Wait, Chapter 26

Objectives

1. Investigate the various forms of wait.

Notes

• When a process exits, the parent receives a SIGCHILD
• pid_t wait (int * _Nullable wstatus)
• pid_t waitpid(pid_t pid, int * _Nullable wstatus, int options);
  • Need sys/wait.h
  • Return on exit
  • waitpid can wait for
    • pid = -1: any child
    • > 0 specific PID
    • other options for process group, if we discuss that.
  • status:
    • Pointer to an int.
    • Macros
      • WIFEXITED(status): true if normal exit.
      • WEXITSTATUS(status) : return value
      • WIFSIGNALED(status): True if child was killed by a signal
      • WTERMSIG(status): Returns signal that caused termination
      • WIFSTOPPED(status): true if the child has been stopped.
      • WSTOPSIG(status): returns signal that stopped the child.
      • WIFCONTINUED(status): True if the child was continued
  • waitpid can be used to
    • Wait for stop and continue signals (WUNTRACED, WCONTINUED)
    • Perform a non-blocking check: WNOHANG
  • Return
    • PID of child exiting or -1 on error.
    • Or if non-blocking : 0 on no exit.
  • We will deal with signals eventually, but this is one way to handle a child process exit.