Wait and waitpid

• This is chapter 26. You should read it.
• pid_t wait(int *wstatus);
  • Needs sys/wait.h included
  • This reports the change in states for a child process.
  • Note that the parameter is a pointer to a int, not an int.

    • int status;
      wait (&status); 

    • Remember this is a c function, and there is no pass by reference, only pass by pointer.
  • If a child has changed state before this call, it will return immediately
  • If not, it will return only when a child changes state.
    • Or it will block and wait on the child.
    • This does not consume CPU time as the process is removed from runnable status.
  • On return,
    • The pid of the child
    • -1 indicates error.
  • The return code
    • There are a series of macros which can be used to inspect the value returned in the status parameter.
    • These take the int, not a pointer to the int.
      • WIFEXITED(wtatus) : returns true if the process exited normally.
        • IE called exit(), _exit(), or returned from main.
      • WEXITSTATUS(wstatus): returns the exit status of the process (ie the value returned by exit, _exit, return, but ONLY IF WIFEXITED is true.
      • WIFSIGNALED(wstatus): returns true if the child was terminated by a signal.
      • WTERMSIG(wstatus): returns the signal number that terminated thechild, BUT ONLY IF WSIGNALED is true.
      • WCOREDUMP(wstatus): returns true only if the child produced a core dump, but ONLY IF WIFSIGNALED is true.
      • WIFSTOPPED(wstatus): returns true if the process was stopped. Special conditions apply
      • WSTOPSIG(wstatus): returns the signal that caused the stop, but only if WIFSTOPPED was true.
      • WIFCONTINUED(wstatus): returns true if the child proces was continued.