Process Termination

Notes

• We know that processes can terminate of their own accord
• In c++/linux/unix
  • return from main
  • call exit, in any of the forms.
• There are other forms of self termination
  • See abort
• But the os can terminate a process as well
  • This is normally done by sending a process a signal.
  • This is a software interrupt
  • Some can be blocked and some can't
  • Some can be ignored, some cant.
• When a child process terminates, a signal is sent to the parent
  • This is how wait works, it waits for the signal.
  • The signal is a SIGCHILD
  • SIGCHILD is normally ignored.
• In linux/unix an interesting process state is zombie
  • When a process exites, the kernel informs the parent that the child has exited.
  • If the parent doesn't handle this, the child becomes a zombie
    • It still has a PCB, but it is greatly reduced.
    • It can not be deleted from the system until this information is no longer needed.
    • If the parent exits without handling this, systemd reparents the process to the closest ancestor that is a subreaper.
    • Which instantly acknowledges the exit status and the child can be deleted.
• By the way, an orphan process is a process whos parent has exited
  • These used to become direct children of init, systemd
  • But now they get reparented
  • But notes suggest this is still systemd.
• Take a look at orphan.cpp and zombie.cpp