Does Process.waitFor() make the process reliant on the java parent? [duplicate]

Question

If waitFor is not used, killing JVM has no effect on its child process. Here is an example.

Bash script:

#!/usr/bin/env bash
echo "Sleeping..." > 'log'
sleep 30
echo "Wake up" >> 'log'

Java code:

public class Code {
  public static void main(String[] args) throws Exception {
    Process process = Runtime.getRuntime().exec("./child.sh");
    // process.waitFor();
  }
}

After Java Code is issued, JVM terminates immediately. And ps -ef | grep 'child.sh' | grep -v grep shows:

jing      3535  2761  0 13:47 pts/15   00:00:00 bash ./child.sh

Then after 30 seconds, I check the content of log file in the current directory. The content is:

Sleeping...
Wake up

And the above grep command shows nothing now. Now I uncomment process.waitFor() and recompile Code.java. After I run java Code, I use the above grep command to verify that child.sh child process is running. Then I issue Ctrl-C, JVM terminates. Now running the above grep command shows nothing. And the content of log file stays as:

Sleeping...

I have checked Process's Javadoc which does not explain this behavior. Then I use the following code to check the behavior of fork , execlp and waitpid system calls. And it shows the same behavior.

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

static void err_sys(const char* msg) {
  printf("%s\n", msg);
  exit(1);
}

int main(void) {
    pid_t   pid;

    if ((pid = fork()) < 0) {
        err_sys("fork error");
    } else if (pid == 0) {
        if (execlp("/home/jing/code/lintcode/child.sh", "child.sh", (char *)0) < 0)
            err_sys("execlp error");
    }

  if (waitpid(pid, NULL, 0) < 0)
    err_sys("wait error");

  exit(0);
}

I am using Oracle JDK 1.8 on Ubuntu 14.04. uname -a produces:

Linux jinglin 3.13.0-108-generic #155-Ubuntu SMP Wed Jan 11 16:58:52 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Can anyone give an explanation about this effect of waitFor and waitpid?

Does Process.waitFor() make the process reliant on the java parent? asks a similar problem on MAC platform. But it lacks details. So I ask this question for my environment here.

Answer 1

There's nothing special about waitPid(), other than the fact that you keep the parent process in the foreground.

---

If you fork and then wait for the child to finish, you have a (simplified) process tree like this:

─┬= 1 init
 └─┬= 2 bash --login
   └─┬= 3 java code
     └─── 4 bash child.sh

java is the foreground process in the terminal and the child is in its process group.

When you hit ^C the entire foreground process group gets terminated.¹

---

If you don't wait, then at first your process tree is the same as the one above. The java process terminates and the child process becomes a child of the process at the root of the process tree.

─┬= 1 init
 ├──= 2 bash --login
 └─── 4 bash child.sh

The child process finishes executing and terminates normally.

---

^{1The process group receives a SIGINT, for which the default action is to terminate. However, a different signal handler may be installed.}