使用等待系统调用时

Question

The theory says that, if wait is not called parent wont be getting information about terminated child and child becomes zombie. But when we create a process, zombies are not created even if we are not calling wait. My question is whether the wait is called automatically?

Answer 1

In many languages, calling a sub process will call wait() for you. For example, in ruby or perl, you often shell out like this:

#!/usr/bin/ruby
system("ls /tmp")
`ls /tmp`

This is doing a bunch of magic for you, including calling wait() . In fact, Ruby must wait for the process to exit anyway to collect the output before the program can continue.

You can easily create zombies like this:

#!/usr/bin/ruby
if fork
  sleep 1000 # Parent ignoring the child
else
  exec "ls /tmp" # short-lived child
end

When we manually fork/exec, there is no magic calling wait() for us, and a zombie will be created. But when the parent exits, the zombie child will get re-parented to init , which will always call wait() to clean up zombies.