Three More Things

• Wait
  • This is a family of functions to let a parent know when a child process exited.
  • This is an extremely primitive form of signal handling.
  • When a child process exits, a signal (SIGCHLD) is sent to the parent.
  • By default processes ignore SIGCHLD
  • SIGCHLD
    • The child terminated
    • The child was stopped (SIGSTOP) (ctrl-z in shell)
    • The cild was continued. (SIGCONT)
  • run zombie.c and top -u dbennett
  • sys/types.h and sys/wait.h
  • pid_t wait(int *wstatus);
    • This waits for any child to exit.
    • The pid of the child process is returned
    • And information about the exit is stored in status.
  • pid_t waitpid(pid_t pid, int *wstatus, int options);
    • Waits for a specific pid, or set of pids
    • the pid argument is
      • Positive, then wait for that pid
      • 0 wait for any child pid.
      • negative: wait for pids in a process group.
    • The options argument is
      • WNOHANG - non blocking, only check to see if a child process has exited.
      • WCONTINUED - check for continued children.
  • For both, status is an int and can be interpreted by the followign macros
    • WIFEXITED(wstatus) : returns true for normal termination
    • WEXITSTATUS(wstatus): returns the least significant 8 bits of the exit value.
    • WIFSIGNALED(wstatus): returns true if the child was terminated with a signal.
    • WTERMSIG(wstatus): The signal that caused the process to exit.
    • WIFSTOPPED(wstatus): true if stopped
    • WIFCONTINUED(wstatus): true if continued
• dup
  • A family of functions which duplicate an open file descriptor
  • unistd.h
  • int dup(int oldfd);
  • int dup2(int oldfd, int newfd);
  • dup creates a copy of the given file descriptor at the next available location.
  • dup2 will close newfd if necessary and open duplicate the file descriptor there.
  • return -1 on failure and the new file descriptor on success.
• Why DUP? - pipe
  • unistd.h
  • int pipe(int pipefd[2]);
    • Creates a unidirectional data channel
    • On success a 0, error -1.
    • pipefd[0] is the read end of the pipe.
    • pipefd[1] is the write end of the pipe.
• Look at cppIOTest.C
• Look at dupTest.C