x                  The process group ID of the session leader of
                       the specified process.
    (pid_t)-1          Indicates an error. The function sets errno to
                       one of the following values:

                       o  EPERM - The process specified by pid is
                          not in the same session as the calling
                          process, and the implementation does not
                          allow access to the process group ID of
                          that process from the calling process.

                       o  ESRCH - There is no process with a process
                          ID of pid.

                       o  EINVAL - The value of pid is invalid.