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.