Description

   The getsid function obtains the process group ID of the process
   that is the session leader of the process specified by pid. If
   pid is (pid_t)0, it specifies the calling process.

   This function requires that long (32-bit) UID/GID support be
   enabled. See 32-Bit UID and GID Macro (Integrity servers, Alpha)
   for more information.