0                  Successful completion.
    -1                 Indicates an error. The function sets errno to
                       one of the following values:

                       o  EACCES - The value of the pid argument
                          matches the process ID of a child process
                          of the calling process and the child
                          process has successfully executed one of
                          the exec functions.

                       o  EINVAL - The value of the pgid argument is
                          less than 0, or is not a value supported by
                          the implementation.

                       o  EPERM - The process indicated by the pid
                          argument is a session leader. The value
                          of the pid argument matches the process ID
                          of a child process of the calling process,
                          and the child process is not in the same
                          session as the calling process. The value
                          of the pgid argument is valid but does
                          not match the process ID of the process
                          indicated by the pid argument, and there
                          is no process with a process group ID that
                          matches the value of the pgid argument in
                          the same session as the calling process.

                       o  ESRCH - The value of the pid argument does
                          not match the process ID of the calling
                          process or of a child process of the
                          calling process.