Process Termination

• This is chapter 25, you should read it.
• exit, _exit, return, abort and exit handlers.
• exit
  • cstdlib, or stdlib.h
  • This is the normal exit function for C
  • void exit(int status);
  • Two standard return values EXIT_SUCCESS, EXIT_FAILURE,
  • Use of numbers larger than 127 may cause issues with the shell.
  • Exit handlers are called.
  • This should not return
  • For the c-99 standard, programs not calling exit, exit with a 0.
  • Before that it was the value of the last statement executed.
• _exit
  • unistd.h
  • void _exit(int status);
  • exit immediately, do not call exit handlers.
• List of actions when exit is called
  • Open file descriptors are closed.
  • Directory streams and other types of streams are closed as well.
  • File locks are released
  • Shared memory is freed.
  • Sighup is sent to any processes in a process group, if this is the process leader.
  • Other things we have yet to discuss.
• abort
  • cstdlib
  • void abort(void);
  • If sigabort is blocked, it unblocks it.
  • Then it raises the abort signal for the calling process.
  • This is an abnormal termination of a process.
• Exit handlers
  • These are mechanisms to clean up memory, libraries, ...
  • Only called from exit, not _exit.
  • Not called during abnormal termination.
  • There are two different mechanisms.
  • But they can be freely mixed.
  • Exit handlers are called in reverse order from when they were registered (stack)
  • And even added during an exit handler call.
    • These additions are pushed on the top of the "stack".
  • If an exit handler fails to return, other exit handlers are not called
    • The exit handler called _exit
    • The exit handler caused a signal to be raised, abort or some signal causing error
    • Another signal was received which caused termination
  • It is undefined for an exit handler to call exit.
• atexit
  • cstdlib.
  • int atexit(void (*function)(void));
  • In this case, we are passing a function pointer.
    • The function must be a void function that takes no parameters.
    • Child process inhered registered exit functions.
• on_exit()
  • cstdlib.
  • This is mostly a linux function.
  • int on_exit(void (*function)(int , void *), void *arg);
  • the function receives the exit value
  • And a pointer to an argument.