Waiting

• This is from chapter 26. Don't worry about 26.14 or 26.3 for now.
• This figure might be helpful
• 
• When a process exits, the parent process is informed.
• The simplest way to receive this message is through the wait system call
  • pid_t wait(int *stat_loc);
  • needs sys/wait.h
  • Needs a pointer to an integer
    • This will contain the exit status.
    • It contains information about why the process exited.
    • And the 8 bit return value of the program.
  • The return value will be the process id of the child.
    • or -1 on error.
    • Most likely the process has no children.
  • Interpreting the status variable.
    • There are a set of macros set up to gain information about the process.
    • These are defined in sys/wait.h
    • Look at them and be in awe
    • But they pick off different bits of the integer.
    • USE THEM, do not write your own.
    • They are :
      • WIFEXITED(status) : return true if the child exited.
      • WIFSIGNALED(status): return true if the child was terminated due to a signal
      • WIFSTOPPED(status): return true if the child was stopped.
      • WIFCONTINUED(status): return true if the child was continued
    • They have corresponding information calls
      • WEXITSTATUS(status) returns the exit code when WIFEXITED is true
      • WTERMSIG(status) returns the signal id that killed the process if WIFSIGNALED is true
      • WSTOPSIG(status) returns the signal id that suspended the process if WSTOPPED is true.
  • Look at simpleWait.C
  • Look at multiWait.C
  • By the way, if a child has exited, the wait call will return immediately.
• A more complex call is waitpid
  • pid_t waitpid(pid_t pid, int *stat_loc, int options);
  • The stat_loc is the same as before
  • But you can specify a specific pid to wait for.
    • If pid = -1, wait for any child
    • If pid > 0, wait for the specified pid.
    • If pid < -1 wait for child in process group (we may discuss this later)
  • Options are the real interesting part
    • These are bit flags and should be combined with a bitwise or.
    • WCONTINUED - report when children are continued
    • WUNTRACED - report when a child process is stopped.
    • WNOHANG - don't block, just check for child termination.
  • If WNOHANG is specified then waitpid returns a 0 if no child has exited.
  • Look at workingWait.C