 'ipc:[4026532497]'
lrwxrwxrwx 1 root root 0 May  6 09:08 mnt -> 'mnt:[4026532561]'
lrwxrwxrwx 1 root root 0 May  6 09:08 net -> 'net:[4026532500]'
lrwxrwxrwx 1 root root 0 May  6 09:08 pid -> 'pid:[4026532498]'
lrwxrwxrwx 1 root root 0 May  6 09:08 pid_for_children -> 'pid:[4026532498]'
lrwxrwxrwx 1 root root 0 May  6 09:08 user -> 'user:[4026531837]'
lrwxrwxrwx 1 root root 0 May  6 09:08 uts -> 'uts:[4026532562]'
~# ls -l /proc/140879/ns
total 0
lrwxrwxrwx 1 root root 0 May  6 09:08 cgroup -> 'cgroup:[4026531835]'
lrwxrwxrwx 1 root root 0 May  6 09:08 ipc -> 'ipc:[4026532497]'
lrwxrwxrwx 1 root root 0 May  6 09:08 mnt -> 'mnt:[4026532563]'
lrwxrwxrwx 1 root root 0 May  6 09:08 net -> 'net:[4026532500]'
lrwxrwxrwx 1 root root 0 May  6 09:08 pid -> 'pid:[4026532498]'
lrwxrwxrwx 1 root root 0 May  6 09:08 pid_for_children -> 'pid:[4026532498]'
lrwxrwxrwx 1 root root 0 May  6 09:08 user -> 'user:[4026531837]'
lrwxrwxrwx 1 root root 0 May  6 09:08 uts -> 'uts:[4026532564]'

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apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  shareProcessNamespace: false
  containers:
  - name: nginx
    image: nginx
  - name: shell
    image: busybox
    securityContext:
      capabilities:
        add:
        - SYS_PTRACE
    stdin: true
    tty: true

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~$ kubectl describe pod nginx
......
Events:
  Type    Reason          Age                   From     Message
  ----    ------          ----                  ----     -------
  Normal  SandboxChanged  3m1s (x2 over 155m)   kubelet  Pod sandbox changed, it will be killed and re-created.
  Normal  Killing         3m1s (x2 over 155m)   kubelet  Stopping container nginx
  Normal  Killing         3m1s (x2 over 155m)   kubelet  Stopping container shell
  Normal  Pulling         2m31s (x3 over 156m)  kubelet  Pulling image "nginx"
  Normal  Pulling         2m28s (x3 over 156m)  kubelet  Pulling image "busybox"
  Normal  Created         2m28s (x3 over 156m)  kubelet  Created container nginx
  Normal  Started         2m28s (x3 over 156m)  kubelet  Started container nginx
  Normal  Pulled          2m28s                 kubelet  Successfully pulled image "nginx" in 2.796081224s
  Normal  Created         2m25s (x3 over 156m)  kubelet  Created container shell
  Normal  Started         2m25s (x3 over 156m)  kubelet  Started container shell
  Normal  Pulled          2m25s                 kubelet  Successfully pulled image "busybox" in 2.856292466s