Chapter 25 Process Termination

Objectives

1. Understand the system calls related to process termination.

Notes

• This is chapter 25, read it.
• A bash note:
  • the variable ? contains the exit code of the last command run
  • echo $? will print that.
  • See simpleExit.cpp
• In general
  • A zero on exit indicates success
  • Anything else indicates an error.
  • Exit codes are only 8 bits. (0 to 255)
  • But apparently shells only expect 7 bits (0 to 127)
    • Bash at least adds 128 to a exit status when the process is killed by a signal.
  • The exit code is delivered to the parent process.
  • See the man page for make
  • We will see more when we get to wait.
• There are an amazing number of ways to terminate a process
  • normal termination is when the process requests an exit.
    • When this occurs, exit handlers are called.
  • abnormal termination is when this process is not followed.
• return, return int, exit(int),
  • These are normal exit conditions.
  • return with no arguments returns 0.
    • This was not always the case, but it is now.
  • All three call eventually call _exit();
• void _exit(int status)
  • Returns satus & 0xFF
  • Children are given a new parent (1), or a subreaper
  • The parent is sent a SIGCHLD
  • _exit does not call exit functions.
  • This is sort of a bail, turn off the engine, quit now function
• Abort
  • void abort(void)
  • Raises SIGABRT
  • Which causes instant termination.
  • This is an abnormal termination.
  • This is sort of like crashing into a wall.
• Throwing an uncaught exception
  • This is similar to abort.
  • This is an abnormal termination.
  • In fact, as far as I can tell, it ends in an abort.
  • kill -l
• ON 533 he provides a list of things both forms of termination do
  • Many involve things we have yet to discuss
  • But closing open file descriptors is one.
• Exit handlers
  • These are user functions that get called during the normal exit process.
  • They allow for
    • General clean up.
    • Unloading libraries.
  • int atexit(void (*func)(void));
    • Needs stdlib.h
    • adds a function to the stack of exit functions.
    • Functions are called in the reverse order in which they are registered.
      • This makes sense, just like unwinding a stack.
    • There can be at least 32 of these, but perhaps more.
    • These are passed on to child functions
    • But cleared on exec.
    • DO NOT CALL EXIT in an exit function.
      • This may result in infinite recursion.
    • These are not called on abnormal termination.
    • Look at the code in myExit.cpp