Description

   The waitpid function suspends the calling process until the
   request is completed. It is redefined so that only the calling
   thread is suspended.

   If the process_id argument is -1 and the options argument is
   0, the waitpid function behaves the same as the wait function.
   If these arguments have other values, the waitpid function is
   changed as specified by those values.

   The process_id argument allows the calling process to gather
   status from a specific set of child processes, according to the
   following rules:

   If the
   process_
   id is          Then status is requested

   Equal to -1    For any child process. In this respect, the
                  waitpid function is equivalent to the wait
                  function.
   Greater than   For a single child proces