Using subprocess concurrency

A subprocess is a command that is started from within another process. As a simple example of this, let's create a parent process with three children. process_a will be the parent. Consider the following code snippet:

use std::process::Command;
use std::env::current_exe;

fn main() {
   let path = current_exe()
             .expect("could not find current executable");
   let path = path.with_file_name("process_b");

   let mut children = Vec::new();
   for _ in 0..3 {
      children.push(
         Command::new(path.as_os_str())
                 .spawn()
                 .expect("failed to execute process")
      );
   }
   for mut c in children {
      c.wait()
       .expect("failed to wait on child");
   }
}

The child process, process_b, runs a loop and prints its own process ID. This is shown as follows:

use std::{thread,time};
use std::process;
fn main() {
   let t = time::Duration::from_millis(1000);
   loop {
      println!("process b #{}", process::id()...