Wait

• A parent process is notified when a child process terminates.
• The easiest way to do this is via the wait call.
  • Needs sys/wait.h
  • pid_t wait(int *stat_loc);
    • returns the id of the process that exited, or -1 on error.
    • stat_loc contains information about the exit status.
  • sys/wait.h contains a number of macros to interpret the exit status.
    • WIFEXITED(stat_val) returns true if the child called exit.
    • WEXITSTATUS(stat_val) should be used only if the previous was ture, returns the lower 8 bits of the exit status.
    • WIFSIGNALED(stat_val) returns true if the child was terminated by a signal
    • WTERMSIG(stat_val) should be used only if the previous is true, returns the signal that caused termination.
    • WIFSTOPPED(stat_val) returns true if the child was stopped. This is only for waitpid.
    • WSTOPSIG(stat_val) returns the signal that stopped the child
    • WIFCONTINUED(stat_val) returns true when the child was continued. Again, only for waitpid.
  • Wait can return immediately if a child has met the conditions
  • Or it can block until this occurs.
  • See simpleWait.C
  • See multiWait.C
• waitpid
  • pid_t waitpid(pid_t pid, int *stat_loc, int options);
    • pid:
      • the process to wait for if positive
      • Or -1 means the next child to exit.
      • Or less than -1 means a process in the process group.
    • stat_loc as before
    • options
      • WCONTINUED reports status of continued children
      • WNOHANG waitpid is not blocking
      • WUNTRACED get stopped children as well.
  • See workingWait.C