The message is generated as a result of an I_SENDFD ioctl(2) (see streamio(7I)) issued by a process to the sending stream head. STREAMS places the M_PASSFP message directly on the destination stream head's read queue to be retrieved by an _RECVFD ioctl(2) (see streamio(7I)). The message is placed without passing it through the stream (that is, it is not seen by any modules or drivers in the stream). This message should never be present on any queue except the read queue of a stream head. Consequently, modules and drivers do not need to recognize this message, and it can be ignored by module and driver developers.