Return_Values

   0                  Indicates success. If the WNOHANG option was
                      specified, and there are no stopped or exited
                      child processes, the waitpid function also
                      returns a value of 0.
   -1                 Indicates an error; errno is set to one of the
                      following values:

                      o  ECHILD-The calling process has no existing
                         unwaited-for child processes. The process
                         or process group ID specified by the
                         process_id argument does not exist or is
                         not a child process of the calling process.

                      o  EINTR-The function was terminated by
                         receipt of a signal.

                         If the waitpid function returns because
                         the status of a child process is available,
                         the process ID of the child is returned to
                         the calling process. If they return because
                         a signal was intercepted by the calling
                         process, -1 is returned.

                      o  EFAULT- The status_location argument points
                         to a location outside of the address space
                         of the process.

                      o  EINVAL- The value of the options argument
                         is not valid.